Originally Posted by ws6transam

.045 inches is supposedly tight. I'm running .046 inches on my new, yet untested engine. As long as crank and rod bearings stay together, and the piston doesn't rock too much from a worn cylinder wall, it should, in theory, be fine. If, however, you destroy a bearing and the rod gets to knocking, the clearance rapidly goes away as the piston/rod wont accurately track the crankshaft. The result is the potential that the piston could make contact with the cylinder head deck surface, causing further destruction. As least, that is my own understanding and is NOT based on any direct experience. I'm currently running a 0.044 inch clearance on my old Feeble305 and it hasn't spit anything onto the pavement yet, even after eating the lobes off a cam two summers ago.