American manufactured engines are the mainstay of hot rodding. They also enjoy one of the industry's best reputations for longevity. Thus, when cared for, they do not need rotating assembly services unless a complete rebuild or hop-up is in order. For purposes of this discussion, the rotating assembly of the engine consists of the crankshaft along with the connecting rods, pistons, wrist pins, harmonic balancer, and flywheel or flexplate. Companion components, such as piston rings and both the crankshaft and connecting rod bearings, are included in this classification.
After a good visual inspection, the crankshaft main bearing journals and crank pins will be measured with a micrometer. The crank pins are the portion of the crankshaft the connecting rod attaches to. When machinists check for taper and out-of-round, they compare the micrometer readings they get with the specification size chart for the crankshaft to determine if it has already been ground undersize. The common sizes to which bearing journals are ground are in increments of .010-inch, like 0.010-, 0.020-, or 0.030-inch. Most machine shops sublet out crank grinding to a facility that specializes in crankshaft repair, since the work is so specialized. Even if a crankshaft is not ground undersize, the journals should be polished to a #15 microfinish before entering service. In this photo, the crankshaft is being spun on a balance machine in the same direction as rotation in the engine. A polishing material that resembles emery cloth is held against the journals. The polishing cloth needs to pull the material away from the direction of rotation. Polishing is done to prepare the journal surface for the proper contact with the bearing material, so the oil will be able to work properly and form the necessary oil wedge that separates the bearings from the journals.