3.0 piston ID?
 

I'm rebuilding a 3.0 that has already had pistons and cyls replaced once. Mahle OE with Nikasils. Is there some way to determine which comperssion ratio I have....8.5 or 9.3? I am assuming the pistons are different, right?
Thansk,
Randy

I believe the difference is in the piston domes. I have a set of 95 mm 8.5:1 Alusils that I could take some measurements on to compare to your pistons.

A 8.5 compression piston will have a dome approximately 29ml.
A 9.3 compression piston will have a dome approximately 36ml.
This based upon a 89ml head chamber volume and 1.0mm deck height.

Thanks Guys.....Tom, I'm not sure how to measure the dome, not knowing how it varies from the 9.3 version...height above wrist pin?
Doug....this is helpful information but without a way to measure the displacement of the dome I'm still in the dark. Any ideas? Isn't there a visual difference that can verify which pistons I have? Part number, casting number, or something?
Thanks,
Randy

Post a picture and I'll tell you what you have.

will do Henry....Thanks!

The part numbers are different for the piston cylinder combinations between the lower CR for '78-'79 and the higher CR for '80-'83. The Porsche PET lists the following:

1979 Schmidt p/n - 930.103.966.0X
1980 Schmidt p/n - 930.103.984.0X

1979 Mahle p/n - 930.103.962.0X
1980 Mahle p/n - 930.103.977.0X

The only markings I can find on my 1978 Schmidt P & C's are cast into the inside surface of the pistons and on the spigot skirts. They do not match either these part numbers or any other reference number listed in the PET.

Not sure if there is a simple visual reference to tell the difference between the different model years/CR.

Here's pics of the pistons in question:http://forums.pelicanparts.com/uploa...1137353766.jpg

Also....the oil rings are 3.5 mm if that helps.
Thanks

Excellent pictures. My best guess from the pictures is the pistons are 8.5:1.
The dome looks to be just under 13mm high. Takes up under half the piston top. Approximately two-thirds of the remaining volume gives me 30ml.
I could be wrong. Henry would know for sure.

For what it is worth... Here's a set of Euro Mahle's 9.8:1 from an '81 engine that I'm curently rebuilding. Lou

http://forums.pelicanparts.com/uploa...1137373195.jpg http://forums.pelicanparts.com/uploa...1137373211.jpg http://forums.pelicanparts.com/uploa...1137373222.jpg http://forums.pelicanparts.com/uploa...1137373233.jpg

8.5 to 1 forged piston fron a Nikasil P&C set.

Thank you Henry....I was hoping for 9.3's but at least they're in nice shape and well within spec.

Henry,

I've hijacked this thread. Can you help me identify my pistons? I have Kolbenschmidt pistons and the top dome looks just like in the pictures of the Euro Mahle's above. The dome has a slight depression where the ++ symbol is in the above pictures. The dome shape looks the same other than that and doesn't look at all like the 8.5 to 1 Mahle's above. The numbers inside the pistons are 095152 and 1275.

Thanks,
-Andy

If it looks very similar to the euro piston but the lowest part of the piston dome is concave then what you have is a 9.3 to 1 80 -83 US piston.

Thanks Henry. I've also looked back through all the classifieds with pictures and I'll try to verify what I have that way as well. The 8.5 KS pistons also have a dome different from the 8.5 Mahles from what I've seen in the pictures but I think mine look more like the 9.3s. Do the 8.5 KS pistons not have a concave section? Mine definitly do.

-Andy