When I'm building a race engine, I weight-match all the moving parts such as pistons, rods, wrist pins, rings, etc. to a tolerance of +/- 1/10 gram. Take a complete assembly for one cylinder, for instance a piston, ring set, wrist pin, wrist pin locks, if applicable, etc. and weight the whole assembly. Do this for all the cylinders, and find the lightest one. Use this as the standard, and file the balance bosses on the other pistons until every assembly is the same weight. Be sure not to mix up components as the engine is assembled. A dollar bill weighs one gram, so every set of reciprocating parts is equal within the weight range of 1/10 of a dollar bill.
Rods are suspended at one end, and the other end is weighed, so both big and small ends get weight-matched individually. Be sure to include the bearing in the measurement of the big end, and keep that bearing with the rod during assembly.
Inline 6-cylinder crankshafts are inherently balanced, so little if any dynamic balancing is required. 4-cylinders, V-8's, and V-6's need "bob weights" attached to the rod journals that represent all the "reciprocating weight",(Pistons & rings, upper rod, etc.) and a certain percentage of the "rotating weight"(Rod big end and bearings) before getting a spin-up. A "fudge factor" is added for the weight of the oil in the assembly. Weight is either removed from the counterweights by drilling, or added by welding to dampen out any vibrations. Add-on parts like the flywheel, clutch assembly, or harmonic balancer can benefit from a spin-balance procedure if the equipment is available and the cost isn't excessive.
Jerry
Last edited by Hotrod Lincoln; 08/28/2010 4:32 PM.
