Mopar 313 Engine Specs

Intro

The Mopar A/Poly 313 is the one that reminds you Chrysler Corporation didn’t build engine history for people who like tidy charts. It belongs to the Plymouth A-block polyspherical V8 family, but it’s mostly a Canadian and export-market story, not the normal U.S. Plymouth path most people remember. Built from 1957 into the late 1960s export era, the 313 gave Canadian, Australian, British, and other non-U.S. applications their own A/Poly displacement while the familiar 318 took over most of the memory bank.

That’s the first cleanup job. The 313 is its own Canadian/export A-block Poly V8, sharing family logic with Plymouth’s 277, 301, 303, and 318 while carrying its own bore, market role, and application spread. Keep it separated from Chrysler Hemi, Dodge Red Ram Poly, DeSoto Poly, and lazy “U.S. 318 with a smaller sticker” talk. Same “Poly” nickname doesn’t prove the short block. Same general family doesn’t make every North American, Canadian, Australian, or Bristol part story line up nice and pretty.

Chrysler needed the 313 because Canada and export markets didn’t always follow U.S. engine calendars. Different assembly operations, tax structures, displacement rules, supply chains, right-hand-drive markets, and local model mixes could keep an engine alive in places where the U.S. lineup had already moved on. The 313 let Chrysler use the A/Poly architecture in markets that needed something between the earlier Plymouth steps and the better-known 318.

In practical use, the 313 served cars that needed a sturdy, modern V8 without pretending they were factory race specials. Canadian Dodges and Plymouths, Australian Chrysler Royals, and Bristol luxury/performance cars all used the displacement in different ways. That doesn’t make every 313 exotic, and it doesn’t make every one a Bristol engine with tea and expensive manners. It means the 313 had a real export and Canadian-market job that doesn’t fit neatly into U.S.-only Mopar storytelling.

The plain-English identity is this: the Mopar A/Poly 313 is a Canadian/export A-block Poly V8, closely related to the Plymouth Poly family but aimed at markets and applications the usual U.S. 318 story doesn’t fully explain. It’s not a Hemi cousin, not a mystery 318, and not a mistake. It’s what happens when Mopar engineering meets borders, tax rules, local production, and a parts book with a sense of humor.

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Overview

The Mopar A/Poly 313 sits off to the side of the normal U.S. Plymouth Poly timeline, and that’s exactly why it needs its own explanation. The 277 started Plymouth’s A-block V8 story. The 301 and 303 handled short early steps. The 318 became the familiar long-runner in the United States. The 313 lived mainly in Canada and export markets, where Chrysler’s model planning didn’t always follow the neat U.S. sequence people memorize from domestic brochures.

Architecturally, the 313 stayed in the A/Poly family. It used Plymouth-style polyspherical heads and the A-block layout, not the Dodge or DeSoto Hemi-derived Poly design and not the Chrysler FirePower Hemi. The bore was smaller than the U.S. 318, while the common A/Poly stroke kept it tied to the same basic engine family. That made the 313 a real A-block Poly, not a badge trick and not a Hemi with cheaper manners. The chamber design, block family, and market use all point to the same answer if the reader lets the hardware talk before the nickname starts yelling.

Its market role is the useful part. In Canada, the 313 appeared in Dodge and Plymouth applications where local production and model differences didn’t match the U.S. catalog. In Australia, it powered Chrysler Royal applications. In Britain, Bristol used the 313 in the 407 and early 408 era, which gives the engine a strange little side passport most American Mopar people don’t expect. That doesn’t turn every 313 into a hand-built British grand-tourer engine. It just proves the displacement traveled farther than the usual American garage stories admit.

The 313’s behavior was never about pure muscle-era theater. It was a practical, medium-displacement A/Poly V8 for markets that needed smooth power, durability, and enough output for larger cars without stepping into bigger Chrysler or big-block territory. In Bristol form, a four-barrel 313 could sound more glamorous because the car around it was wearing a better suit. In a Canadian Dodge or Plymouth, it was more likely doing honest family-car work. Same displacement, different badge, different job, different dinner table.

Family confusion is the biggest problem. The 303, 313, and 318 numbers sit close enough to make people casual, and casual is where old Mopar parts orders go to die. A 313 isn’t the 1956 Fury 303. It isn’t the U.S. 318. It isn’t an LA 318. It isn’t a Dodge Red Ram Poly. It belongs to the A/Poly family, but bore, year, country, application, and actual casting numbers still decide the conversation. The parts book doesn’t care whether the engine crossed an ocean.

Compared with the 301 and 303, the 313 had a broader export life and a more international footprint. Compared with the 318 Poly, it’s less familiar in the U.S. but closely tied to the same A-block family story. Compared with the later LA engines, it’s older, heavier in layout, and not the same wedge-head small-block people think of when somebody says “Mopar 318.” That’s the line that keeps the parts order from wandering into the wrong decade.

The biggest bad assumption is treating the 313 as either an oddball nobody needs to understand or a secret performance prize because Bristol used it. Both are lazy. The 313 is a real working A/Poly displacement with Canadian and export-market importance. It can be correct, interesting, and useful in the right car, but it doesn’t turn into something else because the car wore Dodge, Plymouth, Chrysler Royal, or Bristol badges.

The Mopar A/Poly 313’s real place is as the export-and-Canadian branch of the A-block Poly story: close to the familiar Plymouth family, different enough to deserve its own page, and easy to misidentify if somebody thinks U.S. brochures explain the whole world. Respect the market context, verify the hardware, and don’t let the 318 memory erase an engine that did its work where Detroit’s domestic story didn’t bother leaving good notes.

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What makes it different

The Plymouth/Dodge A/Poly 313 is the Canada-and-export Poly that looks close enough to the 318 to make people lazy. Put it in its own lane: later than the early 277, 301, and Fury 303 steps, smaller-bore than the common 318 Poly, and separate from the LA small-block family even though later Mopar people got used to saying “318” like it solves everything. The 313 is the almost-318: close family, close job, smaller bore, different market story, and plenty of room for wrong assumptions to sneak into the parts order.

Compared with the 277, 301, and 303, the 313 belongs to the later, more settled side of the A/Poly family. The 277 opened the door. The 301 stretched the idea. The 303 brought Fury shine. The 313 went a different direction. It served Canadian and export markets that didn’t follow the exact same U.S. displacement script. That makes it useful history, not automatic hot-rod bait.

Compared with the 318 Poly, the 313 is the smaller cousin people are most likely to blur. Same basic A/Poly direction, same 3.3125-inch stroke, close displacement, and a lot of shared-looking logic from ten feet away. But the 318 became the familiar name: broader recognition, more U.S. use, more builder attention, and a stronger parts-and-knowledge trail. The 313 doesn’t get to borrow all that just because it lives nearby. Close still leaves room for pistons, rings, bore size, compression, and market-specific pieces to make a fool out of somebody.

Compared with the LA small blocks — 273, 318, 340, and 360 — the 313 needs the usual Poly fence, preferably one with barbed wire and a sign big enough for tired buyers. The later LA engines grew from the A-engine direction, but the 313 remains an A/Poly with Poly heads, Poly intake needs, Poly valvetrain concerns, and market-specific parts questions. A 313 A/Poly isn’t an LA 318 that learned Canadian manners. If the order starts with “small-block Mopar” and ends before anybody says Poly, the mistake is already packed and shipped.

Compared with Dodge, DeSoto, and Chrysler Poly or early Hemi engines, the 313 belongs outside the “semi-Hemi” junk drawer. Chrysler FirePower, Dodge Red Ram, DeSoto, Plymouth A/Poly, and export Poly pieces don’t become interchangeable because the valve covers look interesting and the chamber shape sounds fancy. Blocks, heads, mounts, intakes, accessories, and rebuild parts can all split the bill in different directions. “Poly” isn’t a universal password. It’s the part of the conversation where guessing starts getting expensive.

Compared with the U.S.-market 318 story, the 313 is the engine that reminds you Mopar history didn’t stop at the border. Canadian and export Chrysler products often used combinations that make American parts-counter habits look dumber than usual. That’s where the 313 earns its own warning label. Before buying pistons, gaskets, intake parts, or rebuild pieces, verify the engine family, market, bore, year, and actual hardware. The seller’s “basically a 318” line doesn’t get to sign the invoice.

The 313 A/Poly makes sense when Canadian/export correctness matters, or when the car itself deserves the engine it was built with. It can be a solid old Mopar Poly when used honestly, but it’s not the easiest performance path and not the engine to pick if the plan depends on U.S. 318 assumptions doing all the work. The common 318 has the easier name. The later LA engines have better parts comfort. The 313 has its own narrow lane, and it should stay there unless the builder enjoys learning geography through receipts.

That’s the 313’s place. It’s the near-318 Canadian/export A/Poly, not the common U.S. 318, not an LA shortcut, and not a mystery Hemi cousin waiting to become valuable because the word Poly sounds old and clever. Preserve it when the car deserves the right engine. Verify it before buying parts. And don’t let “basically a 318” order pistons, gaskets, or intake parts while the real 313 sits there waiting to make you look stupid.

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General Specs

Displacement	313 cu in / 5.1L	Bore / Stroke	3.875 in × 3.3125 in
Production / Use Era	1957–1967 Canadian/export A-block Poly passenger-car era	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 passenger-car ratings vary by market and tune; Bristol 4-barrel versions are commonly listed around 250 hp / 340 lb-ft	Compression Ratio	Market/tune dependent; verify by exact application
Fuel / Induction	Gasoline; carbureted, 2-barrel or 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 313 sits close enough to the 318 Poly to invite lazy assumptions, and far enough away to punish them when the parts show up wrong.

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Variants

The 313 Poly needs a variant section that keeps the market story nailed down. This wasn’t a normal U.S. Plymouth showroom engine with a clean domestic option ladder. It was mainly a Canadian and export A-series Poly story, with enough Bristol and overseas application noise to make sellers sound educated while the buyer forgets to measure anything.

The useful split is simple: regular Canadian / export passenger-car 313s, higher-output 4-barrel export applications such as Bristol use, 318 and A-series Poly confusion, and service / swap / market-story confusion. That’s enough. Don’t build fake buckets just because the engine crossed borders and made the parts book look like it had been drinking.

Canadian / Export Passenger-Car 313

The regular 313 was a working A-series Poly V8 for Canadian and export Chrysler Corporation use. It gave Dodge, Plymouth, Chrysler Royal, and other market-specific applications a sturdy medium-displacement V8 without following the exact same U.S. engine calendar. That’s the point. The 313 wasn’t a secret American performance engine hiding from brochures. It was a market-specific engine doing market-specific work.

The 313 used a 3.875-inch bore and 3.3125-inch stroke, putting it close enough to the 318 Poly to tempt lazy thinking and different enough that the lazy thinking can cost money. Bore and stroke are the mechanical anchor. If the measurements don’t agree with the story, the story can take a seat.

A 313 in a Canadian Dodge, Canadian Plymouth, Australian Chrysler Royal, or documented export application can be completely legitimate. The same displacement claim in a U.S.-market context needs a harder look. Country, model, year, market, casting information, stampings, and original application all matter. That isn’t paperwork snobbery. That’s how the Geezer keeps a real export engine from getting sold under a domestic fairy tale.

In the right car, the 313 is a good, honest A/Poly engine. It has old-Mopar character, usable displacement, and the kind of market-specific correctness that matters when the car deserves its original story. What it isn’t is a magic 318 substitute, a Hemi cousin, or an automatic rare-engine prize because American hobbyists don’t trip over them every weekend.

Bristol / Higher-Output 4-Barrel Export Applications

The 313 gets more interesting when Bristol and other higher-output export applications enter the room. Bristol 407 and early 408 use gives the 313 a strange little side passport, and that makes the engine sound fancier than the average Canadian family-car version. Fine. The story is real. The proof burden is real too.

A Bristol-related 313 is commonly tied to 4-barrel carburetion and a stronger rating than ordinary passenger-car use. That can make it a neat engine in the right context. But a 4-barrel intake on a loose 313 doesn’t automatically make it Bristol-spec, high-output, or worth sports-car money. Carburetors can move. Stories move even faster.

A real Bristol or higher-output export claim needs application proof. Intake, carburetor, linkage, air cleaner, exhaust, engine details, vehicle history, market context, date clues, and documentation all need to line up. Without that, it’s just a 313 with ambition bolted to the intake.

That doesn’t make an ordinary 313 worthless. It means the special branch has to prove it’s special. A correct Bristol-related 313 is a neat piece. A regular Canadian/export 313 can still be a good engine. A loose 4-barrel 313 with no paperwork gets inspected before it gets complimented.

318 and A-Series Poly Confusion

The 313’s worst everyday problem is 318 confusion. The 318 Poly is the one more people recognize, the one more U.S. hobbyists expect, and the one with the better parts-and-memory trail. That makes the 313 easy to erase under a “basically a 318” shrug, and that shrug has bought plenty of wrong parts.

A 313 isn’t a 318 because the valve covers look familiar. It isn’t a 303 because somebody heard “Canadian” and stopped listening. It isn’t a 301 because the numbers are close enough for bad math. Bore, stroke, casting numbers, stampings, application, and market context still have to carry the identification.

The danger isn’t just naming. It affects pistons, rings, gaskets, compression planning, restoration correctness, value, and build choices. Treat a 313 like a 318 without checking the details, and the parts bill starts correcting the whole room.

The safest rule is simple: identify the exact engine before building the story. The A-series Poly family has enough look-alike relatives to fool anybody who thinks valve covers are legal documents. They aren’t. They’re just lids with attitude.

Service, Swap, and Market-Story Confusion

The 313 is especially vulnerable to market-story confusion because it wasn’t the usual U.S. domestic talking point. That makes one crowd dismiss it as a mistaken 318 and another crowd inflate it into exotic treasure because they haven’t seen one before. Both mistakes are lazy. One undercuts the engine. The other lets the sales pitch grow fangs.

After decades of swaps, rebuilds, imports, exports, and parts migration, a 313 may not be sitting in the car it started in. It may have the wrong carburetor, wrong intake, missing tags, later service parts, or a story that got upgraded every time the car changed hands. That doesn’t make it bad. It means the engine and the car have to prove they still know each other.

For a real 313 claim, check the market context first. Canadian build, export build, Bristol use, service replacement history, and vehicle paperwork matter. Then check the engine: casting numbers, stampings, bore and stroke evidence, heads, intake, carburetion, exhaust, and transmission context. If those pieces don’t line up, the displacement number is just noise wearing greasy paint.

A correct 313 in the right car is worth understanding because it tells a market-specific Mopar story. A loose 313 on a stand with no paperwork may still be useful, but it’s not automatically rare treasure. It’s homework, and homework doesn’t get cheaper because somebody says “export” with confidence.

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Key Points

The Mopar 313 A/Poly is an early A-block Poly with a passport problem. It belongs mainly to Canadian and export Chrysler Corporation use, not the clean U.S. Plymouth 318 story most people keep in their heads. That makes it easy to mislabel, easy to overhype, and easy to under-explain. The Geezer starts with the market before he lets the parts order move an inch.

The basic fingerprint is 313 cubic inches from a 3.875-inch bore and 3.3125-inch stroke. That puts it very close to the 318 Poly, but close isn’t identical. The 318 used a larger bore, and that small-looking difference matters when pistons, rings, displacement claims, machine work, or rebuild planning enter the conversation.

The 313 is A/Poly hardware, not LA-family hardware. LA 318, 340, 360, and Magnum engines may be more familiar, but familiarity doesn’t make the parts fit. Heads, intake, valve covers, front dress, exhaust, mounts, and internal parts need to be treated as A/Poly business until exact parts prove otherwise.

The 313 also belongs outside the Dodge Red Ram Poly, DeSoto Poly, Chrysler FirePower Hemi, and other early Chrysler Corporation buckets that get tossed into the same “old Poly” pile. The word “Poly” describes chamber style. It doesn’t prove block family, heads, crank, intake, exhaust, mounts, or service parts. If the seller can only say “it’s a Poly,” he hasn’t said enough.

Market context carries much of the value. A 313 can be correct in Canadian Dodge, Canadian Plymouth, Australian Chrysler Royal, Bristol, or other export-market use depending on the exact car and documentation. It can also be a swapped engine, service replacement, or loose core with a story that crossed more borders than the hardware did. Make, model, year, country, application, and paperwork all have to matter.

The Bristol connection is real, and that’s exactly why it needs a leash. Bristol 407 and early 408 use can make a 313 more interesting, but it doesn’t turn every 313 into a Bristol engine or a sports-car payday. A Bristol claim needs vehicle context, correct hardware, date clues, and documentation. A 4-barrel and an accent don’t count.

The 313 gets tangled with the 303, 318, and 326 because the A/Poly family shares enough architecture and appearance to fool anyone who stops looking too early. Casting numbers, stampings, bore measurement, original application, market, and matching external pieces have to carry the identification. “Basically a 318” isn’t a spec sheet. It’s a warning label wearing shoes.

As an engine, the 313 is a good old Mopar piece when the car and plan make sense. It has rugged character, decent displacement, and enough Poly breathing to make a period car feel right when the rest of the package is healthy. The trouble starts when someone treats it like the easiest path to cheap horsepower while later, better-supported Mopar small-blocks are standing nearby with both hands raised.

For restoration, the 313 can be exactly the right answer. In a Canadian Plymouth, Canadian Dodge, Australian Chrysler Royal, Bristol application, or documented export-market car, originality can beat convenience. That’s where the 313 justifies the extra work. In a generic hot rod or budget street build, the math can get cruel fast.

Parts support is better than hopeless, but it isn’t casual. The 3.875-inch bore means piston and ring choices need attention, not assumptions based on 318 parts. Camshaft, lifter, valvetrain, intake, exhaust, gasket, mount, accessory, cooling, and drivetrain pieces need verification before anybody promises a cheap rebuild. Missing induction, exhaust, brackets, and accessories can burn more time than the core is worth.

Interchange needs a firm hand. Some A/Poly parts cross around the family, and some later LA-family history makes people think everything is one happy small-block picnic. It isn’t. LA heads, intakes, exhaust pieces, and accessory layouts need to stay out of the plan unless exact fit has been proven. The same warning applies to 318 Poly parts: useful neighborhood, not automatic permission.

A used 313 core needs suspicion plus a little geography. Check for cracks, stuck cylinders, rusty bores, deck corrosion, freeze damage, broken exhaust flanges, missing manifolds, missing brackets, damaged valvetrain parts, butchered mounts, wrong carburetion, market-specific hardware gaps, and previous machine work. An incomplete 313 isn’t charming. It’s a scavenger hunt with invoices.

Completeness matters more here than the seller wants to admit. A running, complete, correct 313 with the right manifolds, brackets, accessories, and application trail is one kind of engine. A bare block with mystery heads and a coffee can full of hardware is another. Both may be 313s. Only one deserves real money standing next to it.

The 313 is easiest to justify when correctness is the point. Verify what it is, where it came from, what’s missing, and whether originality is worth the extra work. If the car needs a Canadian/export A/Poly or a verified Bristol-related package, save it right. If the goal is cheap torque, easy parts, or simple street performance, don’t turn a market-specific Mopar into a wallet fire just because the valve covers look interesting.

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Common Problems

The 313 A/Poly’s trouble starts because it’s the A/Poly engine American Mopar people are most likely to squint at. It belongs to the same broad family conversation as the 318 poly, but it’s not just a 318 with a rounding error. It’s a market-specific Canadian/export A/Poly, and that means the story can get sloppy fast when somebody treats “wide-block,” “poly,” and “small-block Mopar” like they all answer the same question.

• Canadian and Export Identity Confusion
  The 313 needs its market context kept straight. Canadian Dodge and Plymouth use, Australian Chrysler Royal use, and export applications can all show up in the conversation. That doesn’t make every 313 story the same story.
  
  Application, market, year, casting clues, stampings, hardware, and documentation all need checking before the engine gets priced like some rare mystery prize. Canadian or export identity can matter. It doesn’t replace proof, condition, or completeness.
• 313 / 318 Mix-Ups
  The 313 gets confused with the 318 because the engines are close, related, and easy to blend in casual talk. Same A/Poly neighborhood. Similar outside look. Familiar 318 reputation standing nearby with its boots on.
  
  That still doesn’t make a 313 a 318. Bore size, pistons, rings, gaskets, compression planning, application, and parts ordering need 313-aware checking. “Close enough to a 318” isn’t a rebuild plan. It’s a return label waiting for a box.
• A/Poly Versus LA Small-Block Assumptions
  The 313 is A/Poly hardware, not LA-family hardware. LA 318, 340, and 360 engines may be more familiar, but familiarity doesn’t make the parts fit.
  
  Heads, intake, valve covers, front dress, exhaust, mounts, and internal parts need to be treated as A/Poly business. A seller saying “small-block Mopar” has only started the sentence. For a 313, that lazy label can turn a simple order into a pile of wrong parts and muttering.
• Bristol and Export-Application Story Borrowing
  The 313 has enough export and Bristol connection to make some sellers get fancy. Bristol 407 and 408 use is real, but that doesn’t make every 313 a Bristol engine or turn every loose Canadian core into high-dollar Bristol proof.
  
  A Bristol-related claim needs application proof, correct hardware, market context, dates, vehicle connection, and documentation. Without that, the engine is still a 313 A/Poly core, not Bristol money with valve covers.
• Parts Availability and Project Payoff
  The 313 can be useful and interesting, but it’s not always the easiest engine to build in the United States. The 318 poly has better name recognition. LA engines have broader parts support. The 313 makes the builder check before buying.
  
  Pistons, rings, gaskets, bearings, camshaft choices, intake options, exhaust, mounts, water pump, front dress, and accessory parts all need planning. A correct Canadian-market restoration, Australian Chrysler Royal, Bristol application, or market-specific A/Poly build can justify the work. A casual street project may discover that a 318 poly or LA small-block gets the job done with less international detective work.

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 313 A/Poly gets misunderstood because it’s close enough to the 318 to make people lazy and unusual enough to make people dramatic. That’s a bad combination. The 313 doesn’t need fake 318 identity, fake LA interchange, or borrowed export glamour to matter. It needs to be identified by market, application, hardware, and proof.

• “The 313 is basically a 318.”
  No. The 313 and 318 are related A/Poly engines, but they aren’t the same displacement and not automatically the same parts answer.
  
  The 318 may be easier to explain and easier to shop for. That doesn’t make a 313 a mislabeled 318. Bore, pistons, rings, gaskets, application, and documentation still need to agree before the parts bill starts swinging.
• “The 313 is an LA small-block.”
  No. The 313 is an A/Poly engine, not an LA wedge-head small-block. The later LA engines are the ones most people recognize, but the 313 belongs to the earlier poly-head A-engine world.
  
  That difference matters. Intake, heads, valve covers, exhaust, front dress, mounts, and internal parts can’t be chosen by LA habit. The parts need to fit the engine, not the memory of a later catalog.
• “Canadian or export means valuable by itself.”
  No. Canadian and export identity can make the 313 interesting, but it’s not a blank check. Market origin doesn’t replace condition, completeness, correct application, or proof.
  
  A correct Canadian Plymouth, Canadian Dodge, Australian Chrysler Royal, Bristol, or export-market build may need the 313 badly. A rusty loose core with a vague border-crossing story still has to prove what it is.
• “Every 313 is a Bristol engine.”
  No. Bristol use is part of the 313 story, not the whole story. Most 313 conversations don’t automatically become Bristol conversations just because the displacement matches.
  
  A Bristol-related 313 needs the right vehicle context, hardware, dates, and documentation. Otherwise it’s just a 313 with a borrowed story.
• “All A/Poly parts interchange with the 313.”
  No. Same broad family doesn’t mean every part gets a handshake and a free pass. Year, displacement, market, application, and hardware still matter.
  
  The 313 needs 313-aware checking, especially where bore-related parts, gaskets, induction, exhaust, mounts, and front dress are involved. “It’s all A/Poly” is how the wrong parts show up wearing a grin.

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 313 poly is where the early A-poly story crosses the border and starts making U.S.-centric parts logic look stupid. It sounds like a neat step between the 303 and 318. That’s only partly useful. The 313 is a Canadian/export A-series poly, not a U.S.-mainstream LA 318, not a Magnum, not a Dodge Red Ram Hemi, and not a Chrysler Hemi wearing smaller clothes.

The 313 was used in Canadian and export Chrysler Corporation applications, including Plymouth or Dodge contexts where documented. Many references place the 313 near a 3.875-inch bore with the 3.3125-inch A-poly stroke, but the bore still needs to be physically verified. That makes it close to the 318 poly conversation, but not identical enough for lazy ordering. Exact bore, market, year, application, crank, pistons, heads, intake, and hardware still matter. Border-crossing Mopars don’t care how simple your catalog search wanted to be.

The first upgrade limitation is market identity. A 313 may show up in Canadian or export Chrysler Corporation use depending on exact make, model, year, market, and application. That doesn’t make every 313 the same package. The build has to start with proof: casting numbers, stampings, bore, crank, heads, intake, carburetion, date context, market, vehicle application, and surviving hardware. If the engine’s story matters, the tape measure and paperwork matter more than the badge.

Parts support is better than the 277/301 oddball corner, but it still isn’t LA-small-block easy. The 313 shares useful A-poly family logic with the 318 poly, but pistons, rings, compression planning, camshaft work, valve parts, rocker gear, gaskets, intake choices, exhaust manifolds, water parts, brackets, pulleys, linkage, and early drivetrain hardware still need exact checking. Some 318 poly knowledge helps. It doesn’t get to make decisions without measurements.

The second limitation is purpose. The 313 can be a good period engine, especially where Canadian/export correctness matters. It can be rebuilt into a smooth, useful driver. It can even be warmed up if the parts are healthy and the plan is sane. But it’s not the smart choice for cheap horsepower in a U.S.-market build unless the car specifically needs it. If the goal is easy parts, easy upgrades, and easy explanations, the later LA world is waving from across the parking lot.

Compression and camshaft planning need restraint. The 313 has enough displacement to make a decent old-car engine, but it still doesn’t have the parts support or cheap mistake forgiveness of a later small-block. Too much cam, weak compression, poor carburetion, lazy ignition, tight gearing, stock exhaust, or heavy vehicle weight can make it feel dull. It wants cylinder pressure, clean carburetion, correct timing, healthy exhaust, and realistic gearing.

Cylinder heads and valvetrain need inspection before enthusiasm. The polyspherical chamber, rocker gear, pushrods, lifters, valve covers, oiling details, intake fit, and exhaust fit aren’t LA convenience parts. Worn rocker gear, missing shafts, wrong pushrods, tired springs, bad valve seats, and mismatched top-end pieces can make a simple rebuild expensive. The 313’s family resemblance to the 318 poly helps identify the neighborhood. It doesn’t mean the houses all share keys.

Induction can become a mess if the build starts with looks instead of fit. A correct two-barrel or four-barrel setup, where applicable, can make the 313 smooth and useful. A mismatched intake, wrong carburetor, hacked linkage, bad fuel distribution, or fantasy multi-carb plan can make it worse while costing more. The 313 doesn’t need a circus act. It needs the carburetor, intake, ignition, exhaust, compression, and gearing to agree.

Core condition still decides the bill. Rust, stuck rings, worn cylinders, cracked exhaust areas, corroded cooling passages, broken bolts, tired valve seats, damaged rocker gear, missing brackets, and market-specific hardware gaps can turn a “Canadian 318-ish” engine into a long parts chase. Don’t overbore it casually. Don’t assume 318 pistons or rings fix every cylinder. Don’t assume every machine shop knows the 313’s little differences. Measure first, then spend.

The honest upgrade plan is mild, careful, and identity-driven. Rebuild the 313 when Canadian/export correctness, complete hardware, or early A-poly character matters. Keep the cam sensible, the compression real, the induction simple, the exhaust healthy, and the tune sharp. If the goal is cheap horsepower, the LA 318, 340, 360, or even a better-supported 318 poly may be the smarter path. The 313 is interesting because of what it is, not because it’s the easiest way to go fast.

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When It Actually Makes Sense

The 313 makes sense when the engine belongs in the car and the market identity matters. In a documented Canadian or export Chrysler Corporation application, including Plymouth or Dodge contexts where proven, the 313 can be part of the vehicle’s story. Replacing it with a more common U.S.-market engine may make service easier, but it can also erase the thing that made the car unusual.

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 market-specific hardware matter. A complete 313 package is workable. An incomplete 313 isn’t a bargain; it’s a scavenger hunt with an accent.

It also makes sense as a mild period-correct driver. A healthy 313 can be smooth, torquey enough for normal old-car use, and interesting without needing fake performance drama. It needs compression, clean tune, healthy cooling, reasonable exhaust, and gearing that fits the vehicle.

The 313 makes less sense when the goal is performance per dollar. A later LA 318, 340, 360, or a more common and better-supported small-block foundation will usually be easier, cheaper, and more flexible. A 318 poly may also be a cleaner A-poly choice when originality doesn’t require the 313. That doesn’t make the 313 junk. It means its value comes from correctness, market identity, and character.

It also makes less sense when the engine is incomplete, undocumented, or already mixed with mystery parts. If the top-end hardware is missing, the brackets are gone, the linkage is hacked, the drivetrain pieces are missing, and nobody can prove the exact make, model, year, market, or application it came from, the project can get expensive before it gets interesting.

Plain answer: build the 313 when it belongs, when it’s complete, when Canadian/export correctness matters, and when the goal is smooth period driving. Don’t build it because you think it’s just a slightly smaller 318 with easier problems. It isn’t. The 313 is close enough to fool you and different enough to bill you.

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Compatibility Notes

Family identity.
The 313 is an A-series polyspherical V8 most commonly associated with Canadian and export Chrysler Corporation use. It belongs in the A/Poly conversation, not the LA 318, Magnum small-block, Dodge Hemi, Chrysler Hemi, DeSoto Hemi, or other early Mopar V8 buckets. Exact make, model, year, market, and application decide what actually fits.

Bore and stroke proof.
Verify the actual engine before trusting the story. Many references place the 313 near a 3.875-inch bore with the 3.3125-inch A/Poly stroke, but physical measurement is the boss. Check casting numbers, stampings, bore, crank, heads, intake, carburetion, date context, market, vehicle application, and physical measurements before calling it a 313, 318 Poly, 326 Poly, Dodge Poly, export engine, Canadian engine, or Hemi-related swap.

313 versus 318 Poly.
The 313 is closely related to the 318 Poly, but parts planning still needs measurement. Bore size, pistons, rings, compression, year, market, application, and supporting hardware can differ. A 318 Poly recipe can point in a direction. It doesn’t get to finish the 313 build without proof.

313 versus 303 and 326.
The 313 sits in the broader A/Poly family, but the nearby numbers don’t create automatic interchange. Bore, stroke, crank, pistons, heads, intake, market, and application details can differ. Family relationship helps prevent total confusion. It doesn’t create free parts-swapping rights.

313 versus LA small-block.
The 313 is earlier A/Poly iron, not later LA-family hardware. LA intakes, heads, cams, pushrods, pistons, timing parts, exhaust, brackets, accessories, and transmission-related details need to stay out of the order unless exact fit has been proven. The LA engine is the convenient later world. The 313 brought market-specific attitude and Poly-specific parts logic.

313 versus Hemi and other Poly families.
The 313 A/Poly needs separation from early Dodge, DeSoto, and Chrysler Hemi/Poly families. Chrysler Corporation used several different V8 families in this era, and “Poly” can describe chamber style without proving family interchange. Same-looking ideas don’t guarantee the same block, heads, crank, intake, exhaust, valvetrain, or service parts.

Canadian and export claims.
A 313 claim needs exact vehicle and market proof. Canadian or export Chrysler Corporation use, including Plymouth or Dodge contexts where documented, doesn’t create one simple value category. Verify exact make, model, year, market, documentation, date context, stampings, castings, intake, carburetion, linkage, air cleaner, exhaust, transmission pieces, and vehicle history before paying rare-engine money for somebody’s border story.

Bristol and higher-output claims.
Bristol 407 and early 408 use can make a 313 more interesting, but the badge story needs proof before the price gets fancy. Carburetion, linkage, air cleaner, exhaust, application details, date clues, vehicle history, and documentation all need to agree. A loose 4-barrel 313 doesn’t become Bristol-spec just because the story learned to sip tea.

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 313’s top-end hardware is part of its identity. Missing or mismatched parts can turn a simple rebuild into a swap meet subscription with customs fees.

Exhaust and accessory fit.
Check exhaust manifolds, starter clearance, generator or alternator brackets, pulleys, water pump, front dress, mounts, oil pan, steering clearance, and chassis fit. Canadian and export hardware can be stubborn. Later LA or U.S.-market small-block pieces don’t get to walk in and behave just because the word Mopar is printed on the box.

Transmission, crank, and starter fit.
Verify crank flange, crank register, bellhousing pattern, adapter plates, starter location, flywheel or converter fit, clutch parts, linkage, mounts, driveshaft, and rear gear by exact year, market, and application. Early Mopar drivetrain fit is where “close enough” learns exchange rates.

Restoration-correct versus driver-correct versus performance-correct.
For restoration-correct work, the right 313 and its supporting hardware can define a Canadian or export-market car. For a driver, a mild rebuild with correct induction, clean ignition, healthy cooling, and honest gearing can make it enjoyable. For performance work, be realistic. The 313 is interesting A/Poly iron, not an LA small-block convenience package that accidentally learned geography.

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Reality Check

The 313 A/Poly is interesting because it belongs to the Canadian and export side of the Mopar story, not because it’s automatically rare gold. That market identity matters when the car needs it. It doesn’t magically make every loose 313 special, and it sure doesn’t let the engine pretend to be a common U.S. 318 with a smaller number.

The 318 confusion is where the money can get dumb. The 313 is close to the 318 Poly, but close doesn’t order pistons, rings, gaskets, or compression parts. Bore, stroke, casting numbers, stampings, market, application, and hardware still decide the truth. “Basically a 318” is a sentence the Geezer wants removed from the garage before tools come out.

The Bristol and export-performance angle is real, but it needs proof before it gets expensive. A 4-barrel 313 doesn’t automatically become a Bristol engine. A Canadian/export engine doesn’t automatically become exotic. Application, documentation, induction, accessories, and vehicle history all need to line up before the price gets dressed in a tuxedo.

The 313 makes the most sense when correctness matters: Canadian Dodge, Canadian Plymouth, Australian Chrysler Royal, Bristol, or other documented export applications where the engine belongs. In a generic street build, the later 318 Poly or LA small-blocks usually make more sense. That doesn’t make the 313 junk. It means the 313’s best value is identity, not convenience.

The hard truth is simple: the 313 is a good market-specific A/Poly when the car, country, hardware, and paperwork agree. Without that agreement, it’s just an old Poly core with extra explaining to do. Verify it before buying, measure it before building, and don’t let border stories or 318 familiarity write checks the engine can’t cash.

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Bottom Line

The Mopar 313 A/Poly is the Canadian/export branch of the early A-series Poly story. It isn’t a common U.S. 318, isn’t an LA small-block, isn’t a Hemi cousin, and isn’t rare treasure just because it shows up outside the usual American brochure trail. It’s a real market-specific Mopar V8, and it deserves to be judged by its actual market, hardware, and application.

The 313 makes the most sense when it belongs in the car. Canadian Plymouth, Canadian Dodge, Australian Chrysler Royal, Bristol, and other documented export applications can make the 313 the correct engine rather than an oddball inconvenience. In those cases, preserving it can protect the car’s real identity.

It makes less sense when the project needs easy parts, cheap horsepower, or simple U.S.-market small-block logic. A later 318 Poly may be easier inside the A/Poly world, and LA 318, 340, or 360 engines usually make better performance-per-dollar choices. The 313 can be a good engine, but it isn’t the easiest answer unless correctness is doing the asking.

The mistake to avoid is treating “basically a 318” as a plan. The 313’s 3.875-inch bore, market-specific use, and export history need actual verification. Casting numbers, stampings, bore, stroke, heads, intake, carburetion, front dress, application, market, and paperwork all have to support the story before the parts order or price tag gets ambitious.

Completeness matters heavily. A complete, running, correct 313 with its original-style manifolds, brackets, accessories, linkage, and application trail can be worth saving. A stripped, stuck, undocumented 313 missing market-specific hardware can become a border-crossing parts hunt with machine-shop bills riding shotgun.

Bottom line: the 313 is worth respecting when it’s the correct engine for a Canadian or export Mopar story. Treat it honestly, measure it carefully, and verify the paperwork. Don’t inflate it into Bristol gold without proof, don’t dismiss it as a mistaken 318, and don’t let the phrase “old Poly” do a casting number’s job.

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