[hjewell2]

Originally Posted by richard2112

oil viscosity for your engine is determined by the tolerances at your main and rod bearings and also upon the smoothness of the finish of each. A polished finish can work well with tighter tolerances and a thinner oil will develop a good hydrodynamic wedge for the journals to ride on. As your tolerances get looser and the finish a bit more rough a heavier weight oil is necessary. A general rule of thumb for engine building is to allow .001 clearance for each inch diameter of journal.

bingo!!!!