3.8 Pistons - Machine and Flip required?
 

It seems that the RS/RSR pistons are delivered from Mahle with a 1mm offset in the piston pin location. Some (maybe all) engine builders have found that running the pistons as delivered will produce a small difference in where TDC is achieved with on cylinder bank vs. the other. To counteract this, they machine the piston top to prevent interference with the intake valve, and then flip one bank of pistons "upside down" to match the offsets from side to side.

Not sure I got that exactly right, but what have the groups experience been?

Chris

Chris:

All of the 3.6-3.8 Mahle pistons have the same pin offset. This means that 3 of them are correct and 3 of them are offset in the wrong direction in each engine.

Since these engines do not have a favorable rod ratio to begin with, we prefer to flip the "wrong" side upside down and widen the valve reliefs on the piston to clear the larger intake valves. Although the compression is a tad less on that side after this is done, the wear on the rings and top ring lands is reduced for less drag and improved longevity.

Thanks Steve - Wow.

So the stock 3.6 pistons have the same offset issue?

Chris

Indeed, they do,...:)

Fastinating! So this is a very nice to have modification, but is not a must if you asked the factory about it.

Did this get changed in later production with the water cooled engines?

LOL,..The factory cuts many corners these days, all the good name of saving money....:)

I've not had a 996 apart, but the GT-3 motors all have the correct offset pistons.

Can you show us some pics?

Regards,

eye candy.

How long before man perfects these to put into 3.2?

So here is a dumb question :)

Is there any merit to machining the small end rod bushing in an offset manner 1mm (90 degrees from rod length plane) to make up for the 1mm offset in the piston?

Chris

EDIT: I think the answer is that the forces on the rod bearing would be uneven...

And I know the 3.2 doesn't go to 3.8. And there are no dumb questions.