Mopar 318 Engine Specs

Intro

The Mopar A/Poly 318 is the engine that finally made Plymouth’s Poly V8 story stick. The 277 got the family moving, the 301 and 303 handled short early steps, and the 313 carried a Canadian/export branch, but the 318 became the familiar one. Introduced for 1957 and used in U.S. production through 1966, the A/Poly 318 gave Chrysler’s value-brand V8 family enough displacement, production life, and usefulness to become more than a footnote with odd valve covers.

That’s the first cleanup job. This 318 is the A-block polyspherical-head engine, separate from the later LA 318 wedge, Chrysler Hemi, Dodge Red Ram Poly, and DeSoto Poly families. Same displacement doesn’t mean same engine. Same Mopar small-block neighborhood doesn’t mean the intake, heads, cam, pistons, or parts logic all line up like the catalog owed you a favor. The Poly 318 and LA 318 are related in the broader small-block story, but they’re not the same animal.

Plymouth needed the 318 because the early A/Poly engines were stepping stones, not the final answer. The mid-1950s market wanted more V8 power every year, and Plymouth needed an engine that could handle family cars, wagons, better-trim models, Fury performance image, and ordinary daily use without making the car too expensive for its own buyers. The 318 gave Plymouth a practical V8 with enough torque and breathing to feel modern, useful, and competitive.

In normal use, the 318 Poly was the honest worker of the A/Poly family. It could be a regular two-barrel family-car engine, a stronger four-barrel version, a dual-quad Fury V-800 package, a truck engine, or an export-market survivor depending on year and application. That doesn’t make every 318 Poly a Fury performance piece, and it doesn’t make every later 318 story apply backward. Version, year, carburetion, compression, vehicle, and actual hardware still decide what you’re looking at.

The plain-English identity is this: the Mopar A/Poly 318 is the long-running, familiar Plymouth-family Poly V8 that carried the A-block design into broad use before the LA 318 took over. It’s not famous because it was exotic. It’s useful because it was common, durable, and practical enough to keep doing the job after the early oddball displacements had left the stage.

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Overview

The Mopar A/Poly 318 is the anchor of the Plymouth A-block polyspherical V8 family. Earlier displacements got the idea started, but the 318 is the one that lasted, spread, and did real work across cars, trucks, and export applications. Around old Mopars, staying useful is often more impressive than being rare. Rare gets people excited. Useful keeps the car moving after the excitement has leaked onto the driveway.

Architecturally, the 318 stayed true to the A/Poly design. It used the A-block foundation with polyspherical combustion chambers, not true Hemi heads and not later LA wedge heads. The Poly chamber gave Chrysler a practical blend of airflow, cost control, and production simplicity. It wasn’t as glamorous as a Hemi, and it wasn’t as compact and simple as the later LA wedge layout. It lived in the middle: useful breathing, sturdy construction, and enough practicality to make Plymouth’s V8 program look less like an experiment and more like a plan.

With a 3.91-inch bore and 3.31-inch stroke, the 318 gave the A/Poly family the displacement balance it needed. It wasn’t huge, but it was enough for full-size Plymouths, wagons, light trucks, and ordinary drivers who needed smooth torque more than bragging rights. In Fury and V-800 performance form, it could wear dual four-barrels and make real showroom noise. In regular form, it was more likely hauling a family, groceries, luggage, and somebody’s bad decision to buy whitewall tires on credit. Both jobs belong to the same displacement, but they don’t tell the same story.

That split is where the 318 Poly needs proof, not assumptions. A dual-quad Fury 318, a standard two-barrel Plymouth 318, a truck 318, a Canadian/export 318, and a later LA 318 aren’t the same conversation. The first four may live in the A/Poly world, but they still differ by carburetion, compression, camshaft, application, and year. The LA 318 is a later wedge-head engine derived from the same basic small-block direction, but it isn’t a Poly-headed A engine. Same 318 number doesn’t make the parts shake hands.

The A/Poly 318’s reputation is usually quieter than it deserves. It doesn’t have the Hemi’s thunder, the big-block’s intimidation, or the LA 340’s later muscle-car sparkle. What it has is usefulness. It gave Plymouth and later Chrysler divisions a dependable V8 that could power real cars in real traffic without needing a racing story to justify itself. Not every engine earns its keep by scaring people. Some earn it by starting, pulling, and not making every repair feel like a ceremony.

Compared with the earlier A/Poly engines, the 318 is the settled answer. Compared with the 313, it’s the better-known U.S. displacement. Compared with the later LA 318, it’s older, wider at the heads, and more unusual today, but it also carries the Poly chamber character that makes the family different. Compared with Dodge and DeSoto Poly engines, it’s a separate Plymouth A-block design, not the same Hemi-derived family wearing a similar nickname. This is why “Poly” alone isn’t enough information to order anything more complicated than regret.

The biggest bad assumption is thinking the 318 Poly is either just an old LA 318 or just a weird Hemi cousin. It’s neither. It’s a specific A-block engine with its own heads, intake, pushrod layout, parts concerns, and history. It can be correct, durable, and interesting in the right car, but it won’t reward anyone who tries to make later LA parts, Dodge Poly lore, or Hemi mythology do the thinking.

The Mopar A/Poly 318’s real place is as the useful long-runner of the Plymouth Poly family: common enough to matter, different enough to identify carefully, and practical enough to deserve more respect than it usually gets. It’s not exotic, and it doesn’t need to be. Call it an LA, call it a Hemi cousin, or treat every Fury dual-quad story like gospel, and the engine will have you arguing with iron before the first bolt breaks loose.

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What makes it different

The A/Poly 318 is the early Mopar Poly that stuck around long enough to become the name people remember. Keep it in its own lane: later than the 277, 301, Fury-flavored 303, and Canadian/export 313, but still separate from the LA 318 even when the bore and stroke numbers shake hands. The 318 Poly is the working survivor of the A-engine family: common enough to matter, odd enough to punish lazy parts ordering, and useful enough that people still argue about whether it deserves more respect.

Compared with the 277, 301, and 303, the 318 is where the early Plymouth Poly idea finally quit acting temporary. The 277 opened the door. The 301 added bore. The 303 brought Fury shine. The 318 became the engine Chrysler kept using. The smaller engines matter because they show the family’s beginning. The 318 matters because it became the answer that stayed in the toolbox instead of just showing up in a footnote.

Compared with the 313, the 318 is the familiar cousin with the bigger name and broader use. The 313 is the Canada/export almost-318 that makes people trust close numbers too much. The 318 is the one more U.S. buyers, builders, and parts hunters recognize. Close family still needs checking, though. Bore, market, pistons, rings, gaskets, and application details can still make the parts-order mistake wear a nicer hat.

Compared with the LA 318, the A/Poly 318 is where the same displacement gets dangerous. Same bore and stroke doesn’t mean same engine. The LA 318 replaced the Poly with wedge heads, different intake needs, different valve-cover world, different top-end logic, and a much bigger later parts trail. The Poly 318 has its own heads, intake, valvetrain layout, exhaust fit, and early A-engine personality. Calling it “just an old LA 318” is how people end up with parts that look right in the catalog and wrong on the bench.

Compared with the LA 340 and 360, the 318 Poly should stay out of later small-block hero worship. The 340 is the high-winding factory performance small block. The 360 is the bigger-torque later small block. Both have better speed-part comfort and broader performance reputation. The Poly 318 has charm, durability, and a different look, but it doesn’t become a cheap 340 or a funny-headed 360 because somebody wants the story easy. If the goal is common performance parts, the later LA engines win before the first box hits the porch.

Compared with Dodge, DeSoto, and Chrysler Poly or early Hemi engines, the 318 Poly still needs family discipline. Plymouth A/Poly, Dodge Poly/Hemi, DeSoto, and Chrysler FirePower parts don’t turn into one happy junk drawer because the chamber shape sounds related. Blocks, heads, mounts, intakes, accessories, and rebuild pieces all have their own little ways to ruin your week. “Poly” isn’t a password. It’s a warning label with grease on it.

The 318 A/Poly makes sense when the engine’s character matters: correct late-’50s and early-’60s Mopar builds, period customs, oddball driver projects, or cars where the wide Poly valve covers are part of the charm instead of a problem to solve. It makes less sense when somebody wants easy LA parts, cheap horsepower, or modern small-block convenience. The Poly can be made to run well. It can be interesting. It can be cool. But it won’t stop being a Poly just because the builder gets tired of hunting parts.

That’s the 318 Poly’s place. It’s the A-engine that outlasted the little early steps and became the family’s working name, but it still isn’t the later LA 318 wearing old clothes. Preserve it when it belongs. Build it when the Poly character is the point. Verify parts before money leaves the hand. And don’t let “318” do all the talking. The number may sound familiar, but the heads, intake, and hardware are waiting to see who actually did his homework.

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General Specs

Displacement	318 cu in / 5.2L	Bore / Stroke	3.910 in × 3.3125 in
Production / Use Era	1957–1966 U.S. A-block Poly passenger-car and light-truck era; some export use into 1967	Bore Spacing	4.460 in
Engine Family	Chrysler / Plymouth A-series Poly small-block V8	Deck Height	9.603 in A-block baseline
Block Material	Cast iron	Rod Length	6.123 in A-block baseline
Cylinder Head Material	Cast iron	Main Journal	2.500 in
Factory Rating	Common 2-barrel and 4-barrel ratings vary by year/tune; 1957–1958 V-800 dual-quad versions are commonly listed at 290 hp	Compression Ratio	Market/tune dependent; verify by exact year and application
Fuel / Induction	Gasoline; carbureted, 2-barrel, 4-barrel, or dual 4-barrel by version	Rod Journal	2.125 in
Cam Location	In-block camshaft	Main Bearings	5
Valve Layout	OHV / pushrod, 2 valves per cylinder; polyspherical chambers with canted valves	Firing Order	1-8-4-3-6-5-7-2
Common Main Caps	2-bolt	Distributor Rotation	Clockwise
Rear Main Seal	2-piece	Weight	~525–575 lb complete iron A-block Poly, depending on accessories/configuration
Balance	Internal balance	Dimensions	~29H / ~28W / ~30L

Verify the block, bore, stroke, crank, heads, intake, carburetor, exhaust, stampings, market, and vehicle application before ordering parts. The 318 Poly shares a familiar number with the LA 318, but the familiar number doesn’t make the heads, intake, exhaust, or top-end hardware agree.

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Variants

The 318 Poly needs a variant section with enough room for the real spread, but not so much room that it turns into a parts-counter swamp. This is the A-series polyspherical 318, not the later LA 318, and that warning belongs at the front before somebody starts ordering parts by displacement and hope.

The useful split is standard passenger-car 318s, Fury / V-800 / dual-quad performance versions, truck / workhorse / export use, LA 318 confusion, and service / swap identification problems. That’s the map. Not every carburetor, trim badge, and market footnote needs its own parade float.

Standard Passenger-Car 318 Poly

The standard 318 Poly was the normal A-series V8 that finally gave Plymouth and related Chrysler Corporation cars a long-running small-block workhorse before the LA engine took over. It used a 3.910-inch bore and 3.3125-inch stroke, which gave the early A/Poly family enough displacement to feel settled instead of temporary.

In regular passenger-car form, the 318 Poly was built to be smooth, durable, and useful. Depending on year and application, it could wear ordinary 2-barrel or 4-barrel carburetion, reasonable compression, and enough torque to move Plymouths, Dodges, wagons, sedans, and daily drivers without making every hill feel like a committee meeting.

That was the real job. The standard 318 wasn’t there to be exotic. It was there to work. It hauled families, luggage, groceries, and all the other real-world junk that engines actually move when nobody is standing around the fender telling horsepower stories.

A standard 318 Poly can be a good engine, a correct restoration piece, or a useful cruiser foundation. It can also be a tired old lump with stuck rings, wrong parts, missing hardware, and a seller who thinks “wide-block” counts as a technical inspection. The 318 name starts the conversation. Bore, stroke, castings, heads, intake, carburetion, application, and condition finish it.

Fury, V-800, and Dual-Quad Performance 318s

The performance side of the 318 Poly is where the reputation gets louder and the proof burden gets heavier. Plymouth used the 318 to give Fury, V-800, and related performance-minded applications more bite before later big-block and LA small-block legends started hogging the microphone.

The best-known high-performance A/Poly 318 versions are the 1957–1958 V-800 dual-quad packages. Those engines used dual 4-barrel carburetion and stronger performance hardware, and they gave Plymouth serious showroom noise for the era. They deserve respect. They also deserve proof, because carburetors can be bolted on by anyone with a wrench, a Saturday, and just enough confidence to become dangerous.

Golden Commando language needs careful handling. Some period Plymouth performance talk uses that badge around the same general late-1950s performance atmosphere, but the exact engine, year, carburetion, and factory package still need proof. Don’t let one proud name turn every 318 Poly with a shiny intake into a factory hot setup.

A normal 318 Poly with a later 4-barrel intake isn’t automatically a Fury engine, V-800 package, Golden Commando, or factory performance piece. It’s a modified 318 until the car and hardware prove otherwise. Factory identity takes more than a shiny air cleaner and a seller who learned the right words.

For a real performance 318 claim, check the application, engine stamping, casting information, intake, carburetors, linkage, air cleaner, camshaft evidence where possible, exhaust, tags, paperwork, and whether the car itself supports the story. If the seller points at the carburetors while the documentation hides behind the garage, keep the wallet shut.

Truck, Workhorse, and Export 318s

After the early Fury and V-800 spotlight moved on, the 318 Poly settled into the job it did best: sturdy small-block work. It stayed useful in passenger cars, wagons, trucks, Canadian applications, export markets, and enough normal service to become the A-series Poly most people are most likely to find.

That’s not an insult. Workhorse engines keep cars alive after the glamour engines have already been blown up, swapped out, or priced into museum nonsense. A later 318 Poly may not bring dual-quad drama, but it can still be a strong, smooth, period-correct engine in the right car.

Truck, Canadian, and export use can stretch the 318 Poly story beyond the neat U.S. passenger-car timeline. That doesn’t make every late Poly 318 mysterious or exotic. It means the car’s market, year, application, mounts, oil pan, manifolds, front dress, transmission pieces, and paperwork need checking before somebody decides the engine is wrong, rare, or magically valuable.

The standard workhorse 318 Poly is often the sensible one. It may not bring the biggest bragging rights, but it gives displacement, durability, and old-Mopar character without needing to pretend it came from a Fury brochure.

LA 318 and Parts Confusion

The Poly 318’s biggest modern headache is the later LA 318. The LA engine kept the 318 displacement and became common enough that many people now hear “318” and assume every Chrysler small-block part in the known universe should fit. That’s how projects become return-label festivals.

The Poly 318 and LA 318 share the number, but they don’t share the whole engine. The Poly has its own heads, intake, valve-cover appearance, top-end layout, and early A-series personality. Some family relationships may matter in the lower end or broader small-block history, but that doesn’t make the heads, intake, valve covers, exhaust, and bolt-on logic the same.

This matters when buying intakes, heads, valve covers, exhaust parts, gaskets, brackets, accessories, and rebuild pieces. It also matters when evaluating a car. A Poly 318 in the correct earlier application isn’t worse because it isn’t an LA. An LA swap may be easier to build, but it changes originality. The right answer depends on the job, not on which 318 the parts store remembers.

Identify the engine before building the plan. Look at the heads, intake, block, casting numbers, front dress, valve covers, year, and vehicle application. If the plan starts with “all 318s are basically the same,” the plan is already leaking oil onto your shoes.

Service, Swap, and Identification Confusion

The 318 Poly was common enough to survive and old enough to be misunderstood. That’s a dangerous combination. A rare engine makes people cautious. A common engine makes people lazy. The 318 Poly sits in the middle, where somebody can recognize the number and still misunderstand the engine.

After decades of rebuilds, swaps, performance dress-up, Canadian/export movement, truck use, and LA-engine confusion, a surviving 318 Poly may not match the car, the paperwork, or the seller’s memory. It may be a correct original, a solid replacement, a tired core, a dressed-up standard engine, or a performance claim held together by linkage clips and wishful thinking.

For any 318 Poly, the proof burden depends on the claim. A standard cruiser engine needs correct basic identification. A Fury, V-800, dual-quad, truck, export, or special-market claim needs more. Engine stamping, casting numbers, bore/stroke family, heads, intake, carburetion, exhaust, application, market, tags, and paperwork all need to line up before the price gets brave.

A good 318 Poly is a worthwhile engine. A misidentified 318 Poly is a parts-ordering ambush. A fake performance 318 Poly is a seller trying to make carburetors do the work of factory proof. Don’t punish the engine for other people’s stories. Make the metal tell the truth before the money moves.

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Key Points

The Mopar 318 A/Poly is the familiar name in the early A-series Poly family, and that familiarity is exactly why people get sloppy with it. This is the wide-looking, polyspherical-head A-block small block, not the later LA 318, not a Magnum 318, and not a Dodge, DeSoto, or Chrysler Hemi-derived Poly from the earlier corporate soup. “318” isn’t enough identification. It’s just the first clue.

The basic A/Poly 318 fingerprint is 3.910-inch bore and 3.3125-inch stroke. That separates it from the 313, which used a smaller bore, and from the later LA 318, which shares displacement but not heads, intake, exhaust layout, or parts logic. Same cubic inches, different animal. Mopar made sure the argument had teeth.

The biggest buyer mistake is assuming a Poly 318 and an LA 318 are interchangeable because both are Mopar small blocks with 318 on the paperwork. The A/Poly has its own wide polyspherical heads, its own intake layout, different exhaust and valvetrain considerations, and enough unique pieces to punish lazy parts ordering. The parts counter doesn’t care that both engines make the same number on paper.

The “wide-block” nickname helps until it starts doing damage. It points toward the broad A/Poly shape, but it doesn’t mean B/RB big-block, truck-only oddball, or mystery industrial lump. The 318 A/Poly is an early Mopar small-block-family engine with wide heads, not a 383, 413, 426, or 440 hiding under strange valve covers.

Performance claims need proof, especially when the engine is wearing dual quads, a 4-barrel intake, finned covers, or somebody’s Fury / V-800 story. Chrysler did build stronger 318 Poly packages, and the engine earned a real period performance reputation in the right trim. But induction bolts on, carburetors get swapped, and air cleaners lie for free. The block, heads, date trail, induction, application, and documentation need to line up before the price gets brave.

Application context changes the value. A correct 318 Poly in the right Plymouth, Dodge, truck, export car, or early restoration can be worth keeping because it belongs there. A loose engine with missing parts and no original trail is a different conversation. The engine may still be good. The seller’s price may still need a chair and a cold towel.

As a street engine, the 318 Poly is better than many people give it credit for. It has rugged old Mopar bones, decent displacement, and Poly heads that can breathe respectably for the era when the compression, cam, intake, exhaust, and gearing are matched. It isn’t just a weird boat anchor from before the LA era. Treated properly, it can make a solid cruiser engine with more character than the usual belly-button small block.

That still doesn’t make it the easy horsepower answer. The later LA 318, 340, and 360 have better aftermarket support, simpler parts access, and more familiar build paths. If originality or early Mopar character isn’t part of the reason, the Poly 318 can lose the practical argument fast. Cool doesn’t automatically mean efficient, and interesting doesn’t pay the machine shop by itself.

Parts support is where charming turns expensive. Poly 318 rebuild parts exist, and the engine has a following, but it isn’t as effortless as building a later LA small block. Pistons, rings, camshaft choices, valvetrain pieces, intake options, exhaust pieces, gaskets, mounts, brackets, pulleys, and accessory drives need to be verified before money starts leaving the building. Missing parts on a Poly are little invoices wearing work boots.

A used 318 Poly core deserves a hard inspection before anyone gets romantic. Check for cracks, freeze damage, stuck cylinders, rusty bores, worn lifter bores, damaged decks, broken exhaust ears, missing manifolds, missing brackets, damaged rocker assemblies, butchered mounts, truck/export hardware differences, and previous machine work done by somebody who treated measuring tools as optional.

The 318 Poly’s value depends on completeness, correctness, and purpose. A complete, verified engine with its proper manifolds, brackets, accessories, and application trail is useful. A bare block with mismatched heads and a bucket of bolts is mostly a negotiation problem. The 318 name helps because people recognize it. The actual hardware decides whether recognition turns into value or just another greasy story.

The 318 A/Poly earns its keep when the car wants it, the parts are present, and the owner actually likes early Mopar machinery. For a correct restoration or period-flavored cruiser, it can be a smart save. For cheap power, easy parts, or a fast modern build, the later LA engines are usually the cleaner road. The Poly 318 isn’t the wrong engine. It just needs the right reason before the wallet gets involved.

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Common Problems

The 318 A/Poly’s trouble starts because the number survived after the engine changed. People hear “318” and their brains jump straight to the later LA small-block. That’s how the wrong intake, wrong heads, wrong valve covers, wrong assumptions, and wrong internet advice all pile into the same shopping cart. The A/Poly 318 is the wide-block poly. It deserves its own parts conversation.

• LA 318 Confusion
  The A/Poly 318 isn’t the later LA 318. Same displacement number. Different head design, different intake layout, different valve covers, different look, and different parts reality.
  
  That confusion costs money fast. Intake manifolds, heads, valve covers, exhaust fit, front dress, mounts, brackets, and internal parts all need checking by actual engine family. “It’s a 318” isn’t enough. That phrase has sent plenty of good parts back across the counter with an owner muttering at the box.
• Wide-Block Name Confusion
  The 318 A/Poly often gets called a wide-block, and that nickname helps until it starts doing damage. “Wide-block” points toward the broader A/Poly shape, but it’s not a complete parts description.
  
  A seller saying “wide-block 318” has started the identification job, not finished it. Year, application, market, intake, heads, front dress, mounts, transmission fit, and hardware still need checking. Nicknames are fine for conversation. They are lousy at stopping parts from not fitting.
• Fury / V-800 Claims Without Proof
  The 318 A/Poly has real performance history, especially around Fury and V-800 versions. That is exactly why the story gets inflated. Suddenly every 318 poly becomes a factory hot setup because the air cleaner looks ambitious.
  
  A real Fury, V-800, or high-performance 318 claim needs the right year, application, intake, carburetion, linkage, air cleaner, exhaust, camshaft context, stampings, vehicle connection, and documentation. A regular 318 with a swapped four-barrel doesn’t become factory performance hardware because somebody polished the lid and practiced the story.
• Truck, Export, and Service-Replacement Mix-Ups
  The 318 A/Poly lived long enough and traveled far enough to create plenty of identity noise. Passenger cars, trucks, export markets, Canadian use, and later service history can all show up in the same conversation.
  
  That doesn’t make every 318 poly the same parts pile. Oil pans, mounts, manifolds, front dress, cooling pieces, transmission-related hardware, compression, carburetion, and accessory brackets can change the build plan. A good-running 318 poly may still be the wrong version for the car sitting in the garage.
• Common Name, A/Poly Parts Rules
  The 318 A/Poly is common enough to be respected and uncommon enough to surprise lazy builders. It’s not as hopeless as people think, and it’s not as easy as an LA swap crowd wants to pretend.
  
  Parts exist, support exists, and the engine can be a good driver. But it still needs A/Poly-aware planning. Intake choices, exhaust fit, ignition, mounts, cooling, transmission pairing, and rebuild parts need checking before the job turns into three weekends of “why doesn’t this fit?”

For broader Mopar A/Poly problems shared across the engine family, use the Mopar A/Poly family page under Common Family Problems.

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Common Misconceptions

The 318 A/Poly gets misunderstood because the later LA 318 stole the name recognition. That makes people treat the poly like either a weird obsolete lump or an LA engine wearing the wrong hat. Both shortcuts are lazy. The A/Poly 318 is its own engine, with real usefulness, real limits, and real parts rules.

• “The 318 poly is basically an LA 318.”
  No. The A/Poly 318 and LA 318 share the displacement number and some family history, but they aren’t the same engine.
  
  Heads, intake, valve covers, exhaust, front dress, mounts, and many parts questions are different. If the plan starts with LA assumptions, the parts bill is already leaning against the wall with bad news.
• “Every 318 poly is a factory performance engine.”
  No. Fury, V-800, and high-performance 318 poly versions are real, but most 318 poly engines aren’t automatically factory performance packages.
  
  A real claim needs hardware and proof. Intake, carburetion, linkage, air cleaner, exhaust, application, dates, stampings, vehicle context, and documentation all need to agree. The displacement number doesn’t get to wear the whole costume.
• “Wide-block means big-block.”
  No. “Wide-block” doesn’t mean B/RB big-block. It’s a nickname tied to the A/Poly engine’s shape and layout.
  
  A 318 A/Poly is still a small-block-family poly engine, not a 383, 413, 426, or 440. Calling it wide doesn’t make it big-block hardware. It just makes the wrong parts order sound more confident.
• “The 318 poly is worthless because LA parts are easier.”
  Wrong. LA parts are easier in many cases, but easier doesn’t mean better for every car. A correct A/Poly car, period build, or driver that already has a healthy poly may be better served by keeping the engine it came with.
  
  The 318 poly can make good torque, run well, and keep an early Mopar honest. It just needs the right expectations. It’s not junk because the later catalog is thicker.
• “Every A/Poly 318 part fits every 318 poly.”
  No. Same displacement doesn’t cancel year, market, application, truck/passenger differences, or hardware changes.
  
  The 318 poly needs actual identification before ordering parts. Oil pan, mounts, exhaust, front dress, cooling pieces, transmission-related parts, induction, and accessory brackets can all create surprises. “It’s all 318 poly” is how a simple job learns new swear words.

For broader Mopar A/Poly misconceptions shared across the family, use the Mopar A/Poly family page under Common Family Misconceptions.

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Upgrade Limitations

The Mopar 318 poly is the early A-series engine people underestimate because the later LA 318 stole the name and got all the easy parts-store fame. That’s the first mistake. The poly 318 isn’t an LA 318 with funny valve covers. It isn’t a Magnum. It isn’t a Dodge Hemi. It isn’t a cheap modern small-block wearing antique clothes. It’s the A-series polyspherical “wide-block” 318, and it plays by older rules.

The 318 poly uses a 3.91-inch bore and 3.3125-inch stroke. It’s one of the most familiar and better-supported members of the A-poly family, especially compared with the 277, 301, 303, and 313 oddballs. That helps. It doesn’t make it a later LA engine. The block, heads, intake, valvetrain layout, exhaust fit, front dress, brackets, accessories, exact year, market, and application can still make lazy LA assumptions expensive.

The first upgrade limitation is interchange fantasy. Because “318” appears on both the poly and LA engines, people start assuming parts cross over like Chrysler was trying to be helpful. It wasn’t. LA intakes, heads, valve covers, many valvetrain pieces, and plenty of bolt-on expectations don’t simply walk onto a poly 318 and behave. Some parts and service knowledge overlap around the broader small-block world, but the top end and packaging are their own argument.

Parts support is better than the smaller A-poly engines, but it still isn’t LA-small-block easy. Pistons, rings, bearings, gaskets, camshaft work, timing parts, valve parts, rocker gear, intake choices, exhaust manifolds, headers, brackets, pulleys, and water parts all need exact checking. The 318 poly can be built well, but it needs A-poly parts and A-poly thinking. A catalog search that says “318” without knowing which 318 is already halfway to lying.

Compression and camshaft planning need restraint. The 318 poly has enough displacement to be a useful street engine, but it doesn’t have the cheap mistake forgiveness of a later, heavily supported small-block build. Too much cam with low compression, highway gears, weak exhaust, tired ignition, poor carburetion, or a heavy car can make it feel dull. A good poly 318 wants cylinder pressure, clean carburetion, correct timing, healthy exhaust, and gearing that fits the vehicle.

Cylinder heads are the charm and the choke point. The polyspherical chamber gives the engine its look, sound, and identity. It also means the rocker gear, pushrods, valve covers, intake fit, exhaust fit, and oiling details need attention. A good valve job, correct springs, healthy rocker shafts, clean oiling, and proper geometry matter more than pretending the engine is a later wedge-head LA. The poly can breathe respectably when treated right, but it won’t reward sloppy top-end work.

Induction choices can be useful or ridiculous. A correct two-barrel can make a smooth cruiser. A proper four-barrel setup can wake the engine up nicely when the compression, cam, exhaust, and gearing support it. Multi-carb setups and vintage speed parts can look fantastic, but they need real linkage, tuning, fuel distribution, air cleaner fit, and throttle manners. The 318 poly doesn’t need carburetor jewelry. It needs a matched plan.

Exhaust and packaging can slow the job down. The poly’s heads and wider top-end layout are why people commonly call it a “wide-block,” and that can affect manifold, header, steering, starter, accessory, and chassis clearance. Don’t assume LA headers or later small-block exhaust pieces fit because the displacement number is familiar. Measure the actual car, actual engine, and actual parts before the wallet starts clapping.

Core condition still matters. The 318 poly earned a reputation for durability, but old iron still gets old. Worn cylinders, stuck rings, tired valve seats, damaged rocker gear, corroded cooling passages, cracked exhaust areas, broken bolts, missing brackets, and mixed early hardware can turn a simple rebuild into an argument. A complete running poly is a different starting point from a bare core with three coffee cans of bolts and one bad explanation.

The honest upgrade plan is mild to moderate and purpose-built. Rebuild the 318 poly when originality, A-poly character, complete hardware, or period-correct Mopar flavor matters. Use sensible compression, a cam that fits the vehicle, good ignition, proper carburetion, healthy exhaust, and cooling that works. If the goal is cheap horsepower, endless aftermarket choices, and easy bolt-ons, the LA 318, 340, or 360 will usually be easier. The poly 318 is good. It just isn’t the later 318 everyone keeps trying to turn it into.

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When It Actually Makes Sense

The 318 poly makes sense when the engine belongs in the car and the owner wants the early Mopar personality to stay intact. In a correct Plymouth, Dodge, or Chrysler Corporation application where the A-poly 318 is part of the vehicle’s story, keeping it can be the right move. It preserves the period feel without turning the car into another LA swap with apologies.

It makes sense when the engine is complete. Air cleaner, carburetor, intake, exhaust manifolds, distributor, brackets, pulleys, valve covers, rocker gear, water parts, mounts, transmission pieces, linkage, and application hardware matter. A complete 318 poly gives you a real foundation. An incomplete one can still be saved, but it may start charging rent before it runs.

It also makes sense as a mild street engine. A healthy poly 318 can be smooth, durable, torquey enough for normal old-car use, and interesting without being fussy. It likes clean ignition, proper carburetion, decent exhaust, working cooling, and gearing that matches the car. It doesn’t need to become a race engine to be worth keeping.

The 318 poly makes sense when the build wants vintage character more than modern convenience. A well-done four-barrel poly with the right cam, exhaust, and tune can be a satisfying period street engine. It has more personality than a generic later swap, and it can make the car feel right instead of merely easier.

It makes less sense when the only goal is performance per dollar. A later LA 318, 340, or 360 will usually be cheaper to modify, easier to feed parts, and better supported by modern aftermarket choices. That doesn’t make the poly 318 junk. It means the poly’s value comes from durability, character, correctness, and old-Mopar charm, not cheap horsepower math.

It also makes less sense when the engine is incomplete, badly worn, or missing poly-specific parts. Missing rocker gear, intake, exhaust manifolds, brackets, pulleys, water parts, linkage, or drivetrain pieces can take the shine off a cheap core fast. The poly 318 is more supportable than the early oddballs, but missing parts still don’t rebuild themselves out of nostalgia.

Plain answer: build the 318 poly when it belongs, when it’s complete, when period-correct character matters, and when the goal is a strong, smooth old Mopar driver. Don’t build it because you think it’s just an LA 318 with antique valve covers. It isn’t. That little mistake is how the parts bill learns to grin.

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Compatibility Notes

Family identity.
The 318 poly is an A-series polyspherical V8, commonly called the “wide-block” 318. It isn’t the later LA 318, it isn’t a Magnum small-block, it isn’t a Dodge Hemi, and it isn’t a Chrysler or DeSoto Hemi. It belongs in the A-poly family, and that means the block, heads, intake, valvetrain, exhaust, accessories, exact year, market, and application need A-poly-specific checking.

Bore and stroke proof.
Verify the actual engine before trusting the badge. The 318 poly uses a 3.91-inch bore and 3.3125-inch stroke. Check casting numbers, stampings, bore, crank, heads, intake, date context, vehicle application, market, and physical measurements before calling it a 318 poly, LA 318, 313, 326, Dodge poly, export engine, Canadian engine, or Hemi-related swap.

318 poly versus LA 318.
The 318 poly and LA 318 share displacement, not top-end identity. Heads, intake manifold, valve covers, rocker gear, exhaust fit, many brackets, and plenty of swap details differ. Don’t order LA parts and expect the poly to thank you. Same displacement number, different engine family problem.

318 poly versus 277, 301, 303, and 313.
The 318 poly is part of the broader A-series poly family, but it’s not automatically parts-interchangeable with every smaller A-poly engine. Bore, stroke, crank, pistons, heads, intake, year, market, application, and supporting hardware can differ. Family relationship helps identify the engine. It doesn’t turn the whole family into one parts bin.

318 poly versus 326 poly.
The 326 belongs in the A-poly discussion, but exact year, market, bore, stroke, crank, pistons, heads, intake, and application details still matter. Don’t assume a 326, 318, or 313 shares every useful part because the engines look related. Measure first. Buy second.

318 poly versus Hemi and other poly families.
Don’t confuse the A-series 318 poly with early Dodge, DeSoto, or Chrysler Hemi/poly families. Chrysler Corporation reused chamber ideas, names, and visual themes in ways that punish lazy identification. “Poly” describes chamber style. It doesn’t prove the same block, heads, crank, intake, exhaust, valvetrain, or service parts.

Intake, heads, and valvetrain.
Verify intake fit, head casting, rocker gear, pushrods, valve covers, lifters, camshaft, timing parts, and oiling details by exact engine. The 318 poly’s top end is the reason the engine is interesting and the reason lazy LA assumptions fail. Missing or mismatched top-end parts can turn a simple build into a swap meet subscription.

Exhaust and accessory fit.
Check exhaust manifolds, headers, starter clearance, generator or alternator brackets, pulleys, water pump, front dress, mounts, oil pan, steering clearance, and chassis fit. The wide-block layout can make packaging different from later LA small-block expectations. Don’t assume later brackets or exhaust parts will walk in and act civilized.

Transmission, crank, and starter fit.
Verify crank flange, crank register, bellhousing pattern, adapter plates where used, starter location, flywheel or converter fit, clutch parts, linkage, mounts, driveshaft, and rear gear by exact year, market, and application. Early Mopar drivetrain fit isn’t where “same displacement” gets to make decisions.

Restoration-correct versus driver-correct versus performance-correct.
For restoration-correct work, the right 318 poly and supporting hardware can define the car. For a driver, a mild build with correct induction, clean ignition, healthy cooling, honest exhaust, and sensible gearing can be excellent. For performance work, be realistic. The 318 poly can be fun and strong, but it isn’t an LA small-block convenience package with scalloped valve covers.

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Reality Check

The 318 A/Poly isn’t a forgotten mistake just because the LA 318 became more common. It’s the long-running working name of the early A/Poly family, and it earned that place by being durable, useful, and different. The Geezer gives it respect for what it is, not for pretending it’s a later LA engine with funny valve covers.

The performance versions are real, but the proof has to be real too. Fury, V-800, Golden Commando, dual-quad, and other hotter 318 Poly claims need the right car, hardware, year, induction, exhaust, stampings, date trail, and documentation. A standard 318 with a 4-barrel intake doesn’t become a factory performance package because the seller polished the lid and practiced the story.

The standard and workhorse 318 Poly versions deserve their own respect. They can be smooth, durable, and correct in the right Plymouth, Dodge, truck, Canadian, or export application. Not every useful engine has to be a glamour engine. Some of them earn their keep by starting, pulling, and not turning every trip into a parts-store ceremony.

The LA confusion is the wallet killer. Same displacement doesn’t mean same heads, intake, valve covers, exhaust, brackets, or build path. The 318 Poly can be built well, but it wants A/Poly thinking. If the project depends on later LA parts solving every problem, the engine is already waiting to embarrass somebody.

The hard truth is simple: keep the 318 Poly when correctness, completeness, period character, or old-Mopar personality matters. Choose an LA 318, 340, or 360 when cheap horsepower, easy aftermarket parts, and modern convenience matter more. The 318 Poly is a good engine in the right lane. It just hates being shoved into the wrong one.

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Bottom Line

The Mopar 318 A/Poly is the engine that made Plymouth’s Poly V8 story stick. The 277 opened the door, the 301 and 303 handled early special steps, the 313 carried a Canadian/export branch, and the 318 became the useful long-runner. That doesn’t make it exotic. It makes it important in the way old engines actually matter: it did the work.

The 318 Poly makes the most sense when the car deserves its original A/Poly character. Correct Plymouths, Dodges, trucks, export-market cars, and period builds can all justify keeping it. A healthy standard 318 Poly can be a fine cruiser engine. A verified Fury, V-800, Golden Commando, or dual-quad version can be a serious early performance piece when the hardware and paperwork prove it.

It makes less sense when the goal is cheap horsepower or easy modern parts. The later LA 318, 340, and 360 have better support, more familiar build paths, and more aftermarket choices. That doesn’t make the Poly 318 junk. It means the Poly’s best value is correctness, character, durability, and old-Mopar identity, not maximum horsepower per dollar.

The mistake to avoid is letting the number 318 do all the thinking. A Poly 318 isn’t an LA 318. It isn’t a Magnum 318. It isn’t a Hemi cousin. It isn’t automatically a Fury performance engine because somebody added carburetors. Casting numbers, stampings, heads, intake, carburetion, exhaust, application, market, and documentation all have to support the story.

Completeness matters. A running, complete, correct 318 Poly with the right manifolds, brackets, rocker gear, accessories, and application trail can be a good engine to preserve or build mildly. A half-stripped core missing Poly-specific hardware can become a parts hunt before it becomes a motor. The 318 name helps, but it doesn’t make missing pieces grow back.

Bottom line: the 318 A/Poly is a useful, durable, character-rich old Mopar small block when it’s treated honestly. Build it when the car wants it and the parts are there. Don’t confuse it with an LA, don’t inflate every one into a Golden Commando prize, and don’t dismiss it just because the later catalog is easier. The Geezer says the Poly 318 is worth saving in the right lane — just don’t let the familiar number steer the whole truck.

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