Piston to Cylinder Clearance, 95mm J&E 96P

[Walt Fricke]

I have a set of 95mm J&Es. Made, I gather from other posts, of 96P alloy (because that's inside the pistons). These are going into an 8,000 rpm short stroke 2.8 race motor. I am about to send my old Alusil cylinders off to US Chrome for Nikasiling and resizing.

The J&Es came with a sheet stating that the clearance should be 0.0015".

Reading discussions here alerted me to the fact that these pistons come in two alloy flavors, which behave somewhat differently. I had no idea about that when ordering them. And that maybe, for a race motor, one ought to use a bit more clearance. So I started fretting.

What clearance should I specify?

Walt Fricke

[304065]

Walt, just for reference here's the thread where Mike Bruns raises that exact point with regard to the 96P forging for the 95mm application.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/367966-piston-failure-post3488870.html?highlight=bruns+forging#post3488870

[cnavarro]

Walt, I would send the pistons to Calico to have the skirts coated. Specify minimum coating thickness on piston skirts. Then have the cylinders plated and match honed with .0015" +.0005" -0" clearance - That will give you about .001" of margin of safety if the piston does get hot (especially because of the higher RPMs and being it's a race engine) and expands too much (as the cylinders don't expand as fast as the 2618 forging). Thermal barrier on the crown should also help limit the heat soak into the piston, and further increase the strength and reduce its expansion. That should be more than adequate.