Visual and Spec Differences for Pistons - 67/68S, 69S, 906

[beh911]

I will take a stab at this, giving source info. Seems to be a confusing topic for the current active members on the forum

Please help get this right if you feel you have an authoritative opinion...

Let's start with some charts:

Engine chart highlighting engines in question
(source - B. Anderson's book):


Piston chart with some CR, P/N errors and maybe others (e.g. 906 piston numbers)
(source - http://www.early911sregistry.org/for...l=1#post459256)


Some previous discussions on same topic:
69S P/Cs
http://www.pbase.com/9146gt/my9146gt...9_911s_pistons

906 P/Cs
http://www.pbase.com/9146gt/my9146gt...cylinders_set1

Background discussion from April 2010
http://www.early911sregistry.org/for...-ID-Pistons...

Another from Sept 2010
http://www.early911sregistry.org/for...ston-Corrosion

Recently
http://www.early911sregistry.org/for...of-906-pistons

[beh911]

Source pics:
http://www.pbase.com/9146gt/my9146gt...9_911s_pistons
http://www.early911sregistry.org/for...ston-Corrosion

Quote:
"The 69S piston has noticeable straight sides to the valve pocket. The 67-68 S piston has round valve pockets. Thats the easiest way to tell the difference."
http://www.early911sregistry.org/for...l=1#post459487

Based on the assembled pics in these posts, the larger intake valve pocket on the 69S piston is very noticeable when shown against a 67-68S piston, as well as the 'straight sides' to the valve pockets as quoted above.

69S Piston identified by 80P26E






In the side view it is clear this piston is probably no longer usable. But still a decent reference for profile view.

[beh911]

Source pics:
http://www.early911sregistry.org/for...rs-and-Pistons
http://www.early911sregistry.org/for...68-911S-piston

Quote:
"The 69S piston has noticeable straight sides to the valve pocket. The 67-68 S piston has round valve pockets. Thats the easiest way to tell the difference."
http://www.early911sregistry.org/for...l=1#post459487

Notice the top edge of the piston at the intake/exhaust overlap ridge and compare it to the 69S in the above post.

67/68S Piston identified by 80P25E (needs source reference in form of pic/thread)

[beh911]

Source pics:
http://www.pbase.com/9146gt/my9146gt...cylinders_set1
http://www.early911sregistry.org/for...l=1#post480032

906 Piston identified by 80P26 or 5? or 80P25+ and some accounts of 80P26E
The chart in first post above may contain error (906 = 80P26)

Some debate over holes drilled under piston dome and around wrist pin boss, and whether or not that was done to all 906 pistons

[beh911]

Source pics:
http://www.early911sregistry.org/for...l=1#post414905

906 Piston identified by 80P26 or 80P25+

Some debate over holes drilled under piston dome and around wrist pin boss, and whether or not that was done to all 906 pistons

[beh911]

I am not comfortable yet with the chart showing piston numbers mapping to their respective applications.

My quick research shows that there are too many discrepancies out there. Lots of misinformation and confusion.

[blue72s]

Here is a picture of later Mahle 67/68S set (Nikasil). 9.8 to 1 CR, 160 hp.
But the valve pockets look squarer than the originals. Is there a reason for this?

[beh911]

blue72s,
Thanks for the post.

Is there a number on the underside of the piston? Curious if Mahle is even using the numbers in the chart above.

I don't know the answer to your question but the valve pockets appear 'squarer' than original 68S pistons.

Anyone...

[blue72s]

Is there a number on the underside of the piston? Curious if Mahle is even using the numbers in the chart above.

The pic is not mine. I downloaded from the web.

For ages, I have been trying to find answers about the squarer valve pockets but to no avail. My guess is that the pockets are '69S spec (diameter, not depth).

[blue72s]

Some pics of 81mm pistons (not mine)

http://www.early911sregistry.org/for...1&d=1322152431

http://www.early911sregistry.org/for...1&d=1322152954

http://www.early911sregistry.org/for...1&d=1322152509