Piston wear

[Jbgabriel]

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

 

[griffbones]

Grab a hold of the rod and make sure that there is no up and down play in it. Also take feeler gauges and see how much side play is in the big end rod bearing. I don't know the service specs for your bike but they shouldn't be too hard to find out.
BTW that is a difference of over .207! I would have thought the cylinder to piston clearence would have been somewhere around .002, but maybe I am off base here?

 

[Filthy_McNasty]

That piston is worn out, assuming you mic'ed it correctly. It may not be the only sound you were hearing, but that piston had to be making a bit of slap.

 

[kx125412]

I just did the top end on my bike, the piston was worn a lot and it was hitting the powervalve the slightest bit. As the revs came down, in neutral or riding, it would make the worst sounding rattle. With the new piston in and powervalve tab fixed its wayyy quieter. Motor noise went down substantially, no rattle, more compression, more power, starts first kick. You have to remember that the bore has to be bigger than the piston so the piston will mic out smaller.

 

[Jbgabriel]

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.