Chrysler Hemi Engine - Affordable Excellence

Building a 331 hemi

By Ron Ceridono

One of the common criticisms we hear concerning street rod construction is how boring it is to see a small-block Chevy under the hood of just about everything with wheels. Of course, the other common lament has to do with how difficult and expensive it can be to use anything else.

When it comes to practicality, there are probably more reasons to use a small-block Chevy to power a street rod than not. They're readily available, light, compact, and can be made to produce obscene amounts of horsepower for less money than just about any other powerplant. To top it off, they fit an early Ford like Henry planned it that way. Sticking a Chevy in a street rod makes sense, but then this isn't a hobby that's necessarily based on good sense.When it comes to alternative propulsion for a rod, there are a lot of choices, but one of the basic principles of our hobby is that if a little is good, a bunch is better. And there's not a bigger bunch of engine than a Chrysler Hemi. And with the possible exception of the Ford Flathead, there's not another mill that is more steeped in tradition.

A SHORT HEMI HISTORY
Early Chrysler Hemis came in three displacements--331, 354, and 392--and although they are all similar in appearance, there are some dramatic differences.

BLOCKS
A sure way to spot a '51-53 331 Chrysler is by the long extension that housed the fluid coupling for the horrid Fluid Drive transmission. For the '54 model year, a new automatic was offered and the block extension was eliminated, although some '51-53-style blocks have been found in early '54 cars; displacement remained 331 through '55. In '56, the 354 was introduced while the big daddy of them all, the 392, was produced in '57 and '58. Because of the increased stroke, the 392's decks were raised .500-inch; they are often referred to as a raised block, and the 331-354 are standard, or low blocks.

For our project, we used an extended-bellhousing 331-inch '53 block. Why, you might ask, would we use that instead of a 354 or a 392? The simple answer is economics. These are the least popular of the breed, which makes them a bargain as Hemis go (for more on these, you may also refer to our original article on these engines, "Overlooked Excellence." It can still be found under technical articles at www.streetrodderweb.com).

CRANKSHAFTS
All Hemis used forged crankshafts, and, while that's not particularly unique, Chrysler took things one step further. To prevent failures, most crankshafts have a radius where the rod and main journals intersect throws of the crank, as an abrupt 90-degree angle would invite fractures. Usually, a radius is ground into the crankshaft at those points to maintain strength. Chrysler's method to prevent fractures in those areas was the use of rolled fillets, which compressed the grain of the metal, thus increasing strength.

CAMS
Another difference in the early engines is the cam design--'51-54 shafts use a long threaded snout to retain the cam gear while the '55-and-later cams use a threaded hole and a cap screw. We wanted to lose the stock cast-iron front cover and water pump and use a Hot Heads aluminum front cover, which requires a '55-and-later cam. But while all Chrysler Hemi cams will interchange, it's important to note that there are significant differences. The 331 and 354 engines have the same deck height, while the 392, due to its longer stroke, was .500-inch taller. As a result, the lifter bores in the "short-deck" 331/354 engines are located differently relative to the centerline of the cam than the "tall-deck" 392s. That means that mixing a low-deck cam with a high-deck block, or the other way around, will result in improper cam timing.

Along with cam differences, there are several choices in lifters and pushrods for Hemis. There are stock replacement solids and hydraulics, and, in some cases, later Mopar hydraulics are used. Which lifters used, along with the type of rocker arms used, will determine the style and length of the pushrod required. Pushrods for adjustable rockers use one male end (lifter end) and one female end (for the rocker's adjustment screw). Non-adjustable rockers with stock-style hydraulics use pushrods with two 1/4-inch male ends (later hydraulics require 5/16-inch tips on the lifter ends), while some solids require 3/8-inch ends. Pushrod lengths will also vary depending on the combination of parts used. The best bet here is to use a coordinated kit and consult the cam supplier. We used stock, non-adjustable rocker arms and our cam, lifters, and pushrods came from Hot Heads.

HEADS
The 1951-53 heads had small round exhaust ports, water outlets were in the intake manifold face, coolant returned to the radiator through an intake manifold-mounted thermostat housing, intake valves were 1.81-inch, and exhausts were 1.50-inch. In 1954, the heads were redesigned with larger ports, 1.94-inch intake valves, and 1.75-inch exhaust valves, but water still exited the intake manifold face. For 1955, several changes were made in the block and heads. Water jackets were redesigned so now coolant exited through passages in the fronts of the heads. A cast water manifold connected the two heads and housed the thermostat. While displacement stayed the same at 331 cubes, a unique '55-only head gasket was required. Heads for 1956 Hemis remained the same with the exception of a change in the head dowel size.

With the introduction of the 392, intake valves were increased to 2.0-inch; exhaust remained the same at 1.75-inch. With the new raised block, the heads were now farther apart. Chrysler engineers decided to keep the intake manifold's dimensions the same as earlier engines, so a thicker flange was added to the intake face of the heads. This meant there was now a sharper turn in the intake port, and, for that reason, many engine builders prefer the late 331-354 heads because of their straighter and shorter ports. We were lucky and found a pair of '54 heads cheap. And, while they are arguably more desirable, we would have used the earlier heads with no qualms for a street engine.

THE BOTTOM LINE
Sooner or later, just about everything comes down to dollars and cents, and the bad news is no matter how you do it, the parts to rebuild a Hemi (or most other vintage engines) are going to be more expensive than a small-block Chevy. As a comparison, the Hot Heads 331 Chrysler rebuild kit we used sells for $1,590 (354/392 kits are $1,450). It includes cast 8.5:1 pistons, chrome rings, double roller timing set, cam, rod and main bearings, adjustable push rods, performance cam, cam thrust washer, converted 340 high-volume oil pump and pick up; similar kits for an SBC can be had for less than $300. A single four dual-plane intake manifold for a Chrysler will set you back $275, while $129 will get you one for a Chevy; an MSD distributor for a Chrysler is around $350 and about $100 less for a Chevy. And you can plan on spending $200 to $500 on rebuilt Hemi rocker arms and shafts while a new set of Chevy rockers and balls will run around $40. Obviously, carburetors, alternators, and other parts that aren't necessarily make-specific will be the same price, but there may be a few other expenses when building a Hemi. As an example, we opted for a Hot Heads timing cover that also adapts, ironically, to a small-block Chevy water pump for $209. Now the good news--while the parts may cost more, the machine work on a Hemi shouldn't be any more expensive than any other engine, including a Chevy. The trick here is to find a shop with the latest equipment, like John Beck's Pro Machine.

GETTING A SHORT-BLOCK TOGETHER
Although some engine builders would like you to think Hemis are mysterious, a bunch of tricks aren't necessary for a street engine. The bottom end isn't fancy; in fact, ours is pretty close to stock other than the late-model oil pump, Total Seal rings, and balancing. Even the cam is pretty mild. Just like any other engine, checking tolerances and careful assembly is what it takes to build a Hemi. Take a look at how ours went together; next time, we'll take a look at swapping heads, fitting pushrods, and all the finishing details.