[MJN]

Simon

Most modern pistons are barrel shaped or at least they are not round. This is so that when they reach operating temperatures they will fit the bore correctly. As you can imagine the crown or top of the piston will be hotter and therefore expand more than the skirt of the piston. The piston manufacturer will specify where the diameter of the piston should be measured to set bore to piston clearance. Also, forged pistons will typically be run with a larger clearance than cast or hypereutectic pistons. 0.003" sounds about right for a cast piston but too tight for a forged piston. I have Probe forged pistons in my 331 and the specified 'street' clearance is 0.0045" to 0.0050". For racing use they recommend a slightly larger clearance of 0.0055". Probe recommends measuring the piston midway down the skirt at 90 degrees to the wrist pin. BTW I can hear a little piston 'slap' in my engine when it is first started but it is gone within the first minute or so as the pistons heat up and expand. Knocking is not normally piston clearance but bearing clearance.