Mopar Small-Blocks. Larry Shepard

Street Cranks

      An aftermarket performance crank’s most important attribute is that it often has more stroke, so you can create a stroker package and thus add more cubic inches in the engine. Another advantage is having a version that is not otherwise available, such as a 360 crank (3.58-inch stroke) with small mains (318/340 size).

      Most production cranks are cast, but forged cranks are stronger and take more abuse. Cast iron is lighter than steel if all specifications are the same. Performance cast cranks from manufacturers such as Scat and Eagle weighs about 54 to 56 pounds. Forged cranks with these same strokes from Scat, Eagle, K1, and Callies weigh about 58 to 60 pounds.

      The problem is that all of these crank manufacturers offer lightweight and super lightweight forged crank options; however, they do not list weight specifications for these options. The weight is removed from the crank by machining. If you plan on running a high-RPM engine (more than 7,000), you should consider a lighter forged crank.

Race Cranks

      Actual race cranks for the small-block come in all sizes and styles. Although most race cranks tend to be forged or billet, the aftermarket does make cast crank versions because they are much less expensive to build.

      Early in small-block racing, race cranks were made with an eight-bolt crank flange, similar to the Hemi racing block. This adds complexity to the customers’ overall engine package, so in later years, six-bolt cranks were also used in max-performance applications.

      Flywheels and flexplates are available for either flange. Most max-performance or racing cranks use a special stroke to achieve a certain displacement. The 2.96-inch stroke was designed to give the 340 engine a displacement of 305 ci, which was required by the Trans-Am sanctioning body. This is what the 1970 305 Trans-Am racing engine used.

      The second race crank consideration is inertia, or rotating weight. The forged race crank can be made at less weight and less inertia.

      The cast crank is slightly lighter because of the material but not as strong as the forged crank. In general, the amount of stroke is more important than the amount of horsepower. With the 3.58-inch stroke, more than 600 hp might mean a forged crank, but you probably have selected the forged crank earlier because it is lighter. Be sure to balance any new and lighter crank.

       Every rod must have an oiling hole and that hole must be connected to a main journal. These holes are drilled at an angle that has to be calculated by the manufacturer to start and end up in the correct places.

Material

      Production forged cranks made from 1050 or 1053 heavy-duty alloy of mild steel are strong and easy to machine in very large quantities. The aftermarket developed a special alloy forging steel and it offered a 65-percent increase in strength over mild steel. When racing RPM and output went up, strength needed to increase.

      To fill this need a special alloy steel called 5140 was introduced for cranks that offered a 6.5-percent gain in strength over the 1050 steel baseline. Then the aftermarket moved on to 4340 material that offered around a 75-percent gain over the 1050 baseline. These high-carbon steels (5140 and 4340) offer more strength but are more difficult to machine and they wear out tools fast, which makes them suited to low-volume performance/racing applications.

       Note that on the rod journal that is just below the pan rail, with only one rod and rod cap on it, next to the main cap, the journal does not have a groove (undercut) next to the side of the crank . It transitions from the rod journal to the side of the crank in a smooth radius, which is called a full-radius crank. This style of crank is made for racing and generally very popular in the aftermarket because it is stronger than using the undercut. With full-radius cranks, you have to radius the bearing shells.

      Most engine builders have opted for 4130 (125,000 psi) or 4340 (145,000 psi) forged cranks because of their toughness and longevity. The 4340 crankshafts have supported up to 1,500 hp, so unless you need a billet crank, a 4340 forged crankshaft is suitable for extreme and racing builds. Strength numbers are based on SAE tensile-strength data.

      Billet Cranks

      A billet crank starts out as a large round log of steel (an ingot) and then a CNC mill fully machines it into the desired dimensions. This process makes it very easy to change the stroke; therefore, you can produce a special one-off crankshaft that has the exact dimensions you need.

      With a forged crank, small adjustments can be made, but these changes are very limited by the forging. If you are looking for a special stroke length, such as 2.88 or 4.25 inches, you might be looking at a billet crank.

       Measuring the width of the rod journal on the crank with a dial vernier is quite easy and the width will come in handy to estimate rod side clearance. It is difficult to measure rod side clearance directly until the engine is being final assembled, which might be too late to fix any potential problem.

      A billet crank can be used for a high-horsepower street/strip engine, but these are expensive cranks used for their high strength in race engines. Weight considerations must be taken into account if a lighter crank is desired in a race engine. Moldex is one of the premier billet crank manufacturers but has a limited website; Winberg, Callies, K1, and Scat have more information posted on their websites. They can make almost any variation of the Mopar small-block crank.

      For example, the Trans-Am engine used a 4.04-inch bore (stock 340) and a 2.96-inch stroke. Assume that you want to build a 295-ci engine out of the basic Trans-Am package; the stroke has to be 2.88 inches and that length is not readily available. If you want to duplicate it, you must purchase a billet crank. Longer stokes than offered by readily available forgings are more likely, such as 4.050 inches or longer.

      Opinions differ about whether or not a billet crankshaft is stronger than a forged crank. One thing is certain, billet crankshafts are stronger than cast and, in most cases, support up to 1,500 hp. But it can cost up to $3,000 to build a billet crankshaft. Unless you need to construct a billet crankshaft for a special application, you are probably better off with a forged crankshaft. The biggest advantage of a billet crank is being able to build an engine with a unique stroke.

       Notes:

      • The 340 block has thin cylinder walls and it cannot be overbored more than .030 inch. Sonic-test before boring to be sure proper thickness is available.

      • The .030-inch oversize is a common aftermarket piston and ring size for these engines.

      • The 3.79-inch stock was a common performance crank/stroke in the 1980s and 1990s before the 4.00-inch cranks were readily available.

      • Race blocks allow bigger bores than 4.07 inch.

      • Most cranks that increase the amount of stroke over the engine’s standard stroke are generally called stroker cranks. Stroker cranks are an easy way to increase the engine’s displacement for increased torque and horsepower. A stroker crank must be used with other parts that work together as a team. The easy one is a stroker crank and a special piston that adjusts for the extra stroke length by moving the pin up in the piston and using the stock rods. In some cases both the rods and pistons are changed.

      Crankshafts for Your Application

      In an effort to tie all engine hardware together into performance packages, I created five packages that vary from 350 hp to over 700 hp, and all are intended for street/strip applications.