Mahle CIS Pistons

[James Renfroe]

I would like to know if any of the Mahle 911 CIS pistons are forged or can I assume that they are all cast. Many 911S models came with CIS. Some Porsche literature state that the 911S models used forged pistons while the lower performance models, used cast.

I have looked at Alusil and Nikasil CIS pistons, they have features of cast pistons yet they came from S model 911s. I’m pretty certain that the RS, and RSR pistons are forged but I have never seen them up close or from the underside to see their forged features.

If there is a positive way to identify a 911 forged piston and a cast piston, I would love to know. I’ve read that the underside of a forged piston looks different, but how.

Please post any pistons pics for my own education!

Thanks!

[Jubbie]

The way I understood it was the MFI/Carb 'S' or 'RS's had forged pistons, but the CIS 'S' had cast.

[YTNUKLR]

I am not so sure...Whereas the skirts on cast pistons come full-circle, the forged ones only have skirts at two sides. The RSR, etc. are all forged. CIS 'S' might be forged because the bottom looks like the early S pistons...but it might be cast just as well. A junk 67S forged piston:

[afterburn 549]

A lot of folks think forged is the way to go..........but alot of us (at least me) dont think so........hypt. ones are probly better for most stuff,,-but then again I do not know what motivated your question.
With forged there is the large clearance issue..the hypt. pistons treated to some ceramic are probly a better choice for most applications.
choices ,,choices........................................

[Wayne 962]

There are overhangs on the inside of the CIS cast pistons - hard to explain, but you have to have zero angles of incidence in order to forge a piston (no overhangs, except where machined).

Hmm, I don't think that's too clear...

-Wayne

[KobaltBlau]

Wayne, I understand what you are saying.

Here's a relevant thread to this one, with photos:

New Variation on Old Question