Mopar A-Series Poly Small-Block V8 Family Specs and Identification

Intro

The Mopar A-series Poly is what happens when Chrysler builds a perfectly sensible engine and then spends the next decade giving it enough displacements, markets, applications, and relatives to make identification a hobby. The family covered here is the Plymouth-style A-block Poly line: 277, 301, 303, 313, 318, and the Dodge-marketed 326. It is not every old Mopar with funny valve covers, not one of the Hemi-based Dodge or DeSoto Poly families, and absolutely not the later LA 318 just because Chrysler reused a number everybody already knew.

Plymouth needed this engine because by the middle of the 1950s the flathead six had reached the stage where durability was becoming its strongest argument and the competition had discovered horsepower. Chevrolet had a new small-block V8, Ford was selling overhead-valve V8s, and Chrysler’s more expensive divisions already had modern V8 machinery of their own. Plymouth could either join the century or keep explaining why Grandpa had never complained.

The A-series answer was polyspherical rather than hemispherical. Chrysler got a modern overhead-valve V8 with good breathing, shaft-mounted rocker gear, decent torque, and much less cost and bulk than the corporation’s famous Hemis. The Poly chamber was not the poor man’s Hemi somebody settled for after losing an argument with accounting. It was a deliberate compromise between airflow, manufacturing cost, packaging, and the reality that Plymouth still had to sell cars to people who looked at the price tag.

The 277 got Plymouth started in 1956. The 301 added displacement almost immediately. The 303 supplied one of the family’s sharper early performance chapters, particularly in the 1956 Fury story. Then the 313 wandered off into Canadian and export history, the 318 settled down and became the workhorse everybody remembers, and Dodge borrowed the basic idea for the one-year 326 because Chrysler Corporation apparently believed a tidy engine chart would encourage laziness.

Mechanically, the family divides more neatly than the names suggest. The 277 and 301 use the shorter 3.125-inch stroke. The 303, 313, 318, and 326 move to roughly 3.3125 inches, with displacement changing mainly through bore size. Once that pattern is understood, the family tree starts looking like engineering instead of six unrelated numbers somebody pulled from a hat.

The 318 became famous mostly by surviving. It powered ordinary cars, wagons, trucks, export machinery, and some considerably more interesting performance combinations. That broad usefulness made it the natural memory anchor for the whole family, which is also why every unidentified A-block seems to become a 318 after ten minutes of conversation. Sometimes it actually is. Measuring remains cheaper than confidence.

These engines have another virtue modern performance arguments tend to miss: they have character without requiring a collector-auction pedigree. The broad Poly heads, angled covers, shaft rockers, and oddball intake arrangement make one look like something designed when engineers were still allowed to solve problems differently. A decent one is sturdy, torquey, distinctive, and entirely capable of moving an old Mopar the way Chrysler intended.

The bill comes due when parts are missing. Poly-specific heads, rocker gear, intake pieces, exhaust manifolds, early transmission hardware, brackets, pulleys, water pieces, and market-specific odds and ends are not sitting on every speed-shop shelf waiting to apologize. An incomplete rare 313 or 326 may be considerably less useful than a complete ordinary 318. Scarcity is impressive right up until the only missing bracket is owned by a man who knows exactly how scarce it is.

That is the A-series Poly family. It pulled Plymouth into the modern V8 era, carried the corporation through several wonderfully untidy displacement experiments, and eventually handed the small-block job to the simpler LA design. Build one when the car, history, or sheer old-Mopar character gives you a reason. If the only goal is cheap horsepower and easy parts, Chrysler eventually invented several better ways to spend the weekend.

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Overview

The A-series Poly began because Plymouth needed a modern V8 without inheriting the cost and bulk of Chrysler Corporation’s Hemi habit. By 1956, overhead valves and V8 horsepower had stopped being luxury-car novelties. Chevrolet and Ford were making the showroom argument obvious, and Plymouth’s dependable old flathead six could only answer “but it lasts forever” so many times before somebody asked how long it took to reach sixty.

Chrysler’s solution was the A-block polyspherical V8. The basic engine used a cast-iron block and heads, an in-block camshaft, pushrods, two valves per cylinder, shaft-mounted rocker arms, a rear-mounted distributor, and polyspherical combustion chambers. The Poly chamber gave the valves and ports a more favorable breathing arrangement than an ordinary old wedge without requiring the enormous head structure of a full Hemi. It was clever engineering aimed at a car division that still had to care what clever engineering cost.

That distinction from Chrysler’s other Poly engines cannot be treated as trivia. Dodge, DeSoto, and Chrysler had other polyspherical designs tied to different engine families, including Hemi-derived short blocks. The engines covered here are the A-block small-block Polys. Calling all of them simply “Poly” is about as useful as identifying every dog at the park as “brown.”

The later LA small-block creates the other major source of confusion. The LA grew out of Chrysler’s small-block development, but it did not arrive as an A-series Poly with narrower valve covers. The heads, combustion chambers, intake, exhaust, rocker system, valve covers, and plenty of supporting hardware changed. The Poly 318 and LA 318 sharing a displacement is historically interesting and mechanically dangerous if somebody interprets it as a parts guarantee.

The first A-block was the 277, introduced for Plymouth in 1956 with a 3.750-inch bore and 3.125-inch stroke. It was not a giant engine even by 1950s standards, but that misses its assignment. Plymouth finally had its own modern V8: compact enough and inexpensive enough for the division’s cars, yet strong enough to make a full-size Plymouth feel as though somebody at engineering had noticed the horsepower race.

Regular 277s did ordinary family-car work. Power Pack versions could add four-barrel carburetion and dual exhaust for buyers who wanted a little more enthusiasm. That should not be inflated into secret race-engine history. Plymouth was learning to sell performance at the same time it was learning to sell V8s. Sometimes an extra carburetor really was just an extra carburetor.

The 301 followed almost immediately for 1957. Chrysler kept the 3.125-inch stroke and opened the bore to roughly 3.910 inches. That makes the first two engines a tidy short-stroke pair: same crank throw, larger bore, more displacement. The market wanted more power almost before Plymouth had finished congratulating itself on the 277, so engineering did the obvious thing and gave it more cylinder.

The 301 was still a Plymouth engine first, not some miniature muscle-car prophecy. It served the increasingly powerful full-size lineup and appeared in stronger Power Pak and Fury-related contexts, but its short production life tells you where Chrysler was headed. Plymouth had finally joined the V8 race and immediately discovered the finish line kept moving.

The 303 makes the family chart more interesting because Chrysler changed more than bore. It used roughly a 3.810-inch bore with a longer 3.3125-inch stroke, placing it mechanically with the branch that later includes the 313, 318, and 326. In U.S. history it is best remembered for giving the 1956 Fury some genuine machinery behind the gold trim and advertising.

That Fury association deserves respect without becoming contagious. Four-barrel induction, higher compression, and the correct supporting equipment gave the 303 real performance identity in the right car. A loose 303 block does not automatically become a Fury engine after somebody mentions 1956. Chrysler installed parts at the factory; hot rodders have been removing and reinstalling them ever since.

Then there is the 313, proof that North American engine history gets considerably less tidy the moment somebody remembers Canada exists. The 313 used a 3.875-inch bore with the longer 3.3125-inch stroke and lived primarily in Canadian and export-market applications. Australian Chrysler products, Canadian Dodges and Plymouths, British Bristol applications, and other export uses give it a much broader passport than the usual U.S. Mopar chart suggests.

The 313 was not a typo for 318 and was not created merely to confuse American restorers sixty years later. Different markets had their own production schedules, tax structures, parts supplies, vehicle lines, and displacement needs. Chrysler kept using combinations where they made sense locally. Detroit did not organize world production around what would someday fit neatly on an American muscle-car website.

That export history also means the 313 needs more care when parts hunting. Induction, exhaust, cooling, accessories, transmission hardware, and installation details can follow the market and vehicle rather than whatever U.S. Plymouth happens to look similar in a photograph. An engine can be completely correct and still make the American parts catalog shrug.

The 318 is where the family finally settles down long enough to become familiar. Its approximately 3.910-inch bore and 3.3125-inch stroke produced the displacement Chrysler would keep using long after the Poly heads disappeared. In A-series form, though, this is still very much the wide-headed Poly engine with shaft rockers and its own intake and exhaust arrangements.

The 318’s importance came from range rather than rarity. Passenger cars, wagons, trucks, export applications, industrial and marine work all gave the engine places to earn its keep. Two-barrel versions handled ordinary transportation. Four-barrel and performance combinations added more enthusiasm. Fury V-800 and dual-quad hardware showed that the practical engine could dress for Saturday night when Chrysler felt like spending money on carburetors.

That broad use is exactly why the 318 became the safest general-purpose answer within the family. More engines survived, more parts knowledge followed them, and more owners learned how to keep them working. It is still an old Poly, so “easy” remains a relative term. Compared with finding one-year 326 pieces or explaining a 313 to somebody who thinks the number is a misprint, however, the 318 feels almost mainstream.

The 326 occupies the other end of the sanity scale. Dodge used it for 1959 after retiring its earlier Dodge-specific Hemi/Poly family. The new engine belonged to the A-block Poly line, used the longer 3.3125-inch stroke, and carried the family’s largest bore at roughly 3.950 inches. Actual displacement was close enough to the old Dodge 325 that Dodge marketed the replacement as 326, thereby avoiding one kind of confusion and manufacturing another.

The 326 was primarily a Dodge passenger-car engine, not a secret performance package. Hydraulic-lifter hardware in common applications also separated it from several earlier solid-lifter A-block combinations. Its greatest modern importance is usually correctness: if a 1959 Dodge was built with one, the oddball under the hood is not an inconvenience to be corrected. It is part of what makes the car a 1959 Dodge instead of another small-block swap.

Taken mechanically, the family therefore breaks into two stroke groups. The 277 and 301 use the short 3.125-inch stroke. The 303, 313, 318, and 326 use the longer roughly 3.3125-inch stroke. Within each group Chrysler altered bore to produce the various displacements. That is a much better way to understand the family than memorizing six cubic-inch numbers and hoping the parts counter admires the effort.

The displacement progression also explains why block casting numbers alone can be disappointing identification tools. Chrysler reused basic castings across multiple displacements and markets, then machined and assembled them according to need. Some cylinder-head castings wandered across nearly the entire family. The casting tells you what pile of iron Chrysler started with. It does not always tell you what size holes the factory finished boring into it.

That shared hardware is useful until somebody promotes “shared” into “interchangeable.” Bore and stroke affect pistons, rings, crankshafts, compression planning, and rebuild parts. Valvetrain details changed. Export applications can carry their own hardware. The 326 brings year-specific wrinkles. Even when two components physically bolt together, chamber volume, geometry, clearances, and the rest of the combination still have to approve the marriage.

Transmission fit deserves particular suspicion because early Mopar driveline history was apparently designed by people who never imagined anyone would still be swapping these engines after retirement. Crank flanges, registers, bellhousings, starters, flywheels, converters, adapters, linkage, and year-specific hardware all need to be checked together. “Small-block Mopar” is the beginning of a sentence, not a transmission order.

Completeness is therefore unusually valuable. Rocker shafts and arms, correct intake, exhaust manifolds, pulleys, brackets, water hardware, transmission pieces, linkage, air cleaners, and other application-specific bits can cost more time and money to replace than the block itself. A complete 277 that belongs in the car may be a much better buy than a supposedly desirable oddball delivered as two heads, a bare block, and six coffee cans.

These engines are old enough that condition gets the final vote anyway. Rocker-shaft wear, restricted oiling, tired cam lobes, stuck rings, worn bores, rusted exhaust hardware, cooling-system corrosion, leaking seals, broken fasteners, and machine work from several previous civilizations can all be hiding inside a perfectly respectable-looking Poly. Chrysler built sturdy engines. Chrysler did not warranty them through grandchildren.

For restoration work, the choice is usually easy: keep the displacement the car actually deserves. The 277 and 301 belong in early Plymouths where their place in the story matters. A genuine 303 Fury application deserves its correct performance hardware. The 313 belongs in the Canadian and export machinery that explains why it exists. The 326 belongs in the 1959 Dodge that would otherwise lose one of its strangest details.

For a general A-series Poly street build, the 318 makes the strongest practical case. It gives useful displacement, the broadest history, better parts familiarity, and enough torque to make an old Plymouth enjoyable without turning every repair into an international search. If originality does not dictate one of the rarer engines, there is very little glory in choosing the version with fewer available pistons merely to prove determination.

And if the project begins with an empty engine bay and the requirements are cheap horsepower, easy aftermarket parts, lighter weight, and painless transmission choices, the sensible answer may be to leave the entire Poly family to somebody who actually wants one. Later LA engines can accomplish those jobs with far less archaeology.

That does not diminish the A-series Poly. Quite the opposite. Its value today is that it belongs to a specific piece of Chrysler history: the period when Plymouth finally got a modern V8, Chrysler experimented rapidly with displacement and markets, and the corporation had not yet simplified everything into the LA small-block most people recognize. The Poly is wide, strange, sturdy, occasionally inconvenient, and unmistakably from a time when Detroit still solved the same problem six different ways before lunch.

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General Family Specs

Engine Family	Mopar A-Series Poly Small-Block V8	Production / Use Era	Mid-1950s through early 1960s in major factory use*
Configuration	90-degree V8	Valve Layout	OHV / pushrod, 2 valves per cylinder
Cam Location	In-block camshaft	Block Style	Early Mopar A-series / small-block poly architecture
Cylinder Head Layout	Polyspherical combustion chamber heads	Rocker Layout	Shaft-mounted rocker arms
Distributor Location	Rear-mounted	Common Block Material	Cast iron
Common Head Material	Cast iron	Oiling System	Wet sump
Typical Induction	Carbureted	Typical Bellhousing Pattern	Early A-series pattern; verify year, crank flange, and transmission hardware
Major Factory Engines in This Family	277, 301, 303, 313, 318, 326	Major Identification Caution	A-series poly and LA small-block parts don’t automatically interchange

*Use varied by division, market, vehicle line, and application. Canadian, export, service, and regional usage can make year-and-application assumptions dangerous.

Those are the broad family facts. They don’t make every A-series poly part universal, and they don’t turn a later LA 318 into the same engine. The poly heads, intake, exhaust, valvetrain, pistons, compression, front dress, crank flange, and application details can change the plan fast.

The 318 name causes the worst shortcut. A 318 A-series poly and an LA 318 share a famous displacement, not a full parts catalog. The family relationship is real. The bolt-on assumptions still need a leash.

The bellhousing and crank hardware deserve extra suspicion because early Mopar transmission fit can get ugly. Flywheels, converters, starters, crank flanges, and adapters don’t care that somebody said “small-block Mopar” with confidence.

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Identification

The Mopar A-series Poly small-block is usually easy to recognize once somebody stops calling every old polyspherical Chrysler engine a “Poly” and expecting that to settle anything. The A-block has broad polyspherical heads, angled valve covers, shaft-mounted rocker gear, an in-block camshaft, and a rear-mounted distributor. It looks noticeably wider across the heads than the later LA small-block even though both belong to Chrysler’s small-block family tree.

The heads are the loudest clue. An A-series Poly does not have the compact wedge heads and narrow valve covers of an LA 273/318/340/360, and it does not have the enormous central-plug Hemi heads of Chrysler’s early FirePower engines. The intake, exhaust-port arrangement, rocker gear, and valve covers are all Poly-specific.

That gets you to the family. It does not tell you whether the engine is a 277, 301, 303, 313, 318, or the Dodge-marketed 326. These engines share enough architecture, block castings, heads, and external hardware that the exact displacement needs better evidence.

Do Not Confuse the A-Series Poly With an LA 318

The A-series Poly 318 and LA 318 are different engines. The later LA family grew out of Chrysler’s small-block development, but the cylinder heads, intake manifold, exhaust, rocker system, valve covers, and much of the supporting hardware changed.

A rear-mounted distributor does not distinguish them because both families put it there. Look at the heads. The A-series Poly has the broad, canted polyspherical-head arrangement. The LA uses the much simpler wedge-head package.

The shared 318 number has probably bought more wrong Mopar parts than any amount of bad handwriting ever did.

“Poly” Does Not Mean One Chrysler Engine Family

Chrysler Corporation built several polyspherical engines. Early Dodge and Chrysler Hemi-based engines were also offered with Poly-style heads, but those engines use different short blocks and belong to different families.

The 277, 301, 303, 313, 318, and 326 covered here belong to the A-block small-block Poly family. An earlier Dodge 270, 315, or 325 Poly and a Chrysler Hemi-block Poly are not members of this family just because somebody shortened all of them to the same nickname.

The chamber shape is a clue. The block identifies the parts.

Find the Stamped Engine Identification Number

The stamped engine ID is one of the best ways to identify an assembled A-block.

On engines originally sold in the United States, look at the front driver side of the block on the flat machined pad immediately below the cylinder head.

On many Canadian, Australian, British, and other export engines, the stamping is instead found on the front passenger-side block deck on a machined pad. Exceptions exist, so an export engine can occasionally use the domestic location.

That difference matters in this family because the 313 and several 303 applications are heavily tied to Canadian and export production. Failing to find a number on the U.S. pad does not mean Chrysler forgot to identify the engine. Look on the other side before reaching for conspiracy theories.

Useful Factory ID Prefixes

Chrysler’s stamped prefixes changed by model year and market, so there is no single “318 stamp” used for the entire production run. Some useful examples include:

• P29 — 1956 Plymouth 277.
• FP29 — 1956 export Plymouth 303.
• P311 — 1957 Plymouth 277.
• P312 — 1957 Plymouth 301.
• VT — 1957 export 303.
• KV — 1957 export 313.
• LP31 / FLP31 — 1957 318.
• LC — 1958 export 313.
• LP8 — 1958 318.
• MC — 1959 export 313.
• MP8 — 1959 318.
• LD2 / L325 — 1959 Dodge 326.

Later 313 and 318 production used additional prefixes as Chrysler changed model-year coding. The important thing is to decode the stamp for the actual year and market rather than comparing a 1965 engine with a 1957 list and declaring the number wrong.

Block Casting Numbers Help — but Chrysler Shared Them

Block casting numbers are useful for establishing the A-block family and narrowing production period, but several castings were machined into more than one displacement.

Examples include:

• 1618729 — found in 277 and 303 applications.
• 1676329 — found as 277, 303, and 313.
• 1736118 / 1737929 — found under 301, 313, and 318 combinations.
• 1859616 — found under 313, 318, and 326 applications.
• 2268306, 2268509, 2402971, 2468230, 2468260, 2532630, 2532674 — later castings commonly shared by 313 and 318 production.

That is why one raised casting number should not overrule the stamped engine ID. Chrysler often cast one basic block and machined it for different markets or displacements.

The casting tells you what family of iron you found. The stamped pad, year, market, and measurements tell you what Chrysler made from it.

Heads Are Even Less Useful for Displacement

The familiar early 1618721 cylinder head was used across 277, 301, 303, 313, 318, and 326 applications. Later 2268341-family heads appear on 313 and 318 engines.

So a head casting can help establish production era and configuration, but it can be nearly useless for exact displacement.

This family is a particularly good place to remember the Geezer rule: the heads identify the heads.

The 277 and 301 Are the Short-Stroke Pair

The 277 and 301 use the 3.125-inch stroke. The displacement difference comes from bore.

The 277 uses a 3.750-inch bore. The 301 enlarges that to roughly 3.910 inches.

That makes year and stamped ID extremely useful. The 277 belongs primarily to 1956–1957 Plymouth A-block production. The 301 is principally the 1957 Plymouth step-up engine.

A correct P29 or P311 stamp points toward a 277. P312 points toward the 301. Once the stamp and application agree, there is usually little reason to tear into a healthy original engine merely to satisfy curiosity.

The 303 Changes the Stroke

The 303 uses roughly a 3.810-inch bore and 3.3125-inch stroke. That puts it mechanically with the later long-stroke branch rather than the 277/301 crank package.

The 303 is strongly associated with 1956 Plymouth Fury production and certain export applications. That history is useful, but it also creates performance-story problems.

A real 303 block does not automatically make the complete engine a Fury package. Intake, carburetion, heads, exhaust, dates, vehicle application, and other equipment still have to support the performance claim.

The 313 Is the Export Trouble Maker

The 313 uses a 3.875-inch bore and 3.3125-inch stroke.

It appears heavily in Canadian and export-market Chrysler Corporation use, including applications that never appeared in ordinary U.S. dealer literature. That is why a genuine 313 sometimes gets treated as a typo by somebody whose entire reference library consists of American muscle-car catalogs.

Look for the export-side stamping location when appropriate and verify the prefix against the vehicle’s market and year. KV, LC, MC, and later export codes can all belong in the 313 story.

The 313 also appeared outside North America, including Australian, British, and other export applications. “Export” is therefore part of the identification, not an exotic bonus word automatically attached to the price.

The 318 Is the Familiar One

The A-series 318 uses approximately a 3.910-inch bore and 3.3125-inch stroke.

It became the long-running workhorse and therefore the default answer whenever somebody sees an A-block and does not feel like checking.

That shortcut works often enough to keep causing trouble.

The 318 shared several block castings with the 301, 313, and 326 depending on year. Heads overlap even more broadly. The stamped engine code is therefore much more useful than external hardware.

The 318 also appeared in passenger cars, light trucks, heavy trucks, marine service, industrial equipment, and export markets. Those applications can carry very different induction, cooling, valvetrain, exhaust, oil-pan, and accessory hardware while still being legitimate A318s.

The 326 Is a One-Year Dodge Oddball

The engine Chrysler marketed as the 326 was a 1959 Dodge A-block using approximately a 3.950-inch bore and 3.3125-inch stroke.

Useful stamped identifiers include LD2 and L325. The second code is a nice reminder that factory naming and actual calculated displacement do not always agree perfectly. Mopar marketing was allowed near the arithmetic too.

The 326 also commonly carried hydraulic-lifter/nonadjustable-rocker hardware that differs from many earlier mechanical-lifter A-block combinations. That can be a useful supporting clue when original.

It is still bolt-on evidence. A 1959 engine assembled from mixed pieces needs the block stamp and dimensions to settle the question.

Crankshaft Stroke Narrows the Family

If the oil pan is already off, crankshaft identification can divide the family into useful groups.

The 277 and 301 use the shorter 3.125-inch stroke. The 303, 313, 318, and 326 use the longer approximately 3.3125-inch stroke package.

That does not identify every displacement because several engines share each stroke. It does tell you which side of the family tree the rotating assembly belongs to.

Bore finishes the job.

Application Evidence Matters More Here Than Usual

The A-block family has an unusual amount of year, division, and market-specific identity:

• 277: early Plymouth A-block territory.
• 301: principally 1957 Plymouth.
• 303: 1956 Fury and export-related applications.
• 313: Canadian and export-heavy identity.
• 318: broad passenger, truck, marine, industrial, and export use.
• 326: one-year 1959 Dodge application.

That makes an original vehicle, correct stamped pad, appropriate casting period, and matching hardware especially useful. Sometimes the car tells you what the engine should be before the engine confirms it.

Do not reverse the logic. An old Plymouth that was built with a 277 can have a 318 between the fenders today. Sixty years is plenty of time for somebody to discover an engine hoist.

Common Identification Mistakes

• “It is a Poly, so it is a 318.”
  The 318 is only one of six A-block displacements covered here.
• “Poly means all old Chrysler polyspherical engines interchange.”
  No. Hemi-block Poly engines and A-block Poly engines are different families.
• “A Poly 318 and LA 318 are basically the same engine.”
  Same displacement. Different top-end architecture and many different parts.
• “The block casting number proves displacement.”
  Several A-block castings were machined into multiple displacements. Read the stamped pad.
• “The heads prove displacement.”
  Some head castings were shared across nearly the entire family.
• “A 303 automatically means Fury.”
  The displacement is only part of the Fury package. Application hardware and vehicle evidence still matter.
• “313 must be a typo for 318.”
  No. The 313 is a real Canadian/export A-block displacement.
• “326 means the old Dodge 325 Hemi/Poly family.”
  The 1959 326 covered here is an A-block small-block. Similar displacement names do not establish engine family.
• “Rare means valuable.”
  A complete correct 313 or 326 in the vehicle that needs it can matter. A stripped oddball core can simply mean a longer parts hunt.

What Evidence Deserves Trust?

• Wide polyspherical heads, angled valve covers, shaft rockers, rear distributor: strong A-series Poly family identification.
• Stamped front block identification pad: one of the strongest assembled-engine clues for displacement, year, and market.
• Domestic-versus-export stamping location: useful supporting evidence when it agrees with vehicle history.
• Block casting number: strong family and era evidence, but often weak exact-displacement proof because castings were shared.
• Head casting number: useful for head type and period; generally poor displacement proof.
• Vehicle year, division, and market: strong supporting evidence because several displacements had narrow application windows.
• Intake, carburetor, exhaust, rocker gear, front dress, and accessories: useful application evidence when original, but replaceable.
• Crankshaft stroke: useful for separating the short-stroke and long-stroke branches.
• Measured bore and stroke: final mechanical identification.

The A-series Poly is one of those families where Chrysler’s stamped engine number is often more useful than staring at castings. Find the correct front pad, establish whether the engine is domestic or export, compare the code with year and vehicle application, and then use the block casting and hardware as supporting evidence.

If the stamp is gone, the engine has been rebuilt, or the casting was shared across several displacements, the mechanical pattern still helps. The 277 and 301 share the short 3.125-inch stroke. The 303, 313, 318, and 326 use the longer 3.3125-inch stroke. Bore separates the engines within those groups.

When the evidence stops agreeing, quit ordering parts by nickname. Bore and stroke are the ultimate identification test. Old Mopar engine families are confusing enough without helping them.

Bore / Stroke Reference

Displacement	Bore	Stroke		Displacement	Bore	Stroke
277	3.750 in	3.125 in		301	3.910 in	3.125 in
303	3.810 in	3.3125 in		313	3.875 in	3.3125 in
318	3.910 in	3.3125 in		326	3.950 in	3.3125 in

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Shared Design Patterns

The A-series poly family splits into two practical groups: early/less-common displacements that matter mostly for correctness and identity, and the 318, which became the familiar workhorse of the bunch.

The 277 is the early small-displacement engine. It belongs in the correct old-Mopar lane, where originality and period accuracy matter more than power-per-dollar. Building one can make sense in the right vehicle. Building one as a cheap horsepower shortcut is how a machine shop gets paid to teach history.

The 301 is the bigger early cousin, not the magic middle child. It can be right in the correct car, but it doesn’t have the easy-owner logic of the later 318. It’s the kind of engine that rewards the owner who wants correctness and punishes the owner who thinks “close enough” is a build plan.

The 303 is where simple Mopar talk starts tripping over its own boots. It can be part of the correct period story, but the name needs to match the actual block, vehicle, and hardware. Guessing here just turns old-Mopar charm into a wrong-parts pile with prettier valve covers.

The 313 is the Canadian/export troublemaker. It reminds everyone that Mopar didn’t build the entire world around one U.S. parts-counter chart. A real 313 can matter a lot in the right vehicle, but U.S.-centric assumptions can send the parts plan straight into the ditch.

The 318 A-series poly is the practical one. It earned the strongest reputation because it was used widely, worked hard, and stuck around in enough vehicles to become the name people remember. It’s durable, torquey, and worthwhile when originality or period Mopar character matters.

The 326 is the oddball with enough displacement and scarcity to make people start inventing romance. It belongs in the group, but the number needs evidence. When an engine is unusual, guessing doesn’t become smarter. It just becomes more expensive and more embarrassing.

That’s the family pattern: smaller and oddball displacements for correctness and history, 318 for practical ownership, and every single one requiring better identification than “old Mopar small-block with funny valve covers.”

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Choosing Within This Family

The A-series Poly family is mostly a correctness-first engine family today. The 277 and 301 make their strongest case in early Plymouth restorations. The 303 adds genuine early performance identity. The 313 belongs primarily to Canadian and export applications. The 318 is the broad-use workhorse and usually the easiest A-series Poly to justify as a general driver. The 326 is the one-year Dodge oddball that makes sense when that exact identity matters.

• Choose the 277 when preserving Plymouth’s first A-series V8 matters.
  It belongs in a correct 1956 Plymouth or a period build where early-engine identity is part of the point. It can make a smooth mild cruiser when complete and healthy, but its small displacement, limited parts support, and early-specific hardware make it a poor choice when cheap horsepower is the assignment.
• Choose the 301 for the same basic reason, with a little more displacement.
  It is the short-lived 1957 growth step between the 277 and the later engines. A complete 301 can make an excellent period driver and belongs naturally in a correct Plymouth. It still does not offer enough parts support or performance advantage to make much sense as a loose engine chosen strictly for power.
• Choose the 303 when its specific history matters.
  A correct 1956 Fury or other documented market-specific application gives the 303 a stronger performance and collector argument than the ordinary 277 or 301. Its longer stroke and period performance role make it interesting, but rarity and limited parts availability mean it should be rebuilt because the engine belongs to the project, not because somebody wants an inexpensive performance foundation.
• Choose the 313 when Canadian or export correctness is the reason for the build.
  In the proper Plymouth, Dodge, Australian, British, or other documented market application, the 313 can be an important part of the vehicle’s identity. It can also make a pleasant period driver. If originality does not require it, however, the more common 318 Poly is generally the simpler A-series choice and later LA engines make more sense when easy performance is the goal.
• Choose the 318 Poly when you want the most practical all-around member of this family.
  It has the longest and broadest use, better parts support than the smaller oddballs, enough displacement for useful street torque, and a strong record in ordinary cars, trucks, export applications, and period performance packages. A healthy 318 Poly can be an excellent cruiser or mild street engine without pretending to be an LA 318.
• The 318 is also the best A-series Poly choice when originality does not demand one of the rarer displacements.
  It still requires Poly-specific heads, induction, valvetrain, exhaust, and installation hardware, but finding and building one is generally less troublesome than chasing the same result from a 277, 301, 303, 313, or 326.
• Choose the 326 when a correct 1959 Dodge needs one.
  Its larger bore and unusual one-year identity make it interesting, but they do not make it a better general performance engine than the 318. Exact pistons, cylinder-wall condition, hydraulic-lifter details, and application-specific hardware can make the 326 more difficult to rebuild. When originality does not require the 326, a 318 Poly or later small-block is usually easier to justify.

Across the family, completeness can matter more than displacement. Rocker gear, intake, exhaust manifolds, brackets, pulleys, water pieces, linkage, transmission hardware, and other early or market-specific parts can be harder to replace than the bare block. A complete smaller engine that belongs in the car may be a better project than a larger but stripped oddball bought because its cubic-inch number looked more attractive.

The short version is simple. Choose the 277 or 301 for early Plymouth correctness. Choose the 303 when genuine Fury or documented period-performance identity matters. Choose the 313 for Canadian or export correctness. Choose the 318 when you want the strongest general-purpose A-series Poly argument. Choose the 326 when a 1959 Dodge specifically deserves it. If cheap horsepower, easy parts, and modern upgrade choices lead the requirements list, the entire A-series Poly family is probably the wrong place to start.

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Compatibility Notes

The Mopar A-series Poly family shares enough architecture to create useful overlap, but not enough to make the 277, 301, 303, 313, 318, and 326 one interchangeable parts pile. Bore, stroke, crankshaft, pistons, valvetrain, year, market, application, drivetrain hardware, and supporting pieces still have to match the exact engine.

• Keep the A-Series Poly Separate from the LA Small-Block
  The later LA 273, 318, 340, and 360 belong to a different top-end system. Cylinder heads, intake manifolds, rocker and pushrod arrangements, exhaust fit, valve covers, brackets, accessories, and many swap details differ. The Poly 318 and LA 318 sharing the same displacement does not make them the same engine.
• “Poly” Does Not Mean One Chrysler Corporation Engine Family
  Early Dodge, DeSoto, and Chrysler Hemi/Poly engines belong to different block families. Chamber style and similar-looking valve covers do not establish interchange for blocks, heads, cranks, intakes, exhaust, valvetrain, or service parts.
• Watch Reused and Similar Displacement Names
  The Plymouth A-series 301 is not the earlier Chrysler 301 family, and the 1959 A-series 326 is not the older Dodge 325 Hemi/Poly family. Similar numbers and the same corporate badge do not establish parts compatibility.
• The Early 277 and 301 Use the Shorter Stroke Package
  The 277 and 301 use the 3.125-inch stroke, with different bores. Crankshaft, pistons, compression, bore-specific parts, and supporting hardware therefore need to match the actual displacement rather than being ordered simply as early A-series Poly pieces.
• The 303 Uses the Later-Style Longer Stroke but Its Own Bore
  The 303 uses the 3.3125-inch stroke rather than the 277/301 stroke. That changes crank, piston, rod, compression, and rebuild planning. Its relationship to later 313, 318, and 326 engines does not make those rotating assemblies automatically interchangeable.
• The 313, 318, and 326 Require Bore-Specific Parts
  These later A-series Poly engines are closely related, but pistons, rings, compression planning, exact bore, year, market, and application still differ. The 318 is useful reference material for the rarer 313 and 326. It is not permission to order 318 rebuild parts without measuring the engine first.
• Market-Specific Engines Need Market-Specific Hardware Checks
  Canadian, export, Australian, British, and other market applications can carry different induction, exhaust, accessories, cooling pieces, linkage, transmission hardware, and installation details. A 313 or other export-market Poly may share the basic A-series architecture while still carrying hardware that does not match a U.S. passenger-car installation.
• The 326 Has Year-Specific Valvetrain Details
  The one-year Dodge 326 is commonly associated with hydraulic lifters while many earlier A-series Poly combinations used different valvetrain arrangements. Verify lifter type, camshaft, pushrods, rocker gear, timing parts, and oiling before assuming another A-series valvetrain package belongs in it.
• Heads, Intake, and Valvetrain Need to Stay Together
  Poly heads use their own chamber layout, intake requirements, shaft-mounted rocker hardware, pushrods, valve covers, lifters, camshaft relationships, timing pieces, and oiling details. Missing or mismatched top-end parts can turn an otherwise straightforward rebuild into a difficult parts hunt.
• Exhaust Fit Depends on the Engine and the Chassis
  Factory manifolds, headers, starter clearance, steering, vehicle width, and application can all change exhaust fit. Car, truck, export, and one-year applications may carry hardware that does not transfer cleanly to another chassis.
• Transmission Fit Requires Exact Early-Mopar Hardware
  Verify crank flange, crank register, bellhousing pattern, adapter plates where used, starter location, flywheel or converter, clutch components, linkage, mounts, driveshaft, and transmission year together. “A-series Poly” or “small-block Mopar” is not enough information to establish drivetrain fit.
• Front Dress and Installation Hardware Follow the Application
  Water pumps, pulleys, generator or alternator brackets, accessory drives, motor mounts, oil pans, cooling pieces, and steering clearance can vary by year, vehicle, and market. Keep original hardware with the engine until the replacement installation has been proven.

That is the A-series Poly compatibility rule: identify the exact displacement, year, market, and application first, then match the bore and stroke package, heads, valvetrain, induction, exhaust, crank and transmission hardware, accessories, mounts, and cooling around that engine. Family resemblance is useful. “Poly” and “small-block Mopar” are still not parts numbers.

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Common Family Problems

The A-series Poly is sturdy old Mopar iron, and most of its recurring problems have less to do with some hidden engineering disaster than with the fact that these engines have been collecting birthdays since Eisenhower was in office. Worn valvetrain parts, leaking seals, rusty exhaust hardware, neglected cooling systems, sludge, corrosion, and previous repair work are the usual suspects.

The other problem is scarcity. A common worn part is annoying. A worn part nobody has stocked since three presidents ago can turn a simple repair into a nationwide scavenger hunt.

• Rocker-Shaft and Valvetrain Wear
  Rocker shafts, rocker arms, pushrods, lifters, cam lobes, springs, guides, valve tips, and the oil supply feeding the whole arrangement deserve inspection. Worn shafts, restricted oiling, tired lifters, weak springs, damaged cam lobes, and mismatched rebuild pieces can leave the top end noisy, weak, or wearing itself out in a hurry. Shaft-mounted rockers are durable. They are not protected by historical significance, and sixty years of use still count even when the valve covers look interesting.
• Oil and Gasket Leaks
  Valve covers, timing covers, oil pans, rear main seals, fuel-pump areas, intake sealing surfaces, and other old gasket joints are common leak points. Hardened seals, distorted covers, damaged surfaces, crankcase pressure, and repeated previous repairs all contribute. A little seepage from an old engine is not a national emergency. Marking every parking spot like a dog with territorial ambitions means it is time to find the leak.
• Cooling-System Neglect and Overheating
  Clogged radiators, rusty block passages, tired water pumps, weak fans, missing shrouds, wrong pulley combinations, lean mixtures, and bad ignition timing can make an A/Poly run considerably hotter than Chrysler intended. Engines that spent years in trucks, wagons, or other heavy service deserve extra suspicion. Cast iron is tough. It is not impressed by stories about how Grandpa drove it overheated every summer and “never had a problem.”
• Exhaust-Manifold and Fastener Damage
  Rusted studs, broken bolts, cracked manifolds, damaged flanges, and seized exhaust hardware are common age-related headaches. That would merely be irritating on an engine with reproduction exhaust pieces stacked to the ceiling. Poly hardware can be another matter. Breaking an original manifold or snapping several old studs during what was supposed to be a Saturday repair can turn the afternoon into industrial archaeology. Penetrating oil is cheaper than rare cast iron.
• Worn Cores and Previous-Rebuild Damage
  Worn cylinders, stuck rings, tired bearings, damaged valve seats, cracked castings, sludge, damaged threads, corrosion, broken hardware, and old machine work can all hide inside an apparently complete engine. The less-common 277, 301, 303, 313, and 326 deserve especially careful inspection because rebuilding a bad core can become expensive before the engine has produced enough oil pressure to justify the trouble. “Ran when parked” remains one of the automotive world’s most durable statements without a warranty.
• Missing Poly-Specific Hardware
  A complete engine is worth considerably more trouble than an incomplete one because Poly-specific rocker gear, intake pieces, exhaust manifolds, water hardware, pulleys, brackets, linkage, transmission pieces, and market-specific odds and ends are not sitting in every parts warehouse waiting for a credit card. A cheap 313 or 326 missing three peculiar pieces can become more expensive than a complete 318 before the rebuild even begins. Scarcity is charming until the missing bracket belongs to the only man at the swap meet who knows exactly what it fits.
• Mixed Early Hardware and Wrong Replacement Parts
  The A-series covers multiple displacements, years, markets, and applications, which means previous owners have had decades to substitute whatever seemed close. Pistons, rings, gaskets, induction, valvetrain pieces, mounts, cooling hardware, and transmission-related parts need to match the actual engine. The familiar 318 does not automatically donate the right answer to a 277, 303, 313, or 326. “Looks about the same” is not a part number.

The practical rule with an A-series Poly is to inspect completeness almost as carefully as condition. A tired but complete engine can often be rebuilt. A supposedly better core missing rare external hardware may spend years becoming an expensive shelf full of pieces.

These engines are old, unusual, and generally tougher than their parts availability. Treat the original hardware like it matters, measure before substituting, and do not turn a routine repair into an archaeological expedition unless there is no other choice.

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Common Family Misconceptions

The A-series Poly lives in the part of Mopar history where one nickname, several odd displacement numbers, Canadian and export production, and the later LA small-block can make a simple identification job turn into family-reunion chaos. The cure is to identify the engine that is actually there instead of letting the nearest familiar number finish the story.

• “The A-series Poly is basically an early LA small-block.”
  No. The A-series Poly helped lead into Chrysler’s later LA small-block family, but it is not an LA engine wearing wider heads. Heads, intake layout, valve covers, exhaust, valvetrain details, front dress, mounts, and many internal parts differ. The Poly 318 and LA 318 sharing a displacement number does not make them twins.
• “All Mopar Poly engines belong to the same family.”
  No. Chrysler Corporation used polyspherical chamber designs in more than one engine family. Plymouth A-block Polys, Dodge and DeSoto Poly designs, and other Hemi-related Poly engines do not become interchangeable because somebody shortened all of them to “Poly.” The chamber nickname identifies an idea. The block identifies the parts.
• “The 277, 301, 303, 313, 318, and 326 are basically interchangeable versions of the same engine.”
  They share A/Poly ancestry, but bore, stroke, pistons, rings, gaskets, valvetrain details, year, market, application, induction, mounts, and external hardware can differ. The 313 and 326 in particular punish the assumption that a nearby 318 catalog number is good enough. Same family is useful information. It is not permission to stop measuring.
• “Every old A/Poly is really some kind of 318.”
  No. The 318 became the familiar long-running displacement, which is exactly why earlier and market-specific engines get mislabeled as one. The 277, 301, 303, 313, and 326 have their own identities and applications. Familiarity does not get to revise the casting numbers.
• “The performance package comes with the displacement.”
  It does not. A 277 is not automatically Power Pack equipped. A 303 is not automatically a complete 1956 Fury engine. A 301 does not inherit every 1957 Plymouth performance claim. A 318 is not automatically Fury or V-800 hardware, and a 326 is not automatically a performance engine because the number is unusual. Intake, carburetion, exhaust, linkage, dates, application, vehicle context, and documentation have to support the claim.
• “Rare, Canadian, export, or short-lived automatically means valuable.”
  No. A correct 303, 313, 326, or other uncommon A/Poly can be important when the car actually requires it. The same engine as a rusty incomplete loose core may simply be harder to rebuild and harder to sell. Scarcity can raise value when demand follows it. Sometimes scarcity just makes the scavenger hunt longer.
• “The oddball engines are worthless because the 318 is easier.”
  Also wrong. A correct 277, 301, 303, 313, or 326 can be exactly the right engine for an original car or period build. Easier parts support makes the 318 more practical for many projects; it does not erase the historical or restoration value of the engines that came before it or lived in different markets.
• “Every 313 is a Bristol engine, and every export story makes it exotic.”
  Bristol use is real, but the 313 also served Canadian, Australian, and other export-market Chrysler Corporation applications. Market origin and displacement alone do not establish Bristol pedigree or premium value. The vehicle, hardware, dates, and documentation still have to show where the engine belonged.
• “The 326 is just a 318 or the old Dodge 325 under another name.”
  No. The 1959 Dodge 326 belongs to the A/Poly family and uses its own larger-bore combination. It is not the earlier Dodge Red Ram 325 Hemi/Poly family, and it is not merely a mislabeled 318. Mopar created the 326 name partly to keep those identities apart. Future owners promptly volunteered to put them back together.
• “Wide-block means big-block.”
  No. “Wide-block” is a common nickname for the broad-looking A/Poly layout, especially the 318. It does not make the engine a B- or RB-series big block. A wide valve-cover stance cannot turn a small-block-family Poly into a 383, 413, 426, or 440.
• “Any A-series Poly is an easy old-school swap.”
  Not necessarily. Mounts, transmission and crank details, exhaust clearance, cooling, accessories, and less-common parts can make an A/Poly swap considerably more involved than a later LA installation. It can be an excellent choice when period character matters. It is rarely the shortest route to cheap horsepower.

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Bottom Line

The Mopar A-series Poly family is old-Mopar machinery for somebody who actually wants old-Mopar machinery. The 277, 301, 303, 313, 318, and 326 have real toughness, real character, and enough strange-looking hardware to make an engine bay interesting before the key is turned. They also come from an era when parts interchange was apparently considered something future owners could argue about later.

The siblings do not all deserve the same treatment. The 277 is the early correctness-first engine. The 301 is the next step up, still more valuable for belonging in the right car than for winning a horsepower-per-dollar contest. The 303 and 326 are uncommon pieces where exact identity matters more than rarity talk. The 313 carries the Canadian and export side of the family and punishes anybody who assumes a U.S. parts book knows the whole story. The 318 is the practical anchor: the best-known, most useful, and easiest member of the family to justify as an actual driver engine.

For restoration work, preserve the displacement and hardware that belong with the vehicle rather than automatically replacing an oddball Poly with something easier. That historical correctness is where the smaller and less-common engines make their strongest argument. For a general street build with no originality requirement, the 318 Poly usually makes the most practical case inside the family. If the only goal is cheap horsepower and easy bolt-ons, though, the entire A-series Poly family is volunteering for the wrong contest.

The big trap is believing familiar names mean familiar parts. A Poly 318 is not an LA 318 with unusual valve covers. Canadian, export, early, and odd-displacement engines can carry enough differences in induction, mounts, crank and bellhousing details, exhaust, and other hardware to make a reasonable-looking parts order completely wrong. The engine is not being difficult. Somebody skipped the identification work and blamed the casting later.

So choose an A-series Poly because originality, durability, period identity, or plain old Mopar oddball character matters. Verify the exact engine before buying parts, and do not make the rare ones perform jobs the more practical 318 already handles better. A good Poly is tough, torquey, distinctive, and worth keeping in the right car. A guessed-at Poly is just a parts hunt with vintage paint and a machine shop waiting at the finish line.

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