Mopar 426 Hemi Engine Specs

Intro

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

The Chrysler 426 Hemi is what happens when an engine company gets tired of arguing about horsepower and decides to settle the matter on the racetrack. Chrysler did not build this thing because the family sedan needed a little more passing power. The first 426 Hemi arrived in 1964 as a competition engine aimed straight at NASCAR, with drag racing waiting close behind. Street manners were somebody else’s problem. Winning was the assignment.

That origin separates the 426 from almost every Hemi story that came before it. The 331, 354, and 392 FirePower engines had already proved Chrysler knew how to build a hemispherical chamber. The 426 belonged to another generation entirely: 4.250-inch bore, 3.750-inch stroke, later big-block proportions, massive heads, dual rocker shafts, cross-bolted bottom-end strength, and enough breathing ability to make the conventional 426 wedge look like it had shown up to the wrong fight.

The nickname “Elephant” was not advertising department poetry. A complete iron 426 Hemi is roughly three-quarters of a ton’s worth of engine divided by two, and most of the extra width seems to live exactly where steering boxes, brake boosters, headers, and mechanics’ knuckles would prefer it did not. It is big, heavy, complicated, and inconvenient. Nobody has spent sixty years caring because it was convenient.

The Race Hemi came first, and Chrysler did not waste much engineering time making it pleasant. High compression, aggressive cam timing, race induction, free exhaust, and full-throttle priorities suited NASCAR and drag racing just fine. Anybody worrying about cold starts, smooth idle, vacuum accessories, or how well it behaved while creeping through traffic had misunderstood the product before opening the hood.

The Street Hemi appeared for 1966 because Chrysler needed production buyers to have access to the engine as well as racing teams. Compression came down, street calibration and production exhaust appeared, and the whole package gained just enough manners to tolerate license plates. Chrysler rated it at 425 horsepower. Calling that the tame version gives a pretty good indication of where the conversation had started.

That engine went into some of the most desirable Mopars ever built: Chargers, Road Runners, GTXs, Challengers, ‘Cudas, and other cars where checking the Hemi box could change the entire identity of the vehicle. It was expensive when new, unnecessary for ordinary transportation, thirsty, heavy, and harder to live with than lesser engines. Naturally, half a century later those are the cars everybody wishes somebody had ordered.

The problem now is that “426 Hemi” can describe very different things before the seller gets around to the details. A genuine Race Hemi, production Street Hemi, Super Stock engine, service block, later Mopar Performance assembly, aftermarket build, restamped engine, or Hemi sitting in a clone car may all contain legitimate Hemi hardware. They do not carry the same history, originality, or value. Expensive nouns require expensive amounts of proof.

For somebody simply wanting horsepower, the 426 Hemi is often an impressively complicated way to get it. A 440 or well-built wedge can make a brutally fast street car with less width, less specialty hardware, and considerably less financial theater. The Hemi becomes the right answer when being a Hemi is part of the assignment: a correct restoration, genuine race application, serious tribute, or build where the Elephant itself is supposed to be the centerpiece.

The 426 does not need mythology added to it. Chrysler built it to win, racers proved it could, and the Street Hemi brought enough of that machinery onto public roads to permanently change what the word Hemi meant. Verify the block, verify the hardware, verify the car, and let the engine keep the reputation it already earned. There is no reason to pay extra for somebody else’s imaginary Richard Petty story.

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Overview

The 426 Hemi began in 1964 because Chrysler wanted a racing engine, not because the product planners discovered a troublesome gap between “very powerful” and “ridiculous.” NASCAR was the first major target, and Chrysler approached the problem with the useful assumption that airflow and bottom-end strength were cheaper than excuses after the race.

The resulting engine used a 4.250-inch bore and 3.750-inch stroke for 426 cubic inches, with 4.800-inch bore spacing and roughly a 10.720-inch deck height. Those dimensions put it in Chrysler’s later raised-deck big-block neighborhood, but that does not make the Hemi a 426 wedge with expensive cylinder heads screwed on. Heads, pistons, valvetrain, induction, exhaust, and much of the build logic belong specifically to the Hemi.

The block was built as though Chrysler expected people to treat it badly, which turned out to be excellent forecasting. A deep-skirt structure and cross-bolted main caps supported the crankshaft against the cylinder pressure and rpm competition would bring. The engine used five main bearings, 2.750-inch main journals, 2.375-inch rod journals, and 6.860-inch connecting rods. Nobody accidentally wandered into this much bottom-end hardware while designing an engine for errands.

The heads are where subtlety left the building. Hemispherical chambers put the valves opposite one another, central plug tubes drop the spark plugs through the middle of the valve covers, and dual rocker shafts operate all that hardware. The arrangement gave Chrysler enormous airflow potential, especially with the large 4.250-inch bore providing room around the valves. It also made the engine spectacularly wide. Horsepower has always had a habit of using whatever engine-bay space nobody defended.

The 1964 Race Hemi was the first finished answer. High compression, aggressive camshafts, race induction, race exhaust, and competition calibration gave the engine exactly the personality expected from something that was supposed to spend most of its useful life at large throttle openings. Chrysler was not trying to produce the world’s nicest idle. It already had other engines for people who cared about that.

NASCAR supplied the public spectacle, but drag racing quickly became just as important to the Hemi story. Factory-supported Super Stock programs and specialized drag packages showed what the architecture could do when weight, gearing, induction, and chassis were all aimed at the same quarter-mile finish line. Comfort items were not eliminated accidentally. They simply had poor elapsed times.

The 1965 A-990 Super Stock combinations belong in that competition world rather than being lumped casually with later Street Hemis. Lightweight vehicle hardware, dedicated drag-strip intent, specialized drivetrain choices, and race-oriented engines made those cars something very different from a production Hemi car ordered by somebody who still expected to drive home from dinner.

For 1966 Chrysler created the Street Hemi by making the racing package tolerable enough for production use. Compression was reduced, cam timing and calibration became friendlier, production exhaust manifolds appeared, and the engine gained enough low-speed manners to function in traffic without pretending traffic was qualifying. The important word there is “enough.”

The Street Hemi kept the same basic 4.250-by-3.750-inch architecture and used dual four-barrel carburetors on its familiar production intake. Chrysler rated it at 425 horsepower. The engine remained expensive, physically enormous, and entirely unnecessary for most buyers, which explains why ordinary people bought far more 383s and 440s and why collectors now wish those ordinary people had shown less judgment.

Putting the Hemi into production B-bodies and later E-bodies created the part of the story collectors know best. Chargers, Road Runners, GTXs, Challengers, ‘Cudas, and other factory Hemi cars combined the engine with vehicles already designed around serious performance. In those cars the 426 was not merely a larger engine option. It became a major piece of the vehicle’s identity, which is why provenance now changes values by amounts large enough to make grown adults inspect stampings while kneeling in gravel.

That collector value makes one distinction absolutely necessary: a genuine Hemi engine is not automatically the original engine from a genuine Hemi car. Service blocks existed. Warranty replacements existed. Race engines changed parts constantly. Later Mopar Performance blocks exist. Modern aftermarket Hemi blocks exist. All can produce a real 426-style Hemi engine while telling completely different historical stories.

The same goes for a Hemi-swapped car. A properly converted 426 car can be an outstanding machine. It can look right, run right, and embarrass plenty of factory originals whose owners are afraid to use the second carburetor. It still does not become a born-with Hemi car merely because every bolt was tightened correctly. The vehicle’s VIN and documentation get the deciding vote there.

Race engines create an even bigger authenticity problem because racers behave exactly the way racers should. Broken parts get replaced. Better parts go on. Heads move between engines. Blocks get repaired, sleeved, decked, bored, or retired. Intakes change as the combination improves. Fifty years later, somebody wanting every component to remain exactly as it left Chrysler is imposing museum rules on machinery built by people trying to win Sunday afternoon.

That does not mean race history gets a free pass. “Old race Hemi” can mean a documented competition survivor or a damaged block somebody remembers seeing near a drag strip. Cracks, sleeves, main-web repairs, overbores, mismatched heads, altered oiling, and old machine work need inspection before historical enthusiasm becomes machine-shop income.

Street engines deserve the same mechanical suspicion for different reasons. A 426 Hemi is now old enough to have been rebuilt several times, often during periods when original pieces were simply used parts rather than collector artifacts. Pistons, rods, crankshafts, heads, rockers, camshafts, intakes, and carburetors may all have changed. Every component can be genuine Hemi hardware and the finished combination can still be a mechanical family argument.

The valvetrain is one place where that parts-mixing gets expensive quickly. Two rocker shafts per head mean plenty of rockers, stands, pushrods, valves, springs, retainers, guides, and geometry that all need to agree with the camshaft and piston. Worn shafts or mismatched components do not become harmless because the valve covers are worth more than somebody’s first car.

Street camshaft selection deserves restraint as well. Hemi heads can move plenty of air, which has encouraged generations of builders to install enough camshaft to make parking-lot conversations exciting. A heavy street Mopar with weak vacuum, soft low-speed response, fouled plugs, inadequate converter, and miserable traffic manners is not proof the Hemi is temperamental. It is proof somebody ordered parts according to applause.

Cooling deserves similar respect. The engine is large, makes considerable heat, and often lives in an engine bay already crowded by wide heads and exhaust hardware. Radiator capacity, water pump, pulley ratios, fan arrangement, ignition timing, mixture, airflow, and header heat all contribute. The Elephant does not overheat because it resents modern traffic. It overheats when the cooling system loses the argument.

Packaging is where Hemi enthusiasm finally meets the actual car. Headers or manifolds have to clear steering, torsion bars, starter, suspension, and whatever brake equipment happens to occupy the same neighborhood. Spark-plug access can become an exercise in vocabulary. Hood clearance, mounts, oil pan, K-member, cooling, transmission, driveshaft, and rear axle all need to support the engine instead of merely leaving enough room to photograph it.

Compared with a 440, the Hemi is difficult to defend using ordinary street arithmetic. A good 440 supplies tremendous torque, fits more easily, costs less, and asks for fewer specialized pieces. If both engines are sitting beside an empty engine bay and the only objective is a fast street car, the 440 deserves a long look before nostalgia starts signing checks.

The 426 earns the extra aggravation when Hemi identity itself belongs in the project. A real Hemi car needs the correct engine family. A documented race restoration needs the right architecture. A high-end tribute may be built specifically because somebody wants an Elephant under the hood. In those cases a wedge that produces the same dyno number has answered a different question.

That is why the 426 Hemi remains such an unusual engine. Chrysler designed it as a race weapon, then made it just civilized enough to sell to the public without removing the machinery that made it special. The Race Hemi proved the architecture. The Street Hemi made the legend available with license plates. Sixty years of racers, restorers, collectors, and creative sellers have complicated everything afterward.

Treat the engine accordingly. Decide first whether it is supposed to be a race engine, street engine, restoration piece, service replacement, or honest tribute. Then match the compression, camshaft, heads, valvetrain, induction, oiling, exhaust, cooling, and drivetrain to that job. The 426 Hemi is perfectly capable of being magnificent. It is also perfectly capable of becoming an extremely expensive monument to somebody who knew the word “Hemi” but never got around to learning the engine.

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General Specs

Displacement	426 cu in / 7.0L	Bore / Stroke	4.250 in × 3.750 in
Production / Use Era	1964–1971 race and Street Hemi performance era	Bore Spacing	4.800 in
Engine Family	Chrysler Gen II 426 Hemi V8 / RB-based big-block architecture	Deck Height	10.720 in
Block Material	Cast iron; aluminum on race/aftermarket versions	Rod Length	6.860 in
Cylinder Head Material	Cast iron; aluminum on race/aftermarket versions	Main Journal	2.750 in
Fuel / Induction	Gasoline; carbureted, dual 4-barrel common on Street Hemi	Rod Journal	2.375 in
Cam Location	In-block camshaft	Main Bearings	5
Valve Layout	OHV / pushrod, 2 valves per cylinder; hemispherical chambers with dual rocker shafts	Firing Order	1-8-4-3-6-5-7-2
Common Main Caps	4-bolt / cross-bolted mains	Distributor Rotation	Counterclockwise
Rear Main Seal	2-piece	Weight	~765–800 lb complete iron Hemi, depending on accessories/configuration
Balance	Internal balance	Dimensions	~32H / ~32W / ~34L

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Variants

The 426 Hemi needs real boundaries because Chrysler built it for racing first, civilized it for the street later, and then spent sixty years watching people blur the two whenever the asking price needed help. The useful factory split is Race Hemi, A-990 / Super Stock drag hardware, and the 1966–1971 Street Hemi. Service blocks, crate engines, and modern replacements matter for identification, but they are replacement or later-production hardware, not another original factory-performance variant.

1964–1965 Race Hemi

What it was: This was the original 426 Hemi and the reason the engine exists at all. Chrysler built it as a competition engine for NASCAR and drag racing rather than as a production-car option that happened to be fast.

What changed: The Race Hemi used the 4.250-inch bore and 3.750-inch stroke with Hemi-specific heads, cross-bolted bottom-end strength, high compression, aggressive cam timing, race induction, free-flowing exhaust, and competition calibration. Street manners were not missing by accident. Nobody ordered them.

What that did: The engine was designed to breathe hard and live at large throttle openings. High-rpm airflow, cylinder pressure, and durability under race conditions mattered considerably more than idle quality, vacuum, cold starts, or whether somebody could drive it through downtown without fouling plugs.

Where used: Factory-supported NASCAR, drag-racing, and other competition applications beginning in 1964.

Identification caution: A later Street Hemi is not automatically a Race Hemi, and a race-style build is not automatically factory race hardware. Block, heads, induction, compression, camshaft, exhaust, stampings, dates, application, and documented race history all need to support the claim. Race parts also lived race lives, so repairs, sleeves, changed heads, and replacement components do not automatically disprove history — but they absolutely change what is being bought.

1965 A-990 / Super Stock 426 Hemi

What it was: The A-990 package belongs in its own lane because Chrysler built the entire car around quarter-mile work. This was not simply a Race Hemi dropped into an ordinary production body with the radio removed afterward.

What changed: The Hemi remained competition-oriented, but the package combined it with lightweight body and interior equipment, drag-specific drivetrain choices, gearing, suspension, and other hardware selected around Super Stock use.

What that did: Chrysler reduced weight and matched the whole vehicle to the engine instead of asking several thousand pounds of ordinary street equipment to cooperate. That is how factory drag packages work: the engine, transmission, axle, chassis, and body all receive the memo.

Where used: 1965 A-990 and related factory Super Stock drag applications.

Identification caution: A-990 and Super Stock claims require more than a Hemi and lightweight-looking parts. VIN, body details, drivetrain, transmission, rear axle, engine configuration, factory documentation, and race history all need to line up. Race cars changed parts because winning mattered more than keeping an auction catalog happy fifty years later.

1966–1971 Street Hemi

What it was: Chrysler took the race-bred 426 and made it civilized enough to sell through dealerships. “Civilized” is doing considerable work in that sentence.

What changed: The Street Hemi retained the 4.250-inch bore, 3.750-inch stroke, massive Hemi heads, dual rocker shafts, cross-bolted strength, and dual four-barrel induction, but compression came down, cam timing became more streetable, production exhaust manifolds replaced race hardware, and ignition and carburetor calibration were revised for ordinary gasoline, traffic, warranties, and customers who expected the engine to start on Monday.

What that did: Chrysler rated it at 425 gross horsepower. It remained heavy, thirsty, expensive, and considerably less polite than the 383 or 440, but it could survive public roads without requiring every stoplight to become a staging lane.

Where used: 1966–1971 factory Hemi B-body and E-body applications including Charger, Road Runner, GTX, Challenger, ’Cuda, and related production performance cars.

Identification caution: A genuine Street Hemi engine and a born-with Street Hemi car are two different claims. Block casting, assembly stamping, heads, intake, carburetors, exhaust manifolds, date codes, VIN, fender tag, broadcast sheet, transmission, rear axle, and vehicle history all need to agree before a converted car starts charging original-Hemi money.

Service blocks, warranty replacements, over-the-counter race parts, later Mopar Performance assemblies, aftermarket blocks, crate Hemis, and tribute builds belong in the identification discussion rather than the original factory-variant ladder. They can be excellent engines and perfectly honest cars. They simply tell a different historical story.

So the factory 426 map stays clean: Race Hemi, A-990 / Super Stock competition hardware, and Street Hemi. Chrysler built the first one to win races, built the second around a drag-strip mission, and made the third just tame enough to carry license plates. Everything afterward needs to identify exactly which pieces of that history are genuine before the Elephant starts trampling the bank account.

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Key Notes

The Chrysler 426 Hemi is the second-generation race-bred Hemi, not a first-generation FirePower and not a modern Gen III. It isn’t a 331, 354, 392, 426 wedge, Max Wedge, or crate-Hemi identity just because somebody likes the badge. It’s the Elephant, and that means proof matters before the price starts acting holy.

The basic fingerprint is 4.250-inch bore and 3.750-inch stroke. The engine uses massive Hemi heads, dual rocker shafts, cross-bolted strength, RB-based big-block roots, and enough physical width to make engine bays negotiate under protest. Same displacement as a 426 wedge doesn’t make the engines interchangeable.

The first split is Race Hemi versus Street Hemi. Chrysler built the 426 Hemi for racing first, then adapted it for production use beginning in 1966. Race Hemi, Street Hemi, A-990 / Super Stock, service block, crate engine, replacement assembly, aftermarket block, restamp, tribute build, and clone car all need separate identification.

Street Hemi claims need hard proof. The right block, heads, intake, carburetors, exhaust manifolds, distributor, oil pan, date codes, assembly stampings, VIN or body context, transmission pairing, rear axle, fender tag, broadcast sheet, and documentation decide whether the claim holds. Orange paint and dual carbs aren’t evidence.

Race Hemi claims need even colder inspection. NASCAR, drag racing, A-990, Super Stock, altered-wheelbase, and old competition history can add value, but race parts lived hard lives. Blocks were repaired, heads were swapped, intakes changed, sleeves installed, stampings altered, and paperwork lost. Race history without documentation is just a loud story with expensive damage attached.

The 426 Hemi gets cloned harder than almost any Mopar engine. Wedge cars become Hemi cars. Service engines become original engines. Crate Hemis become “real” Hemis. Reproduction parts become “factory” parts. A clone can be a great car, but it needs clone money, not born-with factory Hemi money.

B/RB family roots don’t make wedge parts a Hemi plan. Heads, pistons, valvetrain, intake, exhaust, accessories, cooling pieces, oiling details, and engine-bay fitment are different enough to punish lazy ordering. “Big-block Mopar” gets the search started. It doesn’t finish the parts order.

Packaging is part of the bill. The 426 Hemi is wide, heavy, hot, and rude to tight engine bays. Steering clearance, brake clearance, torsion bars, headers, starter heat, plug access, hood clearance, cooling, mounts, transmission fit, and service access all need planning before the engine starts arguing with the car.

The valvetrain deserves respect. Rockers, shafts, stands, pushrods, lifters, springs, retainers, cam choice, piston-to-valve clearance, oiling, and geometry all have to work together. This isn’t the place to save money with guesses and then call the mess “old-school.”

A used 426 Hemi needs an identity and condition audit before anybody gets excited. Check block casting, stampings, bore size, main caps, heads, rocker gear, intake, carburetors, exhaust pieces, crank, rods, pistons, oil pan, timing cover, date codes, repairs, sleeves, cracks, and previous machine work. Missing Hemi-specific parts can turn a good deal into a box-by-box beating.

Parts availability is better than it used to be, but the prices still live in Hemi country. Blocks, heads, valvetrain, pistons, intakes, exhaust, ignition pieces, restoration hardware, and correct dress parts can be bought or built. None of that makes the 426 cheap. It only means the pain has part numbers.

Build judgment depends on purpose. A 426 Hemi makes sense in a real Street Hemi car, documented race package, A-990 / Super Stock context, serious restoration, high-end tribute, or project built around Elephant identity. It makes less sense when the goal is cheap horsepower, easy packaging, low-stress ownership, or casual bragging rights.

The 426 Hemi deserves its reputation because the iron earned it. It doesn’t need fake race history, clone fog, restamp magic, wedge confusion, or auction-block vocabulary to be important. Verify the generation, block, heads, stampings, dates, application, documentation, and build quality before spending money. The Elephant is real, but it still leaves tracks.

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Bottom Line

The Chrysler 426 Hemi is the Elephant for a reason. It came out of racing, proved itself in NASCAR and drag racing, moved onto the street without becoming ordinary, and turned the Hemi name into one of the biggest pieces of American performance mythology. The reputation is earned. That does not mean every orange engine with wide valve covers deserves to cash the same check.

The 426 makes the most sense when the Hemi identity is part of the reason for the project. A real Street Hemi car, documented race package, A-990 or Super Stock application, serious restoration, high-end tribute, or purpose-built Hemi project can justify the size, cost, and aggravation. In those jobs the engine is not merely a way to make horsepower. It is part of what the whole car is supposed to be.

That is also why provenance matters so much. Born-with Street Hemi, Race Hemi, service replacement, crate engine, aftermarket block, restamp, and tribute-car powerplant are all different conversations. Race pieces may have been repaired, modified, or rebuilt repeatedly because winning the next round mattered more than pleasing an auction catalog fifty years later. None of that automatically makes an engine bad. It does mean the story has to match the metal before the price starts acting holy.

For ordinary street horsepower, the 426 can be a remarkably expensive answer to a question other Mopar big blocks solve more easily. It is wide, heavy, specialized, costly to rebuild, and hungry for Hemi-specific pieces that rarely arrive in the bargain bin. Cooling, exhaust clearance, driveline strength, gearing, service access, and general street manners all need to be part of the plan. The Elephant will fit into plenty of garages. That does not mean it fits every budget or every mission.

So save the real ones, document the correct ones, enjoy the honest clones, and make every valuable claim prove itself before the wallet joins the conversation. The 426 Hemi does not need fake race history, restamp magic, wedge confusion, or auction-room adjectives to matter. It already earned the legend. The engine sitting in front of you still has to earn the money.

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