funhouse –

Bear in mind that these engine designs predate the development of modern concepts of dimensional control, to say nothing of the part-to-part uniformity achievable by modern manufacturing methods. At the time these engines were designed there would have been an implicit assumption that, while some of the parts might end up at the high side of the tolerance range and others would end up at the low side, other parts would fall in the middle, and it would all even out in the end – the machine would go together and perform as designed. It is when you force the tolerances all in one direction that you end up with problems. If we were talking about linear dimensions, an accumulation of tolerances might make some feature at the end of a group of axially connected parts end up improperly located relative to some other part of the machine. In the case of cylindrical fits, I can imagine that slightly loose pistons connected to slightly loose wrist pins riding in slightly loose connecting rods with slightly loose end journals connected to crankshafts which themselves are not as rigidly constrained as intended might result in an engine which is already “tired” the day it leaves the assembly line.

Harald