Piston wear

I have a wiseco piston in my bike. The bore size is 66.4mm. Im ready to order parts for the rebuild. Before I tore it down it was making a horrible knocking/rattling sound from right off idle to max rpm. My main powervalve is worn and I will be replacing it so that might be what made the noise. But I mic'd my piston and got 66.1924mm. Is this too much wear? Since I have it apart should I put a new piston in it anyways? Im really just trying to get to the source of the rattling. Thanks

This is no up and down play in the big end bearing. Crank turn smoothly with no play, etc. Everything else checks out OK besides the powervalves which I will be replacing anyways. I thought that wiseco pistons had looser tolerances to counter the chances of a cold seizure becuase of them being forged. But i agree that some of the noise does sound like piston slap. Im pretty sure I mic'd the piston properly. Just wanted to make sure I was on the right track. The bike still ran pretty well and always started up first or second kick cold. So its looks like ill be ordering a new piston.