Alan G.

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

Bill Verburg

the Mahle design as used in the air cooled 3.6 and 3.8 has the wrist pin o/s 1mm but doesn't diofferentiate between the L/R sides. One side the o/s helps the other side it hurts.

There are supposed to be aftermarket versions w/ correct L/R o/s

Alan G.

John Cramer covered it really well in a cross post on Pellican Engine Rebuilding Forum.

Bill Verburg

Hmm, I didn't know it was an issue w/ earlier engines. I had thought that only the 964 & 993 suffered from this.

Geoffrey

The design is flawed in the pistons including RSR 3.8 pistons.

Bill Verburg

That was the first place I ran into it.

ras62

thread resurection time.....Why cant the piston be turned through 180deg on the problem bank or is common practice?

Steve Weiner-Rennsport Systems

The intake & exhaust valve pockets are a different size to accommodate the different diameter valves.

In the past, we have rotated 3 pistons and simply machined the intake pockets to suit, however you lose a bit of compression on that side which is undesirable in any performance application.

For normal street use, I doubt anyone would be able to tell a difference and correcting the offsets does yield benefits in piston/cylinder wear.

ras62

Cheers Steve, I thought the standard 964 piston crown was symmetrical. So aftermarket pistons with an increased CR and correctly sided pins would be a sensible and fairly inexpensive mod.

Steve Weiner-Rennsport Systems

Mahle finally corrected this and I believe all of the new sets have different offsets for each side. I know all of our custom pistons Mahle makes for us have proper L/R offsets.

Certainly, one can use an aftermarket piston, however the 2618 ones not last anywhere near as long as the Mahle pistons.