924Board.org

emoore924 · Joined: 13 Apr 2004 Posts: 2822

Howdy folks. I'm contemplating a project on a 1980 931. I'm interested in freshening the motor but, surprise surprise, pistons seem to be somewhat unobtanium.

I've read a zillion threads and it looks like there are options out there. But none of them have specifics on part #'s, sources or costs. So I'm hoping you can help me out.

Goals: I don't want to do anything fancy necessarily. Probably about 1.0 bar. Cometic gasket. Reusing the stock rods and crank. No head studs, just bolts. Intercooler. Stock CIS. Cost is important. Reliability as well.

What are my options regarding the following approaches to replacement:

Actual part #'s and sources are what I'm after. I will consider purchase of new or used from an individual if someone has something on the shelf they want to sell (or trade! I have lots of cool toofah stuff). I'd also be interested in hearing your personal experiences, good or bad, especially if there's something that really did NOT work out

What say ye?

WEASEL149 · Joined: 19 Aug 2005 Posts: 595 Location: UK, Sheffield

Stock cast 931 pistons are too expensive, difficult to find and by many accounts, not very strong.

If I were you I'd contact EBS and get a set of JE pistons. Ask them if they can get them made with a crown dish of at least 40cc or preferably 45cc to keep CR around stock.
They normally sell them with a 22mm wrist pin but also sell small end bushes at around $10 each.
I think the pistons were $650 last time I asked.

Your problem then is obtaining rod bolts. ARP for stock rods normally requires a group buy and they're not cheap compared to off the shelf ARP2000 bolts.

I'm currently having some sample rods made by a friend of mine that take 3/8" ARP bolts but they'll take a while as he's making them 'on-the-side' where he works and there are about 70 production stages.
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1979 UK 932

WEASEL149 · Joined: 19 Aug 2005 Posts: 595 Location: UK, Sheffield

If you can stretch to the Crower I-beam rods that Ideola offered I'd go for a set of those.
They're immensely strong and I've seen them used a lot in tractor pulling etc. with very high horse power and torque.
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1979 UK 932

emoore924 · Joined: 13 Apr 2004 Posts: 2822

Good info, thanks.

But with Ideola no longer in the trade, who would be the best source for the Crower rods?

Also, that kind of still leaves me with the rod bolt problem, doesn't it? Or do the Crower rods take a stock ARP bolt of some sort?

Carrera RSR · Joined: 08 Jan 2010 Posts: 2312 Location: Somerset, UK

New rods will come with matched bolts.

Another option but may be pricey for US is Mittelmotor in Germany. Off the shelf parts and proven performance and reliability. Pistons, rods, gaskets, bearings etc. etc.
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252

WEASEL149 · Joined: 19 Aug 2005 Posts: 595 Location: UK, Sheffield

Harm · Joined: 02 Apr 2009 Posts: 1378 Location: Holland

1 Vatican Euro (EUR) is equal to 1.06 United States Dollar (USD)
…could be helpful

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Fwiw, some have expressed concerns about je piston material with cast iron blocks. The theory is they need to start a little loose to make up for their expansion, and this can lead to piston slap.

My motor has probably 10k on it at this point, and I can hear a little slap when cold. Probably won't hurt anything...probably. But it can weaken and break skirts.
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

emoore924 · Joined: 13 Apr 2004 Posts: 2822

Carrera RSR · Joined: 08 Jan 2010 Posts: 2312 Location: Somerset, UK

WEASEL149 · Joined: 19 Aug 2005 Posts: 595 Location: UK, Sheffield

Its the 2618 forged alloy that can suffer from piston slap at cold.
Best thing to do with 2618 pistons is buy them with anti scuff skirt coating. This allows you to run a bit tighter clearance safely. The Omegas have it.

Alternative is order 4032 forged which doesnt need as much clearance.

Emoore - approach JE directly. You can then specify exactly what you want and get them made for the stock 24mm wrist pins (which I believe the Crowers were made for IIRC)
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1979 UK 932

emoore924 · Joined: 13 Apr 2004 Posts: 2822

Anyone know about or have used the VW flat top pistons? If so, what's your experience, what are the specs/part #'s for what you're using and where did you get them?

Kenodog · Joined: 25 Jun 2003 Posts: 2669 Location: Vancouver,B.C.

I have a set of the Gene Berg special VW 88mm low compression flat tops in my new motor. The compression calculator says they should yield about 8:1 which is perfect for me. The are the Cima / Mahle 'forged' pistons,(which are really hypertectic cast) should be good as they were made for forced induction motors. They make the engine non interference as well which is a HUGE bonus. Won't be able to tell you they real skinny on them however until I get my bloody car back from the shop !

Leigh
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1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

leadfoot · Posted: Tue Mar 24, 2015 9:52 am Post subject:

emoore924 · Joined: 13 Apr 2004 Posts: 2822

I found something (sorry, IDK how to link to the exact page). If I enter "88mm piston" into the search feature at the top of the page here:

https://www.actionimport.com/

I find three sets of EMPI VW pistons; one 85.5mm, one 87mm and one 88mm.

So I'm thinking the 87mm set would work with little or no machining for the stock block and the 88mm would work if you punch it out.

Any thoughts on (a) appropriate machining, or (b) rings?

Edit: Looks like sequential was banned a long time ago so other sources would be welcome.

Thanks!!