Wrist pin off sets
 

Hi all: I need some advice to determine how to orient pistons based on wrist pin offset. I know this shouldn't be an issue but my new Mahle 3.8 pistons have both valve pockets cut to the same size...for the exhaust valve. I've had a set of larger intake valves installed in the head to bring the heads up to near RS specs. I knew I had to get the intake valve pockets cut to clear the bigger valves but since both pockets are cut to the same size, I'm unsure which side I need to enlarge. I've read that you need to orient the offset one way on the left bank and another was on the right bank....and that you can play with these offsets to get more power (at the expense of longevity). I just need to be educated on this so I can get on with my project. Does anyone have any thoughts here?

TIA

Felmir

There is an offset on the pistons, and they need to face the flywheel in order to be correct. I read this in the Porsche factory manuals. For example, on my Mahle 3.4 turbo pistons there is a symbol in the middle, and to one side, on the top (face) of the piston. The manual told me this symbol needs to be the closest to the flywheel in order to have the correct pin offset. This would make the Left and Right bank pistons to sit opposite in the cylinders.

I will let the experts weight in on getting more power out of it

Felmir:

Mahle makes all the 3.6 (100mm) & 3.8 (102mm) pistons with one offset and that means one side is correct and the other side is not. :)

If you position the incorrect side to correct the offsets, you need to machine the piston domes to clear the intake valves, especially the 52mm RS ones.

We do this on our bigger engines to extend piston & ring life, not to mention a tad more potential HP and a bit less heat.

Thanks John: My pistons do not have any markings to indicate reference to teh flywheel. Ted Robinson at German Precision also checked them and could not locate any reference marks. Since the valve pockets are the same size (exhaust valve size), I'm thinking that Mahle may have left it up to the builder to determine how they wanted to orient the off set and that's why they did not cut the intake valve pocket.

Avery,

Do you want the engine to be quiet or make more power?

Here is a prior discussion I started that describes the role of offset. I suppose you would measure the offset by inserting a pair of piston pins in the wrist pin hole from the inside out, set the piston in a v-block so it doesn't move, preload the dial indicators on each side to a given value with the tips in contact with the pins, then flip the piston over and measure the difference. That does introduce error from not having the pins level, but it seems easier than trying to measure inside the piston.

Here's the thread I mentioned.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/335700-piston-pin-offset.html?highlight=offset

Pin Offset
 

The Mahle problem really shows up in the larger bores. If a person just wanted to know how the pistons should be oriented in an Automotive application regardless of make. The offset should be in the oposite direction of thrust. That is, in a clockwise rotation (Porsche) 123 should be down 456 should be up, that is the pin offset as designed. Trust me don't go by the marks on a 102mm.

regards

Hi Felmir,
The pin offset is determined by the direction of crankshaft rotation only; not by which bank of cylinders they're in. The marks should be correct as they relate to the offset only. Mahle machined the pockets the same because the exhaust pocket of a piston on the right side would become an intake pocket if installed on the left. if you open up your intake pockets for larger valves you must dedicate three pistons for each side and open the opposite pockets. You will then have three lefts and three rights that are not interchangeable. If you feel like chatting give me a call; I think you have my number.
regards,
Phil (in Santa Cruz)

Thanks!!!!!
 

Based on all your feedback, I have what I need to get going. Thanks a lot. I'm headed to German Precision now.