Mopar LA-Series Small-Block V8 Family Specs and Identification

Intro

The Mopar LA-series is the small-block family most Chrysler people eventually trip over: 273, 318, 340, and 360 cubic inches of wedge-head V8 that spent nearly three decades doing everything from hauling groceries to making big-block owners check the other lane. It replaced the wide-headed A-series Poly, eventually handed the job to the Magnum family, and in between became one of the most useful pieces of machinery Chrysler ever bolted between two fenders.

The LA idea was straightforward. Chrysler already had a good small-block foundation in the A-series, but those polyspherical heads were broad, heavy, and more complicated than the next generation of cars really needed. The LA went to compact wedge heads and a narrower, lighter package. “LA” is commonly explained as “Light A,” and whether somebody wants to argue the letters over coffee or not, the engineering direction is obvious the first time a Poly and an LA sit next to each other.

The 273 started the family in 1964 because Darts, Valiants, Barracudas, and other smaller Mopars needed V8 power without having an iron apartment building dropped over the front axle. It was small, responsive, and perfectly suited to proving the new architecture worked. Chrysler then did what Detroit always did after proving something worked: made it bigger.

The 318 became the engine that everybody owned and nobody bragged about until theirs finally wore out after years of abuse that should have killed it twice. Cars, trucks, vans, police vehicles, fleet machinery, family transportation — Chrysler scattered 318s everywhere because they were cheap enough, strong enough, smooth enough, and durable enough that nobody needed a committee meeting before installing one.

The 340 was the family member that arrived already looking for trouble. Chrysler kept the short 3.3125-inch stroke, opened the bore to 4.040 inches, gave it serious heads, compression, camshaft, induction, and exhaust, and produced a small-block that liked rpm and had enough breathing to make 340 cubic inches feel considerably less apologetic than the number suggested. This is the one collectors remember, which naturally means it is also the one ordinary engines have been pretending to be ever since orange paint became available in spray cans.

The 360 went the other direction. Its bore was actually slightly smaller than the 340’s, but Chrysler stretched the stroke to 3.580 inches and built the biggest factory LA around torque instead of trying to duplicate the 340’s personality. That made it particularly useful in heavier cars, trucks, vans, police work, and later street builds where pulling hard at ordinary rpm is considerably more useful than winning an argument about how quickly an unloaded engine revs in neutral.

That gives the family four genuinely different personalities instead of four numbers stamped onto the same engine. The 273 is the early lightweight. The 318 is the survivor. The 340 is the factory performance piece. The 360 is the torque motor. Chrysler shared enough architecture among them to make swapping useful and changed enough details to make blind swapping expensive.

The other trap is assuming every familiar Mopar small-block belongs here. The earlier Poly 318 is not an LA 318. The later 5.2 and 5.9 Magnum engines are close relatives, not merely LA engines with newer valve covers. Even inside the LA family, early 273 intake hardware, 340 crank and balance changes, 360 main journals and external balance, engine mounts, pans, front dress, and application hardware can turn “all small-block Mopars are basically the same” into a remarkably efficient method of collecting wrong parts.

That is why the LA family deserves more respect than its familiarity usually gets. Chrysler built millions of these things because the architecture worked. A good LA can be a correct restoration engine, cheap cruiser motor, nasty little performance piece, truck workhorse, or serious street build depending on which displacement and hardware are involved. The trick is knowing which LA is actually sitting there before the parts catalog starts making decisions.

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Overview

Chrysler created the LA family because the A-series Poly had reached the point where good engineering was carrying more iron and complexity than the next generation of cars needed. The Poly worked, but its broad polyspherical heads made the engine wide and heavy. Chrysler was moving deeper into compacts and intermediates, and a smaller, lighter wedge-head V8 made considerably more sense than continuing to package yesterday’s cleverness simply because the tooling already existed.

The LA kept the useful small-block fundamentals: a 90-degree cast-iron V8, 4.460-inch bore spacing, roughly 9.600-inch deck height, camshaft in the block, pushrods, five main bearings, rear-mounted distributor, and shaft-mounted rocker gear in traditional LA form. The major change was upstairs. Compact wedge heads replaced the wide Poly chambers, narrowing and lightening the package while giving Chrysler a simpler engine to manufacture, install, and keep developing.

That architecture became the backbone of Chrysler small-block production from 1964 into the early 1990s. Passenger cars, muscle cars, police packages, trucks, vans, fleet equipment, and service replacements all used LA engines. Families that survive that many assignments usually have one of two explanations: the company was desperately stubborn, or the basic engine was pretty good. In this case the evidence is leaning heavily toward the engine.

The 273 opened the family for 1964 with a 3.625-inch bore and roughly 3.312-inch stroke. It was designed around the compact-car job: enough V8 power to make a Dart, Valiant, or Barracuda considerably more entertaining without bringing big-engine weight and packaging trouble along for the ride.

Early 273s also remind everybody that “same family” does not mean Chrysler froze the design on opening day. The earliest heads use intake-manifold bolt details that differ from later LA practice. That is a small engineering footnote until somebody buys a later intake for an early engine and discovers the holes are expressing a different opinion.

Performance 273s showed what the new small-block could do when Chrysler sharpened the package. Commando and Charger versions added compression, four-barrel carburetion, solid-lifter cam timing, springs, and supporting hardware. The D/Dart pushed the little engine into a much narrower factory drag-racing role. The 273 was never going to win the displacement contest, but in a light car with the right pieces it had no obligation to apologize.

The 318 became the practical center of the family. It retained essentially the same 3.3125-inch stroke and increased bore to 3.910 inches, giving Chrysler more displacement without changing the basic short-stroke formula. When the LA 318 replaced the earlier Poly-headed 318 in regular U.S. use, Chrysler kept the familiar cubic-inch number and changed enough of the engine to guarantee future confusion at parts counters everywhere.

Most 318s were deliberately unremarkable. Two-barrel carburetion, hydraulic lifters, mild cams, conservative compression, and ordinary exhaust were exactly what a family car, van, truck, or fleet vehicle needed. People sometimes talk about that specification as though Chrysler had failed to build a 340 by accident. Chrysler was trying to build an engine that started every morning and did not require its owner to know what overlap meant.

That workhorse role explains the 318’s reputation better than any horsepower number. It survived bad maintenance, low-grade fuel, emissions equipment, towing, fleet service, teenagers, and owners who apparently believed the oil filter was a lifetime component. A worn-out 318 is not evidence the design was weak. Sometimes it is evidence the engine finally completed a sentence that started 180,000 miles earlier.

The 340 arrived for 1968 with a very different assignment. Chrysler kept the same nominal 3.3125-inch stroke but opened the bore to 4.040 inches, the largest factory bore of the LA engines covered here. That extra diameter gave the valves more breathing room, and Chrysler backed it with compression, heads, camshaft, induction, and exhaust selected for performance rather than mere transportation.

That big-bore, short-stroke combination gave the 340 its familiar personality. It likes airflow and rpm, especially in the lighter A- and E-body cars where additional engine weight would have worked against the entire point. The 340 did not beat larger engines by secretly possessing more cubic inches. It did it by using the cubic inches it had properly.

The 1970 AAR ‘Cuda and Challenger T/A Six Barrel and Six Pack versions gave the 340 its most collectible factory chapter. Special block, head, induction, and application details make those engines valuable enough that ordinary 340s have occasionally discovered previously unknown racing ancestors during the preparation of classified advertisements.

A real special-package 340 deserves real evidence. Correct block casting, date, stampings, partial VIN where applicable, heads, intake, carburetors, exhaust, and vehicle documentation should all agree. Three carburetors prove somebody has been shopping. They do not prove Chrysler did the shopping.

The ordinary 340 deserves its own respect anyway. Early high-compression four-barrel engines earned the reputation honestly, and later versions remained legitimate 340s even as compression fell, emissions requirements grew, gross horsepower ratings gave way to net numbers, and Detroit entered the portion of the 1970s nobody puts on nostalgic T-shirts.

Late 340 production also introduced another family trap. Most earlier forged-crank 340s are internally balanced, while later cast-crank versions moved to external balance. That external-balance setup is not the same as the 360’s. A weighted damper therefore proves somebody needs to keep investigating, not that the mystery engine has automatically become a 360.

The 360 arrived for 1971 and changed the mechanical progression more than its twenty-cubic-inch advantage over the 340 suggests. Instead of increasing bore again, Chrysler reduced it slightly to 4.000 inches and stretched the stroke to 3.580 inches. More crank throw gave the engine its extra displacement and shifted the natural personality toward low- and midrange torque.

The bottom end changed with it. The 273, 318, and 340 use 2.500-inch main journals, while the 360 uses larger 2.810-inch mains. Factory LA 360s are externally balanced and require the correct damper and transmission-side balance hardware. A 360 is therefore not a 340 that somebody stroked while the parts department was not looking.

That longer stroke made the 360 useful in exactly the vehicles Chrysler increasingly needed to power during the 1970s: heavier cars, police vehicles, trucks, vans, and anything else more interested in getting moving than singing at the top of the tachometer. It arrived during lower compression, emissions restrictions, and net horsepower ratings, which did its reputation no favors even though none of those things changed the fundamental usefulness of 360 cubic inches.

Later four-barrel, police, E58, truck, and other stronger 360 combinations proved the architecture had plenty of performance in it when the surrounding parts were chosen accordingly. The same applies today. A sound 360 with useful compression, decent heads, sensible cam timing, good induction, exhaust, ignition, converter or clutch, and gearing makes an excellent street engine without needing anybody to call it a 340.

Taken mechanically, the family has a wonderfully clean first three steps. The 273 is 3.625 × 3.312. The 318 keeps essentially the same stroke and increases bore to 3.910. The 340 keeps the stroke again and takes bore all the way to 4.040. Chrysler made those three engines largely by keeping the crank throw and opening the cylinders farther.

Then the 360 changes the recipe: 4.000-inch bore, 3.580-inch stroke, larger mains, and external balance. That is why the 340 and 360 can be close in displacement while feeling different in use. One gets its personality from the largest bore and short stroke. The other gets more leverage from the longer crank. Cubic inches tell you how much air the cylinders hold. They do not tell you how Chrysler got there.

Interchange throughout the family is substantial and useful, which is exactly why careless builders get into trouble. Heads can move. Intakes can move when the bolt pattern and port arrangement permit it. Accessories, distributors, timing pieces, transmissions, and other hardware can often be adapted across years and displacements. The fact that something can be bolted onto another LA block does not establish that Chrysler ever intended the combination or that the combination is any good.

The 273’s early intake arrangement is one example. Engine-mount boss spacing between the 273/318 and 340/360 side of the family is another. Oil pans and pickups follow chassis requirements. Timing covers, water pumps, pulley alignment, radiator-hose routing, alternator brackets, power steering, air conditioning, and other front-drive pieces changed as the years and vehicles changed. A complete engine can become astonishingly incomplete the moment it is moved to another chassis.

Transmission fit carries the same warning. The Chrysler small-block bellhousing pattern gets everybody into the same neighborhood, but crank registers, converters, flexplates, flywheels, starters, clutch pieces, year changes, and balance requirements still decide whether the parts should actually be introduced to one another.

The LA family also sits between two related engine lines that cause endless mistaken assumptions. Before it came the A-series Poly. After it came the Magnum 5.2 and 5.9. All three belong to Chrysler’s broader small-block history. They do not form one universal pile of cylinder heads and intake manifolds.

The Poly uses its broad polyspherical heads and different top-end architecture. The traditional LA uses wedge heads, shaft-mounted rockers, and the familiar five-bolt valve covers. The Magnum changes again with pedestal rockers, pushrod oiling, different intake attachment, ten-bolt valve covers, and later fuel, ignition, accessory, and front-drive systems. Family trees are excellent for reunions. They remain lousy interchange manuals.

Choosing among the four LA engines is easier once bragging rights are removed from the discussion. A correct 273 makes sense in an early car where originality and crisp light-car character are the point. It makes considerably less sense when somebody plans to spend machine-shop money chasing horsepower that a larger engine begins with for free.

The 318 remains the sensible cruiser and general-purpose choice. A healthy one can respond nicely to compression, a reasonable camshaft, four-barrel induction, exhaust, ignition tuning, and suitable gearing. The mistake is spending 360 money turning a 318 into an engine that still has 318 cubic inches merely because the original core happened to be free.

The 340 deserves its premium when the car, history, or desired high-revving performance character actually calls for a 340. In a genuine 340 car, especially one with desirable original hardware, preserving it can make considerably more sense than treating collector-value iron as an anonymous foundation for modern horsepower.

The 360 is generally the practical performance answer when originality does not dictate otherwise. More displacement, more stroke, broad availability, and less collector tax make a sound 360 difficult to argue against for a heavier street car or torque-oriented build. Its external balance and other specific hardware require attention. They do not require fear.

Condition can still defeat every tidy recommendation. A complete healthy 318 with the correct pan, mounts, brackets, manifolds, front dress, and transmission hardware may be a better engine than a worn-out 360 delivered as a bare long block and three lies. Likewise, a documented 340 or rare 273 package can be worth preserving even when a different engine would produce more power for less money.

The LA family’s reputation ultimately comes from versatility. Chrysler used this architecture for nearly everything a small V8 could reasonably be asked to do, and several things owners probably should not have asked. The 273 gave light cars V8 power. The 318 spent decades doing the chores. The 340 gave the family real muscle-car teeth. The 360 supplied the torque and displacement that kept the design useful after the original muscle era was over.

That versatility is also the reason “small-block Mopar” should never be the last sentence before ordering parts. Identify the displacement. Know the year. Know the crank and balance arrangement. Check the heads, intake, mounts, pan, front dress, transmission hardware, and application. Then decide what the engine actually needs. Chrysler made the LA family simple enough to be useful. It never promised to make guessing free.

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General Family Specs

Engine Family	Mopar LA-Series Small-Block Wedge V8	Major Factory Engines in This Family	273, 318, 340, 360
Production / Use Era	1964 through early 1990s LA production/use era, depending on displacement and application; later Magnum engines are a related separate family	Configuration	90-degree V8
Valve Layout	OHV / pushrod, 2 valves per cylinder	Cam Location	In-block camshaft
Block Style	Chrysler LA small-block wedge architecture	Cylinder Head Layout	Wedge combustion chamber heads
Rocker Layout	Shaft-mounted rocker arms in common LA production form	Distributor Location	Rear-mounted
Bore Spacing	4.46 in small-block Chrysler pattern	Common Block Material	Cast iron
Common Head Material	Cast iron in common factory use	Oiling System	Wet sump
Typical Induction	Carbureted in classic LA use; later electronic and throttle-body-era applications exist by year and application	Typical Bellhousing Pattern	Chrysler small-block pattern; verify year, crank, converter, flywheel/flexplate, starter, and transmission hardware
Major Family Boundary	Not A-series poly, not B/RB big block, not Gen II Hemi, and not later Magnum small-block family	Major Identification Caution	273, 318, 340, and 360 are related LA engines, but displacement, balance, crank, heads, and application details aren’t universal.

Those are broad family facts, not a permission slip to order parts blind. The table gives the outline. It doesn’t know what block is on your stand, what crank is in it, what heads got swapped, what car it came from, or what somebody did to it in 1987 with a socket set and misplaced confidence.

The LA family shares enough architecture to look simple from across the garage. It also differs enough to punish anyone who treats 273, 318, 340, and 360 as one long part number. The spec sheet points you in the right direction. The actual engine still has to answer questions.

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Identification

The Mopar LA small-block is the compact wedge-head Chrysler V8 family that includes the 273, 318, 340, and 360. The distributor sits at the rear, the cylinder heads are conventional wedge pieces, the valve covers are narrow compared with the earlier A-series Poly, and the whole engine is considerably smaller than Chrysler’s B/RB big blocks.

That rear distributor is a useful first clue because B/RB big blocks and the Gen II 426 Hemi put the distributor up front. It does not separate an LA from the earlier A-series Poly or later Magnum small-block because those engines also live in the rear-distributor neighborhood.

For that, look at the heads and valve covers. The A-series Poly has much broader polyspherical heads and distinctive wide covers. The traditional LA has compact wedge heads with shaft-mounted rocker gear. The later Magnum has a related block architecture but uses a different top end, including pedestal-mounted rockers, pushrod oiling, different intake attachment, and ten-bolt valve covers.

So family identification is usually easy. Exact displacement is where the grease starts earning its keep.

Read the Front Engine Identification Pad

Before crawling around looking for casting numbers, clean the machined pad at the front driver side of the block immediately below the cylinder head. Chrysler stamped engine-assembly information here on LA small-blocks.

Depending on production year, the stamp can include model-year information, displacement, assembly date, plant or sequence information, and additional machining or application codes.

Later small-block stamps can plainly include 273, 318, 340, or 360. Chrysler occasionally made identification almost civilized.

Do not expect one stamping format to cover the entire production history. Chrysler changed code formats as the years passed. Decode the pad according to the engine’s production era rather than forcing a 1973 chart onto a 1965 block.

The pad is one of the strongest assembled-engine clues because it is part of the block rather than something hanging from a bolt. It can still be damaged by decking, corrosion, careless cleaning, or deliberate restamping, so valuable 340 claims deserve the rest of the evidence too.

Find the Block Casting Number

The raised block casting number is found on the driver side of the block, usually low enough that the starter, exhaust, dirt, and fifty years of oil can make reading it considerably less convenient than Chrysler intended.

Many LA castings include the original displacement as part of the raised identification. Common examples include:

• 2465330 and 2536130 — early 273 castings.
• 2536030, 2806030, and related castings — common 318 production.
• 2780930 — the familiar production 340 block.
• 3577130-TA — special 340 casting associated with 1970 AAR ‘Cuda and Challenger T/A production.
• 3418496 — early 360 production.
• 3870230 and later families — later 360 production.

Unlike some Ford families where one block number can start a three-hour argument over five possible displacements, Chrysler’s LA casting numbers are often genuinely useful.

They still identify what the block was manufactured to be. A 360 block with a stroker crank does not grow a new casting number. A 340 block bored oversize does not send Chrysler a change-of-address card.

Check the Casting Date

The casting date is normally found near the raised casting information on the side of the block. It tells you when the bare block was poured.

That date should precede the engine-assembly date and, on an original engine, the vehicle build date.

This is particularly useful when somebody is selling a “numbers-correct” or “original 340” package. A block cast after the vehicle was built has already settled that question no matter how accurately somebody reproduced the orange paint.

Partial VIN Stamping

Later LA blocks can also carry a partial vehicle VIN, which matters when originality is part of the price.

The 1968 model year was a transition period and stamping locations can vary. On 1969 and later production, look for the partial VIN on a machined pad low on the passenger side of the block just above the oil-pan rail.

When that stamping matches the car and the casting date and engine-assembly information also make chronological sense, you have strong evidence that the block belongs to that vehicle.

A service-replacement engine may legitimately lack the original vehicle VIN. Missing numbers therefore require context. Fresh numbers on a freshly ground pad require something else.

273

The 273 is the original LA small-block, using a 3.625-inch bore and approximately 3.312-inch stroke.

Its early production period is one of the easiest clues. A genuine 1964–1966-era LA casting with a correct 273 front stamp and appropriate vehicle application makes a strong identification without opening the engine.

The 273’s small bore is mechanically decisive. A 318, 340, or 360 cannot hide inside a standard-bore 3.625-inch block.

Early 273 Intake-Bolt Trap

The earliest 1964–1965 273 cylinder heads use a different intake-manifold bolt arrangement from later LA heads. That makes an early 273 easy to turn into a wrong-manifold purchase if somebody assumes every LA intake bolts on the same way.

This is useful supporting evidence on an apparently original early engine, but it identifies the head and production period more than displacement. Heads can be changed.

When dealing with an early 273, verify the actual head casting and intake-bolt arrangement before buying a manifold. “It’s an LA small-block” is not enough information.

318

The LA 318 uses a 3.910-inch bore and 3.3125-inch stroke. It shares essentially the same stroke family as the 273 and 340, with displacement coming mainly from bore diameter.

The 318 is by far the easiest displacement to assume and therefore one of the easiest to misidentify through laziness. Chrysler built enormous numbers of them, and that has turned “probably a 318” into a substitute for actually looking.

Casting numbers and the front identification pad usually settle a stock engine quickly. A later block often has the 318 displacement cast or stamped plainly enough that no séance is required.

Do not confuse the LA 318 with the earlier A-series Poly 318. Same cubic-inch number, different heads, intake, valvetrain, exhaust, valve covers, and large sections of the parts catalog.

340

The 340 is the money engine in this family, which means it deserves the least trust from appearance alone.

Factory dimensions are 4.040-inch bore and 3.3125-inch stroke. That bore is actually larger than the 360’s 4.000 inches, while the 340 retains the shorter 273/318-style stroke.

The familiar production block casting is 2780930. A correct casting, correct date, correct front-pad stamp, and proper vehicle/application evidence make a strong 340 identification.

That is considerably better evidence than orange paint, a four-barrel intake, 340 decals, large-port heads, or somebody mentioning that the car “feels like a 340.”

AAR / T/A 340 Claims

The 1970 AAR ‘Cuda and Challenger T/A used the special Six Barrel/Six Pack 340 package. The dedicated block casting commonly carries 3577130-TA.

That casting is valuable evidence, but the expensive claim still deserves the correct production date, vehicle VIN where applicable, heads, intake, carburetors, exhaust, and other application hardware.

Three carburetors prove somebody has three carburetors. The block and vehicle still have to prove the rest of the story.

340 Heads Are Not 340 Proof

Desirable LA heads commonly called X, J, O, U, and other casting-letter families show up constantly in 340 discussions. Large valves and good ports can support a correct-period performance combination.

They do not identify the block.

Those heads have been moved among 318s, 340s, and 360s for decades. A 318 with J-heads is still a 318. A 360 wearing 340 intake and exhaust hardware is still whatever the block and crank say it is.

Forged-Crank and Cast-Crank 340 Balance

Most earlier 340s use a forged crankshaft and are internally balanced. Late production moved to a cast crankshaft and external balance.

The externally balanced cast-crank 340 uses a distinctive weighted harmonic damper, often identified for 340 cast-crank use. Matching flywheel or converter/flexplate balance is also required.

This matters because the 340 external-balance arrangement is not the same as the 360 external-balance arrangement. Both engines may have weighted hardware and still require different weights.

A wrong damper or flywheel can make an otherwise healthy small-block shake hard enough to start a whole new troubleshooting hobby.

360

The 360 uses a 4.000-inch bore and 3.580-inch stroke.

This is the mechanical break in the LA family. The 273, 318, and 340 all use roughly the same short-stroke crank geometry. The 360 gets its extra displacement from a substantially longer stroke.

It also uses larger 2.810-inch main journals, compared with the 2.500-inch mains used by the 273/318/340 branch.

That means a 360 crank is not simply a longer 340 crank waiting to fall into the same block.

The 360’s External Balance Is an Excellent Clue

Factory LA 360 engines are externally balanced. The harmonic damper has an obvious offset weight rather than the simple neutral appearance of a traditional internally balanced 273/318 or forged-crank 340.

The rear of the rotating assembly also requires the correct 360-specific balance through the converter, flywheel, or appropriate flexplate arrangement.

This gives you a useful assembled-engine clue. If somebody claims an apparently stock 360 but it has neutral-balance 318 hardware at both ends, something needs investigating before the ignition key gets a vote.

Do not use the damper as sole displacement proof. Dampers can be replaced, aftermarket rotating assemblies can be internally balanced, and plenty of old engines contain whatever parts the previous owner could make bolt together.

Engine-Mount Bosses Can Help Separate the Small-Bore and Performance Blocks

The driver’s-side engine-mount boss spacing also provides a useful supporting clue. The 273/318 use one left-side mounting arrangement, while the 340/360 use a different spacing. The passenger-side mounting relationship is much more consistent across the family.

This matters during swaps because a 318 bracket does not always sit correctly on a 340/360 block without the proper bracket or spacing.

It is a supporting architectural clue, not enough reason to ignore a clearly stamped casting number.

LA Versus Magnum

The later 5.2 and 5.9 Magnum engines are close relatives of the LA family, but they are not traditional LA engines with different valve covers.

A Magnum uses ten-bolt valve covers, individual pedestal-style rocker arms rather than the LA shaft-mounted rocker system, pushrod oiling, and a different intake-manifold bolt arrangement. Production Magnum engines also bring later front dress, fuel-system, and electronic changes.

A traditional LA has the older five-bolt valve-cover pattern and shaft-mounted rocker gear.

That is enough to separate the families quickly even when somebody has already removed the injection and installed a carburetor on the Magnum.

All 318s, 340s, and 360s Are Not Performance Engines

Displacement identifies displacement. It does not automatically identify the package.

A 318 can be a two-barrel economy engine, truck workhorse, police combination, or modified street engine. A 340 can be an ordinary production 340, high-performance restoration piece, T/A engine, or a block that has spent fifty years collecting somebody else’s hardware. A 360 can be a low-compression smog-era passenger engine, truck engine, police package, performance replacement, or very serious aftermarket build.

Once the displacement is known, exact heads, intake, carburetor, camshaft, exhaust, compression, distributor, balance package, vehicle code, and dates determine what version you actually have.

Common Identification Mistakes

• “Rear distributor means LA.”
  The earlier A-series Poly and later Magnum small-blocks also put the distributor at the rear. Look at the heads and valve covers.
• “Every Mopar 318 is the same engine.”
  The A-series Poly 318 and LA 318 share displacement, not the same top-end architecture.
• “It is orange and has a four-barrel, so it is a 340.”
  Paint and carburetors have bolts and spray cans working against them.
• “J-heads prove 340.”
  They prove J-heads. Check the block.
• “A 360 is just a bored 340.”
  Wrong direction. The 360 actually has the smaller bore and gains displacement through a much longer stroke.
• “340 and 360 external balance are the same.”
  No. Late cast-crank 340 and factory 360 balance requirements are different.
• “A weighted damper proves 360.”
  A late cast-crank 340 also uses weighted balance hardware. Read the damper and the block.
• “The partial VIN proves it is a factory 340 performance engine.”
  The VIN can tie the block to the vehicle. The vehicle code and the rest of the engine package establish what the car was built with.
• “Magnum is just a late LA.”
  Related block family, different top-end architecture and supporting systems. Identify which one you actually have before buying parts.

What Evidence Deserves Trust?

• Rear distributor, compact wedge heads, traditional LA valve covers and shaft rockers: strong LA-family identification.
• Front driver-side engine identification pad: one of the strongest assembled-engine clues for displacement and production information.
• Block casting number and cast displacement: usually strong original-displacement evidence.
• Casting date: strong chronological evidence.
• Partial VIN where applicable: strong evidence tying the block to a particular vehicle.
• Early 273 intake-bolt arrangement: useful production-era evidence.
• 2780930 or 3577130-TA casting: strong 340-family evidence when dates and stampings agree.
• 3418496 / 3870230-family casting: strong 360-block evidence.
• Damper and rear balance hardware: strong evidence of the rotating assembly’s balance requirement when original.
• Engine-mount boss spacing: useful 273/318 versus 340/360 supporting evidence.
• Heads, intake, carburetor, valve covers, exhaust, distributor, and accessories: supporting application evidence only.
• Measured bore and stroke: final mechanical identification.

A stock LA engine is usually one of the easier old V8s to identify before disassembly. Start with the family architecture. Clean the front identification pad. Read the block casting number and casting date. Check the partial VIN when the year calls for one. Then use heads, intake, balance hardware, vehicle application, and other pieces to confirm the story.

That will normally settle whether the engine left Chrysler as a 273, 318, 340, or 360.

What it may not settle is what decades of rebuilders have done inside it. Stroker cranks, overbores, replacement blocks, aftermarket balance work, and swapped performance pieces can move an engine a long way from its factory specification.

When the iron and the story disagree, measure it. Bore and stroke are the ultimate identification test. The 273, 318, and 340 live in the same short-stroke branch and separate primarily by bore. The 360 uses a smaller bore than the 340 but a much longer stroke. Once the measuring tools come out, the decals lose their vote.

Bore / Stroke Reference

Displacement	Bore	Stroke		Displacement	Bore	Stroke
273	3.625 in	3.312 in		318	3.910 in	3.3125 in
340	4.040 in	3.3125 in		360	4.000 in	3.580 in

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Shared Design Patterns

The 273, 318, 340, and 360 all share Chrysler’s LA small-block foundation: 4.460-inch bore spacing, roughly 9.600-inch deck height, wedge-style cylinder heads, an in-block camshaft, shaft-mounted rocker arms, and the compact architecture that replaced the earlier wide A/Poly layout. For the first three engines, Chrysler kept essentially the same short-stroke bottom-end geometry and changed displacement mainly with bore. The 360 is where that pattern changes.

The 273 starts with a 3.625-inch bore and 3.310-inch stroke. That small bore is the defining limitation and personality trait of the first LA. The stroke is essentially the same as the later 318 and 340, so the 273 gets its smaller displacement from cylinder diameter rather than from an especially short crank throw.

The 318 keeps the same basic 3.31-inch-stroke idea and opens the bore to 3.910 inches. That is a substantial increase in cylinder diameter over the 273 while leaving the basic rotating proportions alone. More bore gives the 318 more displacement and breathing room, but it still sits well below the 340 in valve and airflow potential.

The 340 takes that same 3.3125-inch stroke and opens the bore all the way to 4.040 inches. That is the largest factory bore of the LA engines covered here. The extra cylinder diameter gives the valves more room and is a major reason the 340 naturally belongs on the airflow-and-rpm side of the family. Mechanically, the 273-to-318-to-340 progression is mostly Chrysler keeping the crank throw and steadily opening the cylinders.

The 360 breaks that progression. Its bore is 4.000 inches—actually slightly smaller than the 340’s—but its stroke stretches to 3.580 inches. The extra displacement therefore comes mainly from crankshaft stroke instead of simply adding more bore. That longer arm gives the 360 more crank leverage and shifts the engine toward low- and midrange torque rather than the 340’s big-bore, shorter-stroke character.

The 360 also changes the crankshaft side of the LA package more substantially than the displacement number suggests. The 273, 318, and 340 use 2.500-inch main journals, while the 360 moves to 2.810-inch mains. Factory LA 360s are externally balanced, while the 273, traditional LA 318, and forged-crank 340 combinations are normally internally balanced. Late cast-crank 340s are the notable exception, so even inside this closely related family the rotating assemblies are not one universal pile of small-block Mopar parts.

That leaves the family with a clear mechanical map. The 273 is 3.625 × 3.310. The 318 grows mainly through bore to 3.910 × 3.3125. The 340 keeps that stroke and pushes bore to 4.040 inches, giving it the family’s strongest natural breathing advantage. The 360 backs bore down slightly to 4.000 inches but stretches the crank to 3.580 inches, gaining displacement and torque through stroke instead.

Same LA bones, two different ways of making more engine. Chrysler developed the 273, 318, and 340 mainly by giving the same short-stroke idea progressively bigger cylinders. Then the 360 changed the argument with a longer stroke, larger main journals, and a different balance scheme. That is why the 340 and 360 can sit only twenty cubic inches apart and still have noticeably different mechanical personalities.

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Choosing Within This Family

The LA family gives four genuinely different answers. The 273 is the early light-car and restoration engine. The 318 is the durable general-purpose workhorse. The 340 is the factory performance choice. The 360 is the torque engine and usually the strongest practical starting point when originality does not dictate something else.

• Choose the 273 when early-LA identity matters.
  In a correct Barracuda, Dart, Valiant, Commando, Charger 273, or other early application, the little engine can be exactly the right answer. Its small bore and limited displacement reward crisp tuning, sensible compression, modest camshaft, good gearing, and a light car. They do not reward attempts to force 340-sized airflow and horsepower expectations into 273 cubic inches.
• The 273 makes the least sense when horsepower per dollar is the goal.
  The machine work costs real money while the small bore limits valve size, airflow, and ultimate displacement. If originality does not matter, a 318, 340, or 360 generally gives more useful engine before the first performance part is purchased.
• Choose the 318 when reliability, economy, parts availability, and ordinary street manners matter most.
  A healthy 318 is hard to beat for a cruiser, daily-style classic, truck, van, mild street car, or budget Mopar. It can respond well to sensible compression, a mild cam, four-barrel induction, better exhaust, ignition tuning, and appropriate gearing without losing the durability that made the engine useful in the first place.
• The 318 stops being the bargain when the horsepower target requires replacing nearly everything.
  Its smaller bore and displacement mean that expensive heads, pistons, machine work, converter, gears, induction, and exhaust can eventually turn a cheap engine into an expensive way to avoid starting with a 360. Use the 318 when the mission fits the engine rather than when the engine happens to be sitting closest to the garage door.
• Choose the 340 when factory performance identity and sharp small-block response matter.
  The 340’s big bore, short stroke, breathing potential, and factory performance hardware make it the natural LA choice for a period high-performance build. In a correct 340 car, preserving the original engine can also be important to the vehicle’s history and value.
• Pay the 340 premium only when there is a reason to own a 340.
  Genuine engines, especially desirable early or special-package versions, carry collector value that can make them expensive foundations for an ordinary street project. If originality does not matter and the goal is simply strong street power, the 360 can often provide more torque for less money. A 340 earns its price through identity and performance character, not merely through cubic inches.
• Choose the 360 when street torque and practical performance lead the list.
  The longer stroke and larger displacement give it an advantage in heavier cars, trucks, cruisers, towing-oriented combinations, and street machines that spend most of their lives below heroic rpm. A sound 360 with real compression, a sensible cam, good induction, exhaust, ignition, converter or clutch, and gearing can make a very strong street package without requiring big-block weight.
• The 360 is usually the practical performance choice when starting without an originality requirement.
  It offers more displacement than the 318 and generally avoids the collector premium attached to the 340. Its factory external balance and other 360-specific hardware must be respected, but those are compatibility requirements rather than reasons to avoid an otherwise good engine.

Condition and completeness can override the displacement argument. A complete, healthy 318 with the correct pan, brackets, front dress, manifolds, and drivetrain hardware may be a better starting point than a worn 360 missing half its pieces. Likewise, a documented 340 or special 273 package may deserve preservation rather than modification simply because its historical value exceeds the benefit of another fifty horsepower.

The short version is simple. Choose the 273 for early correctness and light-car character. Choose the 318 for dependable, economical, general-purpose street use. Choose the 340 when genuine factory performance identity and high-revving small-block character justify the premium. Choose the 360 when torque, displacement, street performance, and value matter most. Same LA family, four different jobs.

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Compatibility Notes

The LA small-block family has plenty of useful interchange, but “small-block Mopar” is not a universal parts number. The 273, 318, 340, and 360 share the same basic family architecture while bore size, crankshaft, balance, heads, intake, year, vehicle application, transmission hardware, and front dress still decide what actually fits.

• Keep LA and A-Series Poly Engines Separate
  The earlier A-series Poly and later LA engines belong to different top-end systems. Blocks, cylinder heads, intake manifolds, valve covers, valvetrain layout, exhaust fit, and many accessory relationships differ. The Poly 318 and LA 318 sharing the same displacement does not make them interchangeable.
• LA and Magnum Are Related, Not Identical
  Magnum 5.2 and 5.9 engines grew from the same small-block family tree, but traditional LA and Magnum heads, intake bolt patterns, rocker oiling, pushrods, valve covers, front dress, accessories, fuel-system hardware, and electronics can differ. Magnum head or engine swaps can work, but oiling, intake fit, pushrod length, compression, and accessories need to be planned around the exact combination.
• Early 273 Intake Fit Is Its Own Trap
  Early 1964–1965 273 heads use intake-bolt-angle details that do not simply accept every later LA intake. Verify the exact year, head casting, intake pattern, and bolt angle before buying a manifold for an early 273.
• Cylinder Heads Need More Than a Matching Bolt Pattern
  Chamber volume, valve size, port layout, compression effect, rocker and pushrod arrangement, oiling, intake fit, and intended rpm range all matter. A head that physically bolts to another LA block can still be a poor match for the pistons, camshaft, displacement, or intended use.
• The 360 Has Its Own Bottom-End Rules
  The 360 uses the longer stroke, larger main journals, and different rotating-assembly requirements from the 273, 318, and 340. A 360 crank, pistons, damper, converter, flexplate, flywheel, or other balance hardware should never be treated as generic LA parts.
• Factory LA 360s Are Externally Balanced
  The damper and converter, flexplate, or flywheel must match the actual 360 rotating assembly. Do not mix internally balanced 273, 318, or early 340 assumptions into a factory-style 360 combination. If a machine shop has converted a particular assembly to internal balance, prove that before ordering the supporting parts.
• 340 Balance Requirements Can Change with Year and Crank
  Early forged-crank and later cast-crank 340 combinations should not be treated as one balance package. Verify crank type, damper, converter or flexplate, flywheel, pistons, and the complete rotating assembly before mixing early and late 340 hardware.
• Transmission Fit Is More Than the Bellhousing Pattern
  Verify crank flange and register, converter, flexplate, flywheel, clutch hardware, starter, transmission year, linkage, and balance requirements together. The Chrysler small-block bellhousing pattern gets the parts close. The crank and balance package decide whether the combination actually belongs together.
• Oil Pans, Pickups, and Mounts Follow the Chassis
  A-body, B-body, E-body, truck, van, police, service-engine, and swap applications can use different sump locations, pickups, mounts, brackets, and clearance arrangements. An LA engine that bolts to the transmission still has to clear the steering, crossmember, exhaust, and chassis.
• Front Dress Changes Across the Production Run
  Timing covers, water-pump outlet side, pulleys, alternator brackets, power-steering brackets, A/C brackets, radiator hose routing, fan spacing, front seals, and accessory systems can vary by year and application. Keep the original front-drive pieces together until the replacement arrangement is proven.
• Car, Truck, Van, Police, and Emissions Hardware Can Differ
  Engines from different applications may carry different pans, mounts, heads, camshafts, carburetion, ignition, exhaust, brackets, accessories, emissions equipment, and front dress. A complete truck or van engine can still require substantial hardware changes before it belongs in a passenger car.
• Performance Packages Need Exact Hardware Proof
  Commando and Charger 273s, D/Dart engines, early high-performance 340s, AAR/T/A Six Barrel or Six Pack packages, police engines, and other special versions use application-specific combinations. Carburetors, intakes, valve covers, or other visible pieces can be swapped. They do not establish compatibility or factory identity by themselves.

That is the LA compatibility rule: identify the exact displacement, year, crank and balance package, heads, intake, vehicle application, and transmission before ordering parts. Then match the oil pan, mounts, front dress, cooling, accessories, and drivetrain around that combination. The LA family shares plenty. It still expects you to know which LA is actually sitting on the stand.

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Common Family Problems

The LA small-block earned a reputation for durability because most of them deserved it. Fifty years of mileage, neglected oil changes, worn timing parts, tired cooling systems, flat-tappet wear, and previous-owner repairs can still turn a tough little Mopar into an expensive collection of familiar noises.

• Timing-Chain and Timing-Gear Wear
  High-mileage LA engines commonly develop worn timing chains and gears, especially where old nylon-tooth cam gears or long service intervals are involved. Excessive slack can cause wandering ignition timing, weak response, rough idle, poor starting, and cam timing that has quietly moved away from where Chrysler put it. Before rebuilding the carburetor for the third time, make sure the camshaft is still arriving on schedule.
• Oil and Gasket Leaks
  Valve covers, intake sealing areas, timing covers, oil pans, rear main seals, front seals, and other old gasket joints are common leak points. Hardened seals, distorted covers, damaged surfaces, crankcase pressure, and decades of previous repairs all contribute. A small-block that has marked every driveway since 1982 is not demonstrating authenticity.
• Camshaft, Lifter, and Valvetrain Wear
  Flat-tappet cam lobes, lifters, valve guides, springs, rocker gear, pushrods, and related oiling all deserve inspection. High mileage, poor lubrication, long storage, excessive spring pressure, incorrect break-in, or questionable previous rebuild work can produce noisy lifters, weak cylinders, worn lobes, poor valve control, and metal circulating through the oil. Mopar durability does not make worn parts younger.
• Sludge and Lubrication Neglect
  LA engines often survived maintenance that should have killed less forgiving machinery. Short-trip use, infrequent oil changes, cheap oil, long storage, and neglected crankcase ventilation can leave sludge in the valley, pan, oil passages, pickup, and rocker areas. An engine that still runs can be remarkably dirty inside. Survival is not the same thing as cleanliness.
• Cooling-System Neglect and Heat Damage
  Clogged radiators, weak water pumps, tired fan clutches, missing shrouds, wrong pulleys, corroded passages, poor thermostat choices, lean mixtures, and bad ignition timing can make an otherwise sound LA run hot. Repeated overheating can add warped surfaces, damaged gaskets, cracked heads, and tired rings to the repair list. The temperature gauge is cheaper to believe than the machine shop.
• Emissions-Era Vacuum, Carburetion, and Ignition Trouble
  Later 318 and 360 applications can accumulate cracked vacuum hoses, leaking controls, worn distributors, feedback-carburetor problems, Lean Burn-era failures, disabled emissions hardware, and several generations of previous-owner simplification. These engines may run poorly because the surrounding system is worn, disconnected, or incorrectly modified rather than because the basic LA engine is bad.
• High-Mileage and Hard-Service Wear
  LA engines spent decades in passenger cars, police vehicles, trucks, vans, fleet equipment, tow rigs, and ordinary daily service. Worn cylinders, tired rings and bearings, damaged valve seats, stretched timing parts, broken exhaust hardware, corroded cooling passages, damaged threads, and mixed repair parts are common on unknown cores. A 318 that survived everything thrown at it may still have used up most of its useful clearances doing so.
• 360 External-Balance Mismatches
  Factory LA 360 combinations require the correct external-balance hardware. Damper, converter, flexplate, flywheel, crankshaft, and rotating assembly have to belong to the same balance plan. Mixing 318 or early 340 internal-balance assumptions into a 360 can create vibration, bearing stress, and a fresh engine that behaves like something inside is looking for the exit. Bolting together proves only that the bolts fit.

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Common Family Misconceptions

The LA family is familiar enough to make people careless. Four displacements, decades of production, an earlier Poly 318, a later Magnum family, famous 340 performance packages, and piles of interchangeable-looking parts have created the dangerous belief that “small-block Mopar” is a complete identification.

• “Every old Mopar small-block is an LA.”
  No. The A-series Poly small-block came before the LA, and the Magnum small-block followed it. They belong to the broader Chrysler small-block story but are not one universal engine family. Heads, intakes, valvetrain, oiling, front dress, accessories, and other hardware can differ substantially. Family history is not a parts interchange chart.
• “LA parts interchange because they are all the same small-block family.”
  Some interchange is excellent. Universal interchange is fantasy. Early 273 intake bolt details, bore-dependent pistons and heads, different crank and main-journal combinations, oil pans, mounts, front dress, year changes, and the 360 balance system can all change what fits and what works. A part can bolt on and still be wrong enough to make the next problem considerably more expensive.
• “The Poly 318 and LA 318 are basically the same engine.”
  No. They share 318 cubic inches and Chrysler family history, but the earlier A/Poly uses polyspherical heads and different induction, valve-cover, packaging, and parts logic. The LA 318 is the later wedge-head engine. Ordering by “318” without identifying which one is how perfectly good parts develop return labels.
• “The 273 is just a smaller 318 or a baby 340.”
  The 273 started the LA family, but its smaller bore, early hardware, intake details, displacement, and intended use give it its own build requirements. It can be excellent in an early Barracuda, Dart, Valiant, Commando, Formula S, or period build without pretending to be a 340. Small does not mean worthless, and family resemblance does not add cubic inches.
• “The performance package comes with the displacement.”
  It does not. A 273 is not automatically a Commando, Charger, or special performance engine. A 340 is not automatically a T/A, AAR, Six Pack, Six Barrel, X-head, or other premium version. A 360 does not become an E58, police, or other performance package merely because the displacement is correct. Carburetion, intake, heads, camshaft, exhaust, dates, application, vehicle context, stampings, and documentation have to support the claim.
• “The 318 is either smog-era junk or a 340 waiting for bolt-on parts.”
  Neither extreme is useful. Many 318s were mild, low-compression workhorses, but a healthy matched 318 can make a good street engine. It also does not become a 340 because somebody adds large heads, an ambitious camshaft, a four-barrel, and enough noise. Bore, compression, airflow, gearing, vehicle weight, and the whole combination still decide what the engine can use.
• “If it looks like a 340, it probably is one.”
  No. The 340’s reputation and value have encouraged decades of dressed-up 318s, 360s, swapped heads, reproduction induction, orange paint, and optimistic storytelling. Bore, block casting, stampings, dates, heads, application, induction, and documentation need to establish the engine. The air cleaner is allowed to look impressive. It is not allowed to identify the block.
• “Every 340 is the same high-compression muscle-era engine.”
  No. Compression, crankshaft, balance details, heads, induction, emissions equipment, ratings, and application changed during the 340’s production life. Early high-compression engines earned much of the reputation, while later versions lived in a changing emissions and rating environment. A later 340 can still be a good 340. It does not automatically inherit every specification from 1968.
• “The 360 is basically a bigger 318 or a 340 with more cubes.”
  The 360 is its own LA combination with a longer stroke, larger main journals, external balance, different piston and crank requirements, and a torque-oriented personality. It can make excellent street power without becoming either of its relatives. More cubic inches do not transfer 340 identity, and shared valve-cover shape does not make the bottom ends interchangeable.
• “External balance does not matter if the parts bolt together.”
  It matters. Factory 360 balance requires the correct damper and transmission-side balance hardware unless the rotating assembly has intentionally been changed and balanced differently. Wrong components can create vibration and bearing damage while every bolt remains perfectly satisfied with itself.
• “All 360s are weak smog engines.”
  Many 360s were built during the low-compression and emissions years, but that describes factory combinations, not a defective architecture. The displacement and stroke make a sound 360 a useful torque foundation when compression, heads, camshaft, induction, ignition, exhaust, converter, and gearing are matched to the job.
• “A 408 stroker is just a cheap 360 rebuild.”
  No. The popular 408 combination still requires a sound block, correct crank, rods and pistons, clearance work, compression planning, balancing, oiling, heads, camshaft, cooling, converter or clutch, gearing, and sufficient drivetrain strength. Buying a stroker kit establishes the displacement plan. It does not finish the engine.

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Bottom Line

The Mopar LA small-block family earned its place by being useful enough to do almost everything Chrysler asked of a small V8. The 273, 318, 340, and 360 cover everything from mild cruisers and daily drivers to correct restorations, serious factory performance, truck duty, and strong street builds. That versatility is the family’s greatest strength. It is also why people get careless and start treating “small-block Mopar” like one universal part number.

The four siblings have clear personalities. The 273 is the origin engine and makes its strongest case in early cars and period-correct builds. The 318 is the workhorse, common and durable enough to be the sensible answer when dependability matters more than bragging rights. The 340 is the performance piece, with the reputation and value that come from being built for that job. The 360 is the torque engine, bringing more displacement and low-speed pull when a heavier vehicle or stronger street combination needs it.

Choose the engine that matches the mission instead of automatically chasing the most famous number. Preserve a correct 273 when the car deserves it. Use a sound 318 when a dependable street engine is enough. Pay 340 money only when the engine and history actually justify 340 money. Choose the 360 when displacement and torque make more sense than paying a nostalgia premium for a different personality.

The expensive mistakes happen because familiarity breeds laziness. A 318 does not become a 340 because the intake and valve covers look convincing. A 360 is not simply another LA when balance hardware enters the conversation. Earlier A-series Poly engines and later Magnum engines belong to the same broad family tree without becoming interchangeable piles of parts. Buying first and identifying later is how shelves fill up with components that almost fit.

So treat the LA family as four related engines with four different jobs, not one generic Mopar small-block wearing different displacement badges. It is one of Chrysler’s best and most useful engine families precisely because there is an LA for several different kinds of work. Figure out which one is actually sitting there, decide what the vehicle actually needs, and let the iron settle the argument before the wallet starts voting.

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