@ revhead.. imo a broken spring wouldn’t retract the valve all the way back in causing interference with the exhaust valve and piston. There’snit that much clearance between the plug and valves to begin with. A spring broken in two pieces can rotate into the second half to where it’s not the same height anymore. Plus the compression comes and goes which means there is fluctuations which has to be a moving part. A spun rod bearing won’t cause compression issues and I would also have a knocking sound which I do not and it would also be hitting the plug every single time if it’s going to hit it at all.