Mopar 301 Engine Specs

Intro

➤ See the Family Page for specs common to all engines in this family.

The Mopar 301 Poly is what happens when Plymouth finally gets its own modern V8 and discovers almost immediately that 277 cubic inches are already yesterday’s news. The 277 opened the door in 1956. For 1957 Plymouth opened the bore, kept the short 3.125-inch stroke, and came back with 301 cubic inches because Detroit’s horsepower race was not waiting around while anybody admired last year’s engineering.

That larger bore was the whole basic trick. The 277 used a 3.750-inch bore; the 301 stretched that to roughly 3.910 inches while retaining the same A-block Poly architecture underneath. More cubes, more breathing room, more useful pull, and no need to reinvent an engine Plymouth had barely finished introducing. Sometimes progress really is just finding enough iron around the cylinders and using it.

The 301 still had the job of being a Plymouth engine first. It belonged in full-size cars that needed enough torque to move all that Forward Look sheet metal without making the driver wonder whether the fins were producing more thrust than the crankshaft. Standard two-barrel versions handled ordinary duty, while the Power Pak added a four-barrel and dual exhaust for buyers who wanted the machinery to show a little more interest in the styling.

That Power Pak deserves its place without being promoted into something it wasn’t. Better induction and exhaust made a real difference, but a 301 with a four-barrel is not suddenly a Fury race engine because somebody found an old air cleaner at a swap meet. Plymouth sold hotter versions because buyers wanted hotter versions. It did not secretly build every 301 for Daytona and forget to tell anybody.

The engine’s biggest problem today is living between two more memorable relatives. The 277 gets credit for being first. The 318 became the long-running Poly everybody remembers. The 301 occupied one short model-year step between them, did exactly what Plymouth needed, and then watched the family move on again before anyone had time to become sentimental.

That makes the 301 a correctness engine now. In the right 1957 Plymouth, especially with its original supporting hardware, it tells the story better than a later swap ever will. As a loose engine chosen strictly for horsepower, however, it starts with less displacement, thinner parts support, and more early-Mopar peculiarities than the later 318. There are easier ways to make power than proving how stubborn a machine shop can be.

The 301 does not need borrowed Hemi prestige or 318 reputation to justify itself. It was Plymouth’s first enlargement of the A-series Poly, a sensible response to a market demanding more engine almost before the 277 had finished warming up. Brief does not mean pointless. Sometimes an engine’s entire job is to show where the engineers were headed next.

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Overview

The 301 arrived for 1957 because Plymouth’s first A-block V8 had done its job and the market immediately asked for more. The 277 had finally given Plymouth a modern overhead-valve V8 in 1956, but Detroit was adding displacement and horsepower about as calmly as children adding candles to a bonfire. Plymouth needed to keep up without turning its affordable cars into junior Chryslers with Hemi-sized price tags.

So Chrysler kept the basic A-series Poly architecture and enlarged the cylinders. Bore increased to roughly 3.910 inches while stroke stayed at 3.125 inches, producing 301 cubic inches. Bore spacing remained 4.460 inches, deck height about 9.603 inches, and the basic package continued with cast-iron block and heads, five main bearings, an in-block camshaft, pushrods, shaft-mounted rockers, and a rear distributor.

That shared 3.125-inch stroke is the useful mechanical connection to the 277. These two form the short-stroke end of the A-series family. The 301 simply wraps a larger bore around the same basic crank throw. Later 303, 313, 318, and 326 versions move to roughly a 3.3125-inch stroke, which is where the family starts changing its personality as well as its displacement.

The Poly heads remained the reason Plymouth could have a modern V8 without paying for full Hemi architecture. Polyspherical chambers and canted valves gave the engine useful breathing while keeping the package simpler and cheaper than Chrysler’s hemispherical machinery. That was good engineering for Plymouth’s place in the corporate food chain. The customer got a modern V8. Chrysler did not have to install a FirePower and then apologize at the cashier.

Most 301s were still passenger-car engines with passenger-car obligations. They had to start cold, idle cleanly, pull away without drama, and move a full-size Plymouth carrying passengers, luggage, chrome, fins, and whatever else Virgil Exner had convinced management belonged on the car. The standard two-barrel version handled that job without pretending every grocery run required a timing slip.

The Power Pak sharpened the same engine with four-barrel carburetion and dual exhaust. Better breathing gave the 301 more useful performance and gave Plymouth salesmen something stronger to talk about than upholstery patterns. In 1957 that was worthwhile hardware, and it deserves to be recognized as such without being inflated into a factory competition package sixty years later.

That distinction becomes important because the hardware moves easily. Intakes, carburetors, exhaust systems, linkage, and air cleaners can all be changed during decades of ownership. A four-barrel sitting on a 301 today establishes that somebody wanted four barrels. A genuine Power Pak claim needs the rest of the application, equipment, date clues, and vehicle history to stop arguing.

The 301 also sits dangerously close to the 1957 Fury 318 story. Both use roughly the same 3.910-inch bore, but the 318 gets its additional displacement from the longer approximately 3.3125-inch stroke. That difference is buried inside the engine where a decal cannot fix it. A 301 does not become a 318 because somebody adds dual quads and develops confidence.

That longer-stroke 318 quickly became the family workhorse, which is why the 301 disappeared from normal memory almost as quickly as it appeared in production. The 318 had more displacement, more torque potential, a much longer useful life, and eventually enough production volume that generations of Mopar owners actually knew what one was without consulting a manual.

None of that makes the 301 a mistake. It was exactly the sort of rapid development Detroit practiced in the 1950s. Introduce a new engine, enlarge it next year, change the stroke shortly afterward, and keep going until the marketing department stops demanding another number. The 301 was a stepping stone because Plymouth was moving quickly, not because the stone broke.

Identification deserves care because early A-series engines share enough external architecture to invite bad guesses. A 301 is not identified merely by wide Poly heads, and some family castings and hardware overlap. The engine stamping, year, vehicle application, bore, stroke, heads, induction, and supporting parts need to tell one coherent story before the displacement becomes more than an educated guess.

The engine also has to stay separated from Chrysler Corporation’s other Poly machinery. Dodge and DeSoto used different Poly families tied to different short-block architecture. The later LA small-block is another design development again. “Poly” tells you something about the combustion chamber. It does not hand the parts counterman a complete shopping list.

Compared with the 277, the 301 is simply more of the same useful idea: larger bore, twenty-four additional cubic inches, and a stronger answer for the same general Plymouth mission. If somebody is restoring a correct 1957 car, that progression is historically interesting because it shows how quickly Plymouth was trying to catch up in a horsepower market that had stopped standing still.

Compared with the 318 Poly, the 301 loses the practical argument once originality leaves the room. The 318 brings more displacement, more torque, a longer production history, and better parts familiarity. Starting with loose engines and no requirement for correctness, choosing the 301 because it is rarer can be an excellent way to make the parts search harder without making the car faster.

A complete 301 changes that equation. Original rocker gear, intake, exhaust manifolds, pulleys, brackets, transmission pieces, linkage, and other early hardware can be worth preserving because replacements are exactly the sort of things nobody thinks are scarce until the garage suddenly needs one. Early Mopar projects have produced many expensive lessons that began with somebody throwing an ugly old bracket in the scrap pile.

Condition still outranks nostalgia. Worn cylinders, tired rocker shafts, weak valve gear, plugged oil passages, cracked exhaust hardware, leaking seals, broken fasteners, and previous machine work all deserve inspection. A 1957 engine has had plenty of time to be rebuilt by somebody excellent, somebody adequate, and somebody who owned a hammer.

The best 301 build therefore starts by asking why the engine is there. If it belongs in the car, preserve its identity and build it into the smooth, useful Plymouth V8 it was intended to be. If it has legitimate Power Pak hardware, keep the package together rather than turning scarce correct pieces into swap-meet inventory.

If the project is merely hunting cheap horsepower, let the 301 keep its dignity and find another engine. Its importance is not that it can be forced into modern performance duty with enough money. Its importance is that Plymouth enlarged its first A-series V8 almost immediately, gave buyers more muscle for 1957, and then kept developing the architecture until the 318 finally stayed around long enough for everybody to remember the name.

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General Specs

Displacement	301 cu in / 4.9L	Bore / Stroke	3.910 in × 3.125 in
Production / Use Era	1957 Plymouth 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
Fuel / Induction	Gasoline; carbureted, 2-barrel or 4-barrel Power Pak 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

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Variants

The 301 keeps the variant story simple. Plymouth built the 1957 engine with the same 3.910-inch bore and short 3.125-inch stroke underneath, then offered it in ordinary two-barrel form or as the stronger Power Pak with four-barrel carburetion and dual exhaust. That is the real factory hardware split. Fury dual quads belong to the longer-stroke 318 story, no matter how ambitious the seller gets.

Standard 301 — Two-Barrel

What it was: This was Plymouth’s regular 1957 passenger-car 301, replacing the smaller 277 as the stronger early A-series Poly choice.

What changed: The 301 retained the short 3.125-inch stroke but enlarged the bore to 3.910 inches. Standard engines used polyspherical heads, mechanical lifters, and two-barrel carburetion.

What that did: More bore and displacement gave ordinary Plymouths stronger torque and better acceleration without turning the engine into something it was never intended to be. Plaza, Savoy, Belvedere, and wagon buyers got a more capable V8 rather than a factory race motor hiding behind a quiet air cleaner.

Where used: 1957 Plymouth passenger-car applications.

301 Power Pak — Four-Barrel / Dual Exhaust

What it was: The Power Pak was the genuine factory performance step above the standard two-barrel 301.

What changed: Plymouth added four-barrel carburetion and dual exhaust while retaining the same basic 301 short-stroke A-series architecture.

What that did: Better induction and exhaust let the engine breathe harder and pull more strongly when the throttle opened. In 1957, that was meaningful performance equipment. It gave the machinery enough extra bite to keep up with all that Forward Look styling without pretending every Plymouth had suddenly joined the competition department.

Where used: 1957 Plymouth passenger cars ordered with the Power Pak equipment.

Identification caution: A four-barrel intake and dual exhaust can be added to an ordinary 301. Correct intake, carburetor, exhaust layout, engine stamping, vehicle application, option history, tags, and documentation need to support a genuine factory Power Pak claim.

The big trap is the 1957 Fury 318. The 301 and 318 share roughly the same bore, but the 301 uses the 3.125-inch stroke while the 318 uses the longer 3.3125-inch stroke. A 301 Power Pak is a perfectly good performance version of the 301. It is not the Fury dual-quad 318, and no amount of carburetor polishing adds seventeen cubic inches to the crankshaft.

So the useful 301 ladder is short and honest: standard two-barrel and Power Pak four-barrel with dual exhaust. The engine only lasted briefly, but rarity does not need help from borrowed Fury history. Measure the stroke, identify the hardware, and let the 301 be the engine Plymouth actually built.

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Key Notes

The Mopar 301 A-series Poly is one of those engines people mention for about eight seconds before jumping to the 318, which is exactly why it needs its own warning label. This isn’t an LA 318, isn’t a later parts-friendly small-block, and isn’t a 277 with a little more displacement and a better story. It’s an early Plymouth A-series polyspherical-head V8, and it belongs to the short-run early stretch of Chrysler’s first Plymouth small-block V8 learning curve.

The basic fingerprint is 3.910-inch bore and 3.125-inch stroke in the early A-series Poly architecture. That makes it a larger-bore, short-stroke step up from the 277 before the later longer-stroke A engines made the family more familiar. It sits in that early Plymouth zone where displacement, model year, and application all matter. Calling it “a Poly” still isn’t enough to know what you’re buying.

The 301’s short-stroke layout gives it a different personality from the later 318 Poly. It can be smooth and willing when built correctly, but it doesn’t have the same torque, parts support, or recognition as the 318. That matters because people love borrowing the later engine’s reputation when they’re trying to sell the earlier one. A 301 is interesting. It isn’t automatically the Poly everyone actually wants.

The A-series Poly chamber is the reason this engine exists. Chrysler wanted a cheaper, simpler alternative to Hemi architecture while keeping better breathing potential than a basic wedge-style chamber. That gave Plymouth a modern V8 without dragging full Hemi cost into every engine bay. Good idea, but don’t confuse clever engineering with easy ownership. A 301 still brings early-design parts hunting and limited interchange.

Don’t confuse the 301 A/Poly with the later LA small-block. The LA engines became the familiar Mopar small-block path, with better parts support and easier build planning. The 301 A/Poly is earlier, heavier, and different where the wrench actually lands: heads, intake, valve covers, timing/front dress, mounts, exhaust, pistons, brackets, and accessories. Some family resemblance doesn’t make the parts bin suddenly friendly.

The 301 also needs separation from the 277 and 318 Poly. The 277 is the smaller first Plymouth version. The 318 Poly is the better-known later A-series engine with broader usefulness and stronger recognition. The 301 sits between them as a short-run displacement that can matter in the right car, but it doesn’t have the broad appeal of the 318 or the origin-story clarity of the 277. It needs exact identification, not family guesswork.

The 301 also needs separation from the 303. The numbers sit too close together, and the early Plymouth Poly world already has enough fog without adding lazy rounding. The 301 and 303 don’t become the same thing because both sound like late-1950s Plymouth trivia. Bore, stroke, year, application, and actual hardware still have to settle the argument.

Application context carries most of the value. The 301 is mainly a 1957 Plymouth engine, and its value depends heavily on whether it belongs in the car, matches the year, and still has the correct pieces. In a proper restoration or period-correct build, it can be worth saving. Loose on a stand with missing accessories and a seller saying “old Mopar V8,” it becomes a small-displacement parts hunt with machine-shop bills attached.

Factory performance claims need proof. Power Pak equipment can make a 301 more interesting, but a 301 doesn’t become special just because somebody remembers Fury trim, a 4-barrel, dual exhaust, or a gold air cleaner. The intake, carburetor, heads, exhaust, engine numbers, date codes, application, and documentation need to support the claim. Old Plymouth glamour isn’t a substitute for hardware.

The Fury 318 story is the big value trap. A Power Pak 301 can be correct and desirable in the right car, but it isn’t the dual-quad Fury 318 and shouldn’t be priced like one. The 318’s longer stroke, higher-profile Fury connection, and stronger performance identity belong to that engine. The 301 doesn’t get to borrow the trophy because it parked nearby.

Parts availability is where the romance gets expensive. Pistons, gaskets, bearings, rings, valvetrain pieces, intake hardware, water pumps, timing components, exhaust, brackets, and correct dress hardware can take real hunting. Some pieces may cross within the A-family world, some won’t, and some will make you learn how small the old-Mopar parts market really is. Missing parts on a 301 are not minor details. They’re the beginning of the bill.

A used 301 needs inspection before the nostalgia starts singing. Check casting numbers, bore, stroke, heads, intake, carburetion, coolant passages, oiling condition, crank, pistons, accessories, rocker gear, and previous machine work. These engines are old enough to have been rebuilt badly, parked wet, stuck, swapped, stripped, or misidentified by three owners and a cousin with a trailer.

Build judgment needs a cold head. A 301 can be a good choice when it preserves the car, fits a correct Plymouth project, or supports a period build where the early Poly character is the point. It’s a bad choice for cheap horsepower, easy aftermarket support, or modern convenience. A later 318 Poly, LA small-block, or more common Mopar V8 will usually be easier to build and easier to keep alive.

The 301 can still be satisfying if expectations stay honest. It’s not a stoplight bruiser, and it’s not a throwaway if it belongs in the right Plymouth. Build it as a clean, correct, early Mopar V8 with sensible compression, proper carburetion, good ignition, sound cooling, and realistic gearing. Try to turn it into cheap speed, and it’ll punish you with parts scarcity before it ever scares anybody on the street.

The Mopar 301 A-series Poly deserves respect as a short-lived early Plymouth V8 and a step between the little 277 and the better-known 318 Poly. It doesn’t need LA assumptions, fake Fury magic, or “rare engine” pricing to matter. Verify the A-series Poly identity, displacement, year, application, parts completeness, Power Pak claim, and rebuild condition before spending money. A 301 can belong in the right old Plymouth, but it’s a poor place to confuse uncommon with valuable.

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Bottom Line

The Mopar 301 A/Poly is the growth step in Plymouth’s early V8 story. The 277 got the program started, the 301 added enough displacement and showroom muscle to move the idea forward, and the later 318 became the version everybody remembers. That puts the 301 in an important middle spot without requiring anybody to inflate it into something more glamorous.

Its strongest argument is correctness. A standard two-barrel 301 belongs in the right 1957 Plymouth just as honestly as a Power Pak version belongs in a car that actually carried the stronger factory hardware. The four-barrel and dual-exhaust setup can add interest and value when the parts and documentation support it. It still does not become a Fury 318 just because the seller found a better adjective.

For a general performance build, the 301 has a much weaker case. It is early, short-run, and less supported than later Mopar V8s, so a 318 Poly or later small-block usually makes more sense when easy horsepower and parts availability are the real goals. The 301 is worth rebuilding because the car deserves its original engine, not because rare automatically means fast, valuable, or easy to live with.

Completeness matters heavily. A correct 301 with its rocker gear, brackets, intake pieces, accessories, and application still together can justify the effort. A stripped mystery core can become expensive before the machine work even starts. Old Poly parts do not materialize because somebody called the engine rare, and rarity without demand does not volunteer to pay the invoice.

So preserve a good 301 when the car and history call for it, verify Power Pak claims before paying extra, and let the engine keep its own identity. It does not need 318 Fury mythology, LA convenience, or Hemi perfume to matter. The 301 was Plymouth’s short-lived step toward a better V8. That is a perfectly respectable job without asking it to carry somebody else’s legend.

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