Piston Pin Offset
 

When I took the Pistons and Cylinders off of my 964-3.6 I noted the direction of the piston by recording what direction I slid the pin, so I'm pretty confidant I know how they came out. Seems to be small (E) marks on the upper sides

The piston pins are offset in the piston, and the offsets are on the high side of both banks. With clockwise rotation I would expect 4-6 To be on the upside, and 1-3 to be offset on the underside.

Maybe I'missing something. How is it supposed to be designed/installed?

Thanks,

Alan

All explained here:

http://forums.pelicanparts.com/showthread.php?t=335700&highlight=piston+offset+th rust

Thanks John
 

Perfecto! :)

The 964 (and many 993) 3.6 engines unfortunately use the same offsets on ALL pistons.

This means that 3 of them have the correct offsets and the others are wrong; its been that way since 1990,.....:)

It practically took an "Act of God" to get Mahle to finally correct this on the latest 3.8 P/Cs'.

Steve, after reading the thread linked above and then your comment, I am wondering if the early cars had side-specific piston offsets. Were the offsets on 90mm Mahles both to the exhaust side or were the offsets working together to produce the same effect on both cylinder banks?

Wouldn't cam timing be slightly different (relative to the crankshaft position) between the cylinder banks if the piston pin offsets were both on the bottom? I suppose that is why cams are timed according to TDC on 1 and 4 cylinders separately.

I do realize .8mm is not going to do much.

The only pistons that I recollect have the same offsets are the 3.6, 100mm ones and most 3.8 102mm versions.

Early stuff like your 90mm ones had side-specific offsets,.... :)

Cam timing is not affected but piston & cylinder wear are affected on the "wrong" side of those P/C sets.

Thanks for the interesting information. :)

So, the 90mm Mahles have the offset to reduce piston slap on both sides? Or is it the greater push on the power stroke?

Indeed, the 90mm Mahles do have offset pins, specific for each side, that reduce both thrust losses and friction on the power stroke,....:)

Piston skirts last longer as well as reduced top ring land wear.

Steve, wow, I didn't know that.

Tom Butler, do you have a photo of the top of a 90mm Mahle handy?