340, and 5.2L is one group; the 3.58-inch stroke for the 360 and 5.9L is the other.
Only a few production crankshafts are offered for the Magnum and A-engine. This cast 3.31-stroke crank has a casting number on the second counterweight on the left. All street and street/strip packages use the 3.31 stroke crank, and it can typically support 500 to 600 hp. But chances are if you plan on building this much power, you would want at least a 3.58-inch stroke.
The 3.31-inch-stroke crank weighs about 54 pounds; the 3.58-inch-stroke crank weighs about 58 pounds. Cast cranks are generally lighter than forged cranks. The long-stroke 360 crank also has larger diameter mains by .310 inch. The radiused, sealing surface on the end of the number-5 main is smaller on the large-main 360 oil pan.
Small-block cranks become complicated in relation to external balancing. All 360/5.9L cranks are externally balanced, but the 5.9L uses less weight than the 360. The 318 is not externally balanced in either A-engine or Magnum versions. The forged-crank 340 (1968–1971) is not externally balanced, but the cast-crank 340 (1972–1973) is externally balanced.
When building an engine, consider your performance targets. Production crankshafts are suitable for up to about 600 hp for a high-performance street build. If you’re planning to build a race engine, start with a race block and use a performance crank (typically forged or billet).
Both forged and cast crankshafts are internally balanced and have been installed in 318 production engines. When a crankshaft is externally balanced, non-symmetrical weights have been added to the vibration dampener and the flywheel/torque converter/flexplate. The A-engine generally added weight to the torque converter face; the Magnum family adds the weight to the flexplate. In manual transmission cases, the weight is removed by drilling holes in the engine side of the flywheel.
An Eagle forged crankshaft is ideal for performance applications. This 3.31-inch-stroke crank should be good for any street or street/strip package. Customers that want the high-RPM, high-output model probably would want a longer stroke, at least 3.58 inches. The key for cranks, after the stroke, is if the journals are full radiused. Production cranks are under-cut and performance cranks often are fully radiused, and the full radius requires the bearings to be clearanced on the sides, offered by Sealed Power.
It is very difficult to differentiate between a forged crank and a cast crank. If you have a cast crank and it has a casting number on one of the counterweights, you are in good shape. Forged cranks generally do not have 5-, 6-, or 7-digit numbers on the counterweight that can be used for identification. The basic forging process tends to wipe away any number, so they are very rounded and difficult to read. Cast cranks tend to have sharp edges, whereas forged cranks do not.
External Balancing
Vibration and unwanted harmonics are the enemy of any engine, and if left unresolved, they lead to catastrophic engine failure. Thus, any engine, particularly high-performance engines, must be properly balanced. For a max-performance or race engine, I recommend internal balancing performed by a machine shop.
My main reason for recommending internal balance for max-performance engines is that flywheels and converters are often swapped or replaced for performance reasons. Therefore, each time, it must be externally re-balanced. With Magnum engines, external weight is added to balance the flexplate, so converters can be swapped (tested) easily without adding weights to each converter.
With many max-performance applications, SFI dampeners are required. There are SFI externally balanced dampeners, but the choice tends to be limited. Most of these racing applications need lighter-weight dampeners and the external weight limits this approach.
Max-performance engines equipped with manual transmissions present another challenge. These often use a clutch with an aluminum flywheel; it’s very difficult to use aluminum flywheels with any external-balance setups. The drilled holes in the flywheels are used to create the proper amount of external weight, but it assumes that the hole is drilled in steel or cast iron. Because aluminum is so much lighter than cast iron or steel, the balance holes need to be very large.
Solutions are possible, but they are expensive. One solution is to add steel weights to the engine side of the flywheel, but they must be machined into a relief so they do not hit the block. They are then screwed into the flywheel, which makes it expensive.
To explain why some small-blocks are externally balanced and some are not, you have to look at the engines in more detail. The size of the production counterweights on the crankshaft is designed for the production piston, rod, and stock stroke of 3.31 inches in standard forged steel.
Cast iron is slightly lighter than forged steel. When the Chrysler 340 switched from forged steel to cast iron in 1972–1973, the counterweight lost weight and the balance was lost. A small amount of weight was added externally to put the engine assembly back in balance. With a longer stroke, the counterweight has to become larger (heavier).
Space inside the crankcase is also a limiting factor. The long-stroke 360 needed a lot of weight, so it was added externally. When the Magnum 5.9L was introduced, it had much lighter pistons; therefore, the amount of external weight was reduced.
The number-5 main journal is the widest journal on the crank, and therefore it is the least loaded on a load-per-area basis. It is also where the rear seal is located. It is to the left of the bearing surface. The bearing surface is smooth, while the rear seal surface has oiling slots cut at an angle, at approximately 45 degrees. The rear seal groove is the trapezoid-shaped area cut into the block below the seal surface. It is wider at the top and narrower at the bottom. The rear seal sits in this area. If the new crank has these oiling slots cut too deeply or the edges are left too sharp, the neoprene (rubber) seal will leak. This requires a rope seal, which fits in the same groove.
The crank journal oiling holes are in every journal, several per journal in most cases. The crank gets oil from the main oil galley down to the main journals. From there the crank gets the oil out to the rod journals. There are typically two holes per journal, one for each rod. Each one of those holes comes from one of the mains. The production cranks basically let the machining enter the journal and then add a small radius. The aftermarket cranks take this a step further and taper the entrance in the rotation directions (left and right as shown). This helps get oil into the bearings.
These are the top half of the main bearing shells. The top shells get the oiling hole and the groove that helps spread the oil around the bearing. Grooving the crank weakens the crank. Most of the bearing loads are on the bottom shell because the pistons are trying to push the crank out the bottom, so the top shell, the one in the block, is loaded lightly, relatively speaking. This means that the groove doesn’t really weaken the bearing and is very important to getting oil around the bearing. Sealed Power and Clevite are two good choices.
To tell a 360 crank from the 3.31-stroke group, measuring the main bearing diameter and looking for 2.81 inches is probably easier than measuring the 3.58-inch stroke.