Best Crank
Note: Any one of these packages could use a lightweight forged crank. But neither a lightweight forged crank nor a billet crank is required.
Almost all race cranks use full-radius journals on the cranks; all production cranks, both cast and forged, use undercut journals. The full-radius journal makes the crank much stronger. Undercut-radius cranks make it easier to assemble the engine, which is very important when you are building 50 engines per hour or 1,500 engines per day. The bearings must take a full radius into account either by radiusing the bearing individually or by obtaining bearing manufacturer information for full-radius journals.
When you buy a new crankshaft, you don’t have much prep work before you install it, but you should carefully inspect it for nicks, scratches, and any damage. You also need to verify a few measurements. I recommend doing a quick check on the crank’s end play by installing the number-1, -5, and -3 thrust bearings and lowering the crank into place, torqueing the three main caps, and checking the end play with a dial indicator.
In most cases, a new crank comes polished. In some cases, a used crank can be polished to remove very light scratches and normal wear; then reinstalled.
Repair
If the wear is high and/or there are scratches that are too deep to polish out, the crank is typically sent out for repair, which means to grind it undersize. This might be .020 inch. It is common to grind the crank .020/.020-inch undersize and then use matching undersize bearings.
A technician uses a special machine to balance your crank, rods, pistons, and dampener and flexplate/flywheel. You need to balance the assembly and you can’t change pieces after the balance numbers are set. Sometimes the dampener (at far right) and flexplate/flywheel (about in the center) are optional. If you can provide all of the weights needed, the manufacturer can balance the crank. With a used crank, if the rods are staying the same, and the pistons are made to service weight, then balancing may be optional. However, many performance pistons are lighter and you might want to rebalance for the lighter hardware.
A machine shop might put the crank in V-blocks to measure the stroke. However, if you want to measure it yourself, it is most easily done in the engine. With one piston-and-rod assembly installed (rings optional), rotate the piston to BDC (bottom dead center) and use a dial vernier to measure from the top of the block to the top of the piston. Use a straight edge at the top of the block to help keep the vernier straight. Rotate the piston up to TDC and measure from the top of the piston to the top of the block (piston’s deck height). Subtract the two numbers for the stroke. (Photo Courtesy R. Koffel)
If the crank needs more serious repair and the amount of undersize grinding is .040 or .060 inch, you should consider having the crank re-heat-treated by a process called nitrating. Grinding the crank more than .020 or .030 inch grinds much of the crank’s surface hardness away. The nitrating process can add hardness.
A qualified local machine shop should check the balance of your new crankshaft (in this case, it’s an Eagle forged crank). Most cranks are ground to an assumed common package, but most engines are somewhat different, so there can be problems that your local shop will find because they have the actual hardware. If extra-light pistons were assumed and only light pistons are being used, the crank’s counterweight may have a lightening hole that is not needed. The shop will press in a cast iron/steel plug, cut to size, and then weld it in (the dark spot at the top left of the first counterweight). Aftermarket manufacturers usually use the stamped numbers on the front face on forged cranks for identification purposes.
Lubrication Grooves
The rear main seal surface, just before the crank flange, contains small grooves cut at an angle to the centerline of the crank. Almost all production cranks have these grooves. Aftermarket cranks may or may not have these grooves. Chrysler replaced the original rope seal in the late 1960s, and the newer rubber (neoprene) seal with the lip design that must point at the center of the engine does not require these grooves. If you find a new crank that has the grooves and they have sharp edges, the grooves should be polished to help protect the rear seal from damage.
On a new crank, if the grooves are cut too deep or the edges of the grooves are too sharp, the rubber rear main seal may not seal and the engine leaks oil. The solution is to use a rope seal, but they are difficult to find. It has been so long since rope seals were used that most manufacturers just put the rubber seal in the various gasket kits. You can obtain a new rope seal from Best Gaskets, among other suppliers.
Crank Mods
Crank manufacturers offer special modifications. Some modifications offer advantages in windage and rotating inertia, but they can be very expensive. When it comes to making the crank lighter, I prefer to cut down the counterweights’ diameter because it gets the most weight off.
Once the block and crank are prepped, you should check the crank’s endplay at the final assembly to make sure it’s within spec. The crank is rotated so that one of the flat counterweights is up and access is available. Set up the dial indicator with the pointer parallel to the crank centerline and the pointer on the flat surface of the counterweight. Use a large screwdriver between the counterweight and the bulkhead and lever the crank rearward. Zero the indicator and then lever the crank forward. The reading on the indicator is the crank endplay.
The main cap just to the right of center has the special cap screws used with the windage tray. If a windage tray is going to be used, these special headed bolts are required. If you are planning on using a stroker (long-stroke crank) then you may have to clearance the rotating assembly to the windage tray. This holds true for special aftermarket scrapers and windage trays by companies such as Milodon.
Knife-edging also makes a crank lighter. For this approach to be helpful, you have to include high RPM in the equation. For most people, knife edging isn’t worth the expense, but a lighter crank at a competitive price is worth it!
Only two bearing sizes are offered for the small-block: the standard 273-318-340 and the large main 360. Bearings have been made of tri-metal, aluminum, or babbit, but tri-metal is the most popular today. Clevite, Sealed Power, Mahle, King, and Dura-Bond make bearings for the Mopar small-block.
Most make a version of a tri-metal bearing (copper-lead mix typical), but several manufacturers offer an aluminum mix. Mahle offers a moly-graphite coating, which may provide a wear benefit.
Some bearing companies offer bearings that have been radiused for use with full-radius cranks. Most bearing companies offer various under-size bearings for the repaired cranks (.020-inch undersize is common).