Hi,
Mike is right, if it isn't a loose or broken rocker, then it may be a rod or main bearing, could be a 'spun bearing' or a 'piston wrist pin' with the wrong clearance. I would plastigauge all the bearings, should be .0015 to .002 for mains and .002 for rods. Racing clearances are .0005 more. Startup is when the assembly lube saves your bearings until the
oil takes over, need LOTS of assembly lube on bearings or they can stick to the crank and spin.
Good Luck,
Perry.
