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Where is everyone getting Pistons from for their 931's
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Mike9311  



Joined: 14 Dec 2004
Posts: 986
Location: Chicago-ish

PostPosted: Tue Nov 24, 2020 4:53 am    Post subject: Reply with quote

The biggest issue when looking for alternative pistons for these cars is the wrist pin diameter and compression height.

There are a ton of 86.5 to 88mm pistons out there especially from modern cars but they have really low compression heights and look more like F1 pancake pistons compared to our soup cans. This is also why Dan's looked cool. He either stroked or long rodded his creations (or both with his DProd). This lowered his compression height and therefore overall piston height.

Dan looked and researched like crazy and I have been doing it too as of late since I need a custom config. Our wrist pin size is rare as well as the Compression Height (Center of wrist pin to top of piston). This is why going with a longer manley rod (or other brand of which there are tons as well) is so enticing. Yes, you are forced to grind down the crank rod journals (also allows you to stroke if you want) but you get to use a smaller, more available wrist pin size and other pistons do become available as a choice because the compression height is smaller/shorter. They are not perfect but they become a option.

CarreraRSR those pistons are interesting because the Wrist Pin is smaller at .866" (22mm) vs stock .944" (roughly 24mm). You would need to bush the rod end with Alu-Bronze or similar which is totally doable. Saves some weight in the end

Gegge might chime in, I hope, since I know he has researched this topic to death as well over the years. He always has some interesting take of things
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA M650 Herausnehmbares Dach
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car
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lildude4life  



Joined: 27 Apr 2015
Posts: 24
Location: Milwaukee WI

PostPosted: Tue Nov 24, 2020 5:24 am    Post subject: Reply with quote

I got my JE's via topend performance in CA. Went with 22mm wrist pin's and new pauter rods so I didn't have to deal with the odd wrist pin size. Also could then use standard ARP rod bolts as I know those are hard to find as well.

Only thing I wished I did first was have my machinist verify everything as I think I ended up with a bit more CR than I wanted as the deck height seemed off for some reason once everything was put together.
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"931 EFR" 81 931 S2 w/ EFR 6258 turbo
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safe  



Joined: 18 Mar 2017
Posts: 210
Location: Sweden

PostPosted: Tue Nov 24, 2020 5:33 am    Post subject: Reply with quote

wrist pin size isn't a big issue, you can have the rods rebushed to 22mm.
Compression height is a bigger thing. But there was BMW rods that was a bit longer, it "only" needed some sort of spacers to center them.
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/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, under EFI and turbo conversion
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Mike9311  



Joined: 14 Dec 2004
Posts: 986
Location: Chicago-ish

PostPosted: Tue Nov 24, 2020 9:51 am    Post subject: Reply with quote

safe wrote:
..... But there was BMW rods that was a bit longer, it "only" needed some sort of spacers to center them.


That's right. I remember that being mentioned before
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA M650 Herausnehmbares Dach
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car
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View user's profile Send private message
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