I’ve spent years studying, admiring and researching big block Mopar specs, especially the 426 Hemi. The name Hemi transcends car brands making many people wonder about its specifications. Therefore, let’s answer, what are the 426 Hemi specs?
The 426 Hemi has 426 cubic inches, a 4.25″ bore and 3.75″ stroke, 425 horsepower @ 5,000 RPM and 490 ft. lbs. of torque @ 4,000 RPM. It has a compression ratio of 10.25:1, 2X4 bbl carburetors, 2.25″ intake valves and 1.94″ exhaust valves. The Hemi’s cam has a lift of .484 intake and .475 exhaust.
This article will dive deeper into the specs and include more details about the famous heads and the valve sizes. In addition, the camshafts, intake and exhaust manifolds, the type of carbs, crankshaft, valve covers, pistons, casting numbers and torque settings.
The following Hemi specifications and facts were learned from my personal experience and research, Mopar brochures and engine manuals, webinars, videos, books and articles.
426 Hemi Specifications
The following table includes all the specifications for the 426 Street Hemi.
|426 Hemi Specs
|426 (actual 425.6)
|Cylinder head material
|Inline 2X4 bbl (Carter)
|425 HP @ 5,000 RPM
|490 Ft. Lbs. @ 4,000 RPM
|1966-1967: .467″ Intake / .473″ Exhaust
1968-1969: .490 intake / .481 exhaust
1970-1971: .490″ Intake / .481″ Exhaust
170 cc combustion chamber
|Cylinder head valves
|2.25″ Intake / 1.94″ Exhaust
Cylinder numbering (front to rear)
Left Bank: 1-3-5-7
Right bank: 2-4-6-8
|1966-1969: 5 qt oil pan plus 1 qt for oil filter
1970-1971: 6 qt oil pan plus 1 qt for oil filter
|3/8″ fuel line
|Engine weight with intake and water pump
Why not start with what makes the engine legendary, the cylinder heads. Chrysler began using Hemi heads in the 1940s for military tanks and aircraft.
The V-12 AV-1790-5B engine was used to power M47 Patton tanks and the XIV-2220 to power the P-47 Republic Thunderbolt fighter aircraft.
The XIV-2220 surpassed 500 mph during test flights and was rated at 2,500 hp. Little did those fighter pilots know the engines powering their planes were the beginning of an automative legend.
The first Hemi head engines went into Chrysler cars for the first time in the 1950s and are known as the first generation Hemi. They include the following:
- Chrysler FirePower (331 cu., 354 cu., 392 cu.)
- DeSoto FireDome and FireFlite (276 cu., 291 cu., 330 cu.,341 cu.)
- Dodge RedRam (241 cu.,270 cu., 315 cu., 325 cu.)
These early engines were the beginning of history and were used until 1959 in their trucks. The Hemi returned to Nascar in 1964 and the 426 street Hemi began in 1966.
What makes the 426 Hemi heads so special?
The hemi head has a hemispherical shaped combustion chamber. This allows for optimal combustion chamber pressure, larger valves and increased intake and exhaust air flow. The center positioned spark plugs provide better ignition spark.
The casting number for 426 Hemi heads from 1966 to 1971 is:
The head casting number can be found on both sides of the head.
The combustion chamber shape is like half of a sphere. It’s like cutting a softball in half and hollowing it out. The large combustion chamber and width of the head are able to fit larger valves.
The combustion chambers produce a faster rise in combustion chamber pressure compared to a typical wedge shaped head.
Since the spark plug tip comes down in the center of the chamber between the two valves, the reflected chamber pressure results in a rapid pressure peak.
The intake and exhaust valves are opposite each other, perpendicular to the length of the engine, instead of next to each other like in a wedge head. The Hemi intake valve measures 2.25″ and the exhaust valve 1.94″ in diameter.
The larger sized valves means more air and fuel can get into the combustion chamber and more of the exhaust can leave.
The flow of intake and exhaust air are more direct and with less bends allowing for better air flow.
The Hemi’s intake and exhaust valves are tipped sideways, so they open away from the cylinder wall and also avoid hitting each other.
The upper row of cylinder head bolts (closest to the middle of the engine) are actually 4 studs that are threaded into the head from underneath. When the heads are placed onto the block, the threaded studs fit through bosses in the engine valley and tightened from underneath with nuts down near the lifter bores (See video below)
I set this video to start at the exact time he unbolts the Hemi head and the studs.
Since the chamber is dome shaped, the top of the piston is also domed to provide better combustion. The top of the piston is notched on both sides to avoid the intake and exhaust valves from hitting the top of the piston.
The pistons are made of forged aluminum and have three rings. The top two rings are compression and the bottom one is an oil ring.
Learn more details about the pistons in my article, 426 Hemi Wrist Pin Size and Pistons.
The valvetrain consists of the tappets (lifters), rocker arms, rocker shafts, pushrods, valves and valve springs.
The size of the 426 Hemi valves are:
- 2.25″ intake
- 1.94″ exhaust
The rocker arms pivot on rocker shafts, one shaft for the intake side and one shaft for the exhaust side. Therefore, the shafts are parallel to each other spaced inches apart.
A 440 type head only has one shaft per head compared to the Hemi which has two shafts per head.
- The intake rocker shaft is shorter than the exhaust rocker shaft.
- The intake rocker arm is shorter than the exhaust rocker arm.
- The rocker arms have a 1.5 ratio.
The intake pushrods are shorter than the exhaust pushrods. Also, the rocker gear needs to be positioned in place for the cylinder head to be tightened.
Horsepower and Torque
The 426 Hemi was rated at 425 hp @ 5,000 RPM and 490 ft. lbs. of torque @ 4,000 RPM. Pretty respectable numbers even if they were totally true, but they weren’t.
It’s common knowledge many of the horsepower, and torque numbers of the era were underrated by the car manufacturers. Either they were misleading or in the case of the Hemi rated only at 5,000 RPM.
At higher RPM the engine produced much more horsepower. The horsepower numbers at the higher RPM weren’t reported.
Many engine builders have tested original specification 426 Hemis on the dyno proving Chrysler’s power numbers were rated too low.
One machine shop located in Massachusetts hooked up a rebuilt (stock specs) engine to the dyno. The following are the power numbers obtained:
- 459.3 horsepower @ 5,000 RPM (34.3 more than factory number @ 5,000)
- 494.4 horsepower @ 5,950 RPM
- 498.3 ft. lb. of torque @ 4,500 RPM
The engine in the video below was rebuilt to factory specifications except it was bored .030 over. The dyno test only revved the engine to 5,100 RPMs but you’ll see how the horsepower number at 5,000 was 436.8 and at 5,100 was 439.6.
I’m sure if the test was done to 6,000 rpm the horsepower numbers would have kept going higher.
This powerplant in the following video pulled 491.3 horsepower at 6,100 RPM and 493 ft. lbs. of torque at 4,500 RPM.
In all fairness, the pistons were 0.30 over to clean up the cylinders increasing the cubic inches from 426 to 432. Also, the compression ratio was 10.6:1 instead of 10.25:1 because of piston availability and honing.
Even though, the power numbers measured in the dyno far exceed factory numbers. There’s no doubt about it, when you open up all 8 barrels by stomping the gas pedal to the floor, you better hold on and get ready to be planted into the seats.
There’s no better feeling in the world!
Carburetors and Intake Manifold
Carter AFB carburetors were used on all street Hemis from 1966 to 1971. The letters AFB meant aluminum four barrel. Each 4 barrel carb is rated at 625 cfm for a total of 1,250 cfms.
The two carbs were bolted on the intake manifold inline, one behind the other. When the gas pedal is floored, the sound of the air rushing through the 8 barrels may only be matched by the 440 six pack.
Under slight throttle only the rear (primary) carburetor opened up. Under heavier throttle, the progressive linkage opened up the front carburetor also.
The rear carburetor is the primary carburetor and the front carburetor is the secondary carb. The engine idles on both carburetors.
The fuel to the carburetors is delivered by a mechanical fuel pump and a 3/8″ gas line.
My uncle had two Hemi carburetors on the shelf in his garage. Every time we visited him I would venture into the garage and stare at them. He didn’t know what car or year they were from. He acquired them from someone he knew years earlier.
426 Hemi carburetor identification numbers differs by year:
- 4139S Front
- 4140S Rear
- 4139S Front
- 4343S Rear
- 4430S Front
- 4431S Rear Manual
- 4432S Rear Automatic
- 4619S Front
- 4620S Rear Manual
- 4621S Rear Automatic
- 4742S Front
- 4745S Rear Manual
- 4746S Rear Automatic
- 4971S Front
- 4969S Rear Manual
- 4970S Rear Automatic
The fuel filters on the 1966 and 1967 were mounted inline above the intake manifold right before the carbs.
After 1967 the fuel filter was mounted next to the fuel pump. It had a fuel vapor separator because the vapors would get returned from the filter housing.
Learn more about the carburetors in my article, The Carburetors on a 426 Hemi.
The street 426 Hemi intake manifold is a dual-plane design made of aluminum. The intake had a dual casting number, meaning each manifold had two casting numbers.
The two intake manifold casting numbers for the street 426 Hemi are:
Both casting numbers can be seen on the intake manifolds in the photos below.
The pentastar emblem was added near the front during 1967 and remained throughout 1971. Therefore, intakes without the pentastar are 1966 to early 1967 like in the photo below.
The intake manifold bolts to the heads using sixteen 1/4″ coarse thread fasteners. Nine of them are 2 1/2″ bolts and seven are 3″ bolts.
There is a heat shield underneath the manifold shown in the photo below. The intake gaskets are port only and not a valley pan like in the 383 or 440 engines.
The Race Hemi prior to the street Hemi didn’t have a heat crossover passage in the heads, therefore there is no heat crossover in the intake manifold.
The street version intake manifold needed heat for cold operation and drivability. To solve this problem, the street Hemi has a heat passage in the rear of the manifold.
Tubing connects the right side exhaust manifold to the rear of the intake manifold providing exhaust heat. Take a look at the picture below showing the tubing.
Fun fact: The painted heat riser was installed on the engine prior to it being painted. The unpainted tube was added after the exhaust system was installed on the car and why it’s not painted.
The camshaft in the engine changed over the years. From 1966 to 1969 the camshaft was mechanical while in 1970 and 1971 the cam was hydraulic.
They changed to a hydraulic cam to make the engine more drivable for the street. Hydraulic cams also have less maintenance as the valves don’t need adjusting. When the change to hydraulic was made the cam lift and duration were kept the same.
Three different camshafts were used in the engine. The following are the 426 Street Hemi camshaft specifications:
|.467″ Int / .473″ Ex
|.490″ Int / .481 Ex
|.490″ Int / .481 Ex
|276° Int / 276° Ex
|284° Int / 284° Ex
|284° Int / 284° Ex
After 1967 the intake lift was increased. In addition, the intake and exhaust duration were increased from 276 to 284 degrees.
Find out the all differences between a Race and Street Hemi in my article, The Difference Between a 426 Street Hemi and a 426 Race Hemi.
All 426 Hemi production engines used the double roller chain cam drive. This means the chain is twice as wide as the standard width chain. The crankshaft and camshaft sprockets have two rows of gears each.
Whichever cam is in the engine doesn’t seem to matter much as the idle, power and sound of all of them is incredible.
All 426 HEMI Engines have a forged steel crankshaft. The main bearing caps are 4 bolt mains. Two of the four bolts are cross threaded from the side of the block.
This is only for the middle three main bearings which are the most likely to experience cap walk, #2, #3 and #4.
I love the cross thread design compared to other 4 bolt main caps where all 4 bolts thread straight down in the same direction. Securing the main caps from two different angles instead of one is stronger.
This design was a carryover from the Race Hemi blocks used in Nascar in 1964.
A common question asked is what is the connecting rod length in a 426 Hemi?
A 426 Hemi connecting rod length is 6.859″-6.863″ center to center.
The following is the weight of the 426 Hemi connecting rod:
- Large end weight: 770 grams plus or minus 2 grams.
- Small end weight: 314 grams plus or minus 2 grams.
The piston pin bushing bore diameter is 1.0314″-1.0317″
The engine block is made of cast iron. For added strength the block is skirted and has 4 bolt (cross bolt) main bearing caps for the crank.
The blocks for 1970 and 1971 were modified in the crankshaft saddle area, where the crank sits when placed in the block.
They were modified because some blocks would develop a crack there and run towards the cylinder bore. To strengthen the area, they made the saddle area thicker.
Fun fact: The slant six Chrysler engine and the 426 Hemi shared the same crankshaft main journal sizes and many Mopar experts claim the main bearings are interchangeable.*Disclaimer: Only use 426 Hemi bearings for your 426 Hemi
How to Identify Your 426 Street Hemi Block
The easiest, most common way to identify a 426 Hemi block is to check the front ID pad, partial VIN and the casting number.
On the Hemi, and all Mopar big blocks, the ID pad is located to the right of the distributor hole. The pad is flat and measures approximately 2″ x 3″.
The pad typically has 2 rows of stamped numbers on it. The first row contains 2 letters and the number 426 like this, G H 426. In this example G stands for the year, 1971, H for hemispherical and 426 for the cubic inches.
The 426 street Hemis used the following letters for each year:
- B for 1966
- C for 1967
- D for 1968
- E for 1969
- F for 1970
- G for 1971
The 2nd row of numbers identify the month, day and which number engine it was built for that day. For example, if the 2nd row was 11-02-15, it would stand for the engine being built on November 2nd and the 15th engine built that day.
Many 1968 blocks don’t have the letter or 426 on the pad.
Other symbols found on the 426 Hemi ID pad include the following:
- A Maltese cross on the ID pad will indicate .001″ undersized main bearings. Any numbers following the cross indicate the specific bearings that were undersized.
- A Maltese cross and X indicate the same as above but .010″ undersized bearings.
- The letter A on the ID pad indicates .020″ oversize cylinder bores.
- A diamond symbol on the ID pad indicates all the tappets are oversized by .008″
- The letters O.S. on the ID pad indicate .005″ oversize valve stems.
The casting number is located on the left side of the block. The numbers are raised and not stamped in. The casting number for a 1964 to 1971 Hemi block is:
Along with the casting number, is the casting date of the block in the same area. The casting date indicates the day, month and year.If the casting date is 11 22 70, it means the block was cast on November 22, 1970. However. this block would be for a 1971 model year car, not a 1970 model.
The casting date year and model year will not always match. Typically, if the block is cast after July, it is intended for use in the next model year.
If the block is cast before July, then the casting year usually matches the model year of the car.
Be sure not to confuse the casting number with the partial vin number located on the right side of the block.
The partial vin matches the vin from the car it was originally installed into. It can help identify the car it was installed into from the factory.
Partial vins began on 1968 cars.
They are the last 8 digits of the car’s vin number the engine was installed into.
- The 1st number will indicate the model year.
- The 2nd digit is a letter indicating what plant the car was built.
- The last 6 digits (numbers) indicate the vehicle sequence number at that plant.
1968 Hemi blocks may have a partial vin number stamped at the top rear of the block near the oil sending unit. It will be where the transmission or bell housing bolts up.
Some 1969 blocks may have the full vin instead of the partial vin.
Engine Identification Number (EIN)
The EIN started in 1968. Therefore, you won’t find them on the older Hemi blocks.
426 Hemi blocks have the engine serial numbers stamped on the cylinder block oil pan rail at the left rear corner below the starter opening. They will be facing the ground with the engine installed in the car.
The engine serial number contains 14 characters and digits. The first letter or two indicate the assembly plant.
A 426 Hemi should start with MN or MV, which is where they were built.
The plant codes are as follows:
- PM or M: Mound Road
- PT or T: Trenton
- MV or MN: Marysville
- W: Windsor
The next three digits or numbers are the cubic inch displacement. For a 426 Hemi, it would be 426.The next 4 digits represent the 10,000-day calendar. This indicates the exact date it was built.
For a complete breakdown of the Chrysler 10,000 day calendar, check out my 440 Six Pack Specifications article.
The last 4 digits or numbers of the EIN are the daily sequential number of engines built.
Find out why the Hemi was banned twice in my article, Why the 426 Hemi was Banned From Nascar.
The 426 Hemi has a dual breaker distributor, meaning it has dual points. Two points, instead of one, is much better than a single point distributor in any high performance or racing setup.
A single point setup is fine up until approximately 5,500 RPM. I’m not sure about you but if I bought a brand new Hemi car back in the day, you bet I’ll be revving it more than 5,500 RPM.
Therefore Chrysler engineers installed a dual point distributor in the higher performance engines.
The stiffer spring load on the dual point distributors is efficient up to 6,000 to 6,500 RPM.
Oil Pan and Oil Capacity
From 1966 to 1969 the 426 Hemi engine came with a 5 qt oil pan. The oil capacity would be 5 quarts plus 1 quart for the oil filter. Therefore, if you were changing the oil and filter, you would have to add 6 quarts.
The 1970 and 1971 426 Hemi came with a 6 quart oil pan. The oil capacity would be 6 quarts plus 1 quart for the oil filter. If you were changing the oil and filter, you would add 7 quarts.
Windage trays for the engine help keep the oil down in the oil pan and to prevent the oil from splashing around.
These help keep the oil off of the crankshaft and connecting rods which can steal some horsepower. This is more important at higher RPMs than lower.
|Engine Lubrication Specifications
|Oil pump type
|Rotary full pressure
|Oil pump drive
|Oil filter type
|Minimum oil pump pressure @ 500 RPM
|Operating pressure @ 1,000 RPM
|Pressure drop resulting from clogged filter
|1966-1969: 5 qt oil pan plus
1 qt for oil filter
1970-1971: 6 qt oil pan plus
1 qt for oil filter
The windage tray also keeps more oil available for the oil pickup and has been shown during dyno testing to add approximately 15 horsepower at 6,000 RPM.
Another feature of the oil pan is an acceleration baffle to the rear of the oil pan and a deceleration baffle at the front. The baffles help to keep the oil at the bottom of the pan and covering the oil pickup.
It’s mentioned earlier about how the heads can flow a lot of air and how much air the carbs allow into the engine. Let’s talk about how the air passes through to the carburetors.
The earlier street Hemis like in a 1966 Charger, used a large, round, chrome air cleaner lid with two wing nuts. The chrome lid fit over a dull colored base which sat on the carbs. This type of air cleaner assembly was closed in and used in cars with no fresh air scoops.
Next came the large, oval air cleaner assembly which had an orange lid and black base. The air filter on these is exposed all around.
Some of them become enclosed and sealed under the hood when it’s closed to allow for fresh air from various air scoop designs. One is the air grabber hood seen on Roadrunners and GTXs.
The air grabber was activated by the driver pulling on a lever inside the car. In 1970 it changed to vacuum operation.
It may be the most intimidating stop light feature on a muscle car. If it was combined with a Hemi emblem, the race was already half won. The Dodge version was called Ramcharger but was less intimidating.
The same oval lid and air cleaner was used on twin air scoop designs like in the ’69 Super Bee. This air cleaner assembly also sealed under the hood when closed into a metal structure which captured the air from the scoops.
Then there’s the shaker hood scoop. The shaker is a classier scoop which also says this car has plenty of horsepower.
The scoop is functional and allows air through the left and right vents located in the front. The flaps behind the vents can be open or closed by the driver inside the car with a lever mounted under the dash. The lever is similar to the air vent controls.
- The shaker scoop was standard on all 1970 Hemi Cudas.
- The fender tag or build sheet code for a shaker hood is N96.
Find out the real reason it’s called the elephant in my article, Why the 426 Hemi is Called the Elephant Engine.
The enormous valve covers for a 426 Hemi are made of stamped steel. They have a black and dull wrinkle finish.
It may not be the most attractive finish but it grows on you over time and sets the Hemi crowd apart from the typically shiny paint or chrome.
What sets the Hemi valve covers apart from non-hemi ones is the holes in the middle for the spark plugs.
The right side cover (passenger side) has the oil fill with a cap towards the front of the car and a breather towards the rear. The left side has a smaller hole for a PCV valve.
The earlier covers have a large covered PCV on one side and the oil fill on the other. There is no small PCV hole.
Check out all the Hemi engines produced prior to the 426 in my article, Hemi Engines Made Before the 426 Hemi.
With such a great engine, you’d think millions of them were put into cars for sale to the public. Unfortunately, that’s not true and a street Hemi is very rare. Although it’s one of the reasons why it’s so special today.
One of the main reasons is the price tag. Depending on the car and the year purchased, the Hemi costs an extra $839 extra. Back then it was a lot of extra money.
Therefore, how many 426 Hemi engines were built for production cars?
There was a total of 10,669 426 Hemi engines placed in production cars between 1964 through 1971.
- 150 426 Hemi engines were placed in the A body cars in 1968.
- 9,311 426 Hemi engines were placed in B-body cars from 1964-1971.
- 1,208 426 Hemi engines were placed in E-Body cars from 1970-1971.
For a complete list of all cars having the 426 Hemi check out my article, What Cars Had the 426 Hemi: Street and Race Hemis.
Engine Parts Casting Numbers
The following are castings numbers for the 426 Hemi engine:
|426 Hemi Engine Part
|426 Hemi Block (1964-1971)
|426 Hemi Heads (1966-1971)
|426 Intake Manifold (1966-1971)
The following table is the torque specifications for the 426 Street Hemi:
|Part or Bolt
|Main bearing caps
|100 ft. lbs.
|Main bearing cap tie bolts
|45 ft. lbs.
|Crankshaft rear bearing seal retainer bolt
|25 ft. lbs.
|Connecting rod nut (7/16)
|75 ft. lbs.
|Camshaft bolt (3)
|40 ft. lbs.
|Cylinder head bolts & stud nuts
|70-75 ft. lbs.
|Cylinder head cover
|Distributor clamp bolt
|Rocker shaft brackets
|30 ft. lbs.
|Oil pan screw
|Oil pan drain plug
|20 ft. lbs.
|Chain case cover (upper)
|15-18 ft. lbs
|Carburetor to manifold
|Middle 4 – 72 in. lbs.
Front 2 & rear 2 -48 in.lbs
|24 ft. lbs.
|Centerbolt (Vibration Damper)
|135 ft. lbs.
|Clutch cover to flywheel
|Flexplate/Flywheel cap screws
|70 ft. lbs.
|Torque converter to flexplate bolt
|65 ft. lbs.
|Flywheel housing to block
|50 ft. lbs.
|30 ft. lbs.
|30 ft. lbs.
|30 ft. lbs.
|Fan attaching bolt (4)
|15 ft. lbs.
|Starter mounting bolt
|50 ft. lbs.
|Water pump to housing bolt
Water pump to cylinder block bolt
|30 ft. lbs.
There’s more to the head design that makes the engine so special. Find out more in my article, What Makes the 426 Hemi So Special.
Find out about the mythical 426 Hemi DOHC in my article, The 426 Hemi Dual Overhead Cam Engine – The “Doomsday Hemi”
Check Out More 426 Hemi Articles!
Any questions or if you have more information you’d like to contribute, send us an email found on our contact page.
- Google books: How to Rebuild and Modify Chrysler 426 Hemi Engines
- Google books: The Definitive Plymouth Barracuda and Dodge Challenger Guide: 1970-1974
- The Mopar Blog: Mopar.com
- GTS: Galen’s Tag Service
- Hemmings: Hemmings.com
- Wikipedia: Chrysler Hemi engine
- Dodge: Performance 2022 Dodge Challenger