Help with piston size!

[dsm_mike]

I took my block to the machine shop for a rebuild. I took along a set of wiseco pistons (6559m855) which are listed as .020 over, or 3.366 in. The cylinders were measured at 3.346, however the pistons measure 3.362. The machinist said he had never seen pistons with an odd measurement like that (0.016 over). Is this normal? I found a thread http://www.dsmtuners.com/forums/new...-right-way-measure-piston-wall-clearance.html in post 28 treebonker says: No, "clearance built in" means when you buy 20 over pistons for 3.366(85.5mm) bore, the pistons are made .004 smaller and will measure 3.362.
It appears this is the case with me, but i dont quite understand. What should the cylinders be bored out to?

 

[DSMReviver]

The best way to know for sure is to contact wiseco itself. when i had my motor build, we contacted wiseco to give us the clearances. Everything work out good.

 

[Only2G4Me]

From what I understand the "built in" clearance in forged aluminum pistons is there for heat expansion. As the engine warms up and the pistons get some heat in them they expand slightly and take up the extra clearance. My guess would be to have the cylinders bored to match the piston plus clearance. You will probably have some piston slap on start up when its cold, but you wouldn't want the pistons expanding too much and ruining the rings/cylinder walls.

 

[dsm_mike]

Thanks, I guess I will be calling wiseco again this morning. Hopefully I can speak to someone, yesterday all I got was the heavy call volume, leave a message recording.

Well I was able to tall to somebody at wiseco, he said to bore the cylinders out to 3.366 (.020 over) finished hone. He said that the pistons should measure 3.363 and that the machinist's instrument was probably off. This will allow me to run .003 piston to wall clearance as he recommended. So that makes sense now. Thanks for the replies