924Board.org

[letsgomattmoore]

I just got a 931 as a project car last week. It's my first venture in Porsche's I don't know much about them. I got it with the whole engine and everything out of it. The only thing I can see I need right now until I paint it and put it back together is pistons. I found a 924 that is mostly put together in a nearby junkyard. I could get them for a reasonable price and would rather do that but I don't want to get them if they don't work. If anyone can help me with this it would be greatly appreciated.

[ideola]

No, the normally aspirated pistons have a much lower compression height to accommodate the heron (flat) head of the 924; the 931 pistons have a higher compression height, with a dish, to accommodate the 21.5cc of combustion chamber dished into the head. Using NA pistons on a 931 would result in something like 6:1 compression ratio.

If you need used 931 pistons, they are fairly easy to come by. I probably have at least one or two sets available for sale, and I know there are others here as well that have extras.

But before you commit to buying ANYTHING, you need to have the cylinder bores inspected by a competent machine shop to see whether the cylinders will require an overbore. If the cylinders are OK, then getting used pistons with a fresh set of TotalSeal rings (which I can supply at ~$241 per set) will be your cheapest option. On the other hand, if the cylinders need to be bored out due to scoring, damage, or being beyond the wear limit, there is no cheap option; oversized OEM pistons are nearly unobtanium; I can provide new pistons from Diamond in any bore and compression ratio you want @ $695 per set, which includes new rings, wrist pins, and retainers; this will be your least expensive option if you need to bore out the cylinders.
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erstwhile owner of just about every 924 variant ever made

[letsgomattmoore]

Thanks. When I get everything a little more put together I will be searching more for pistons but the cylinders aren't scored at all (surprisingly).

[Raceboy]

Euro NA pistons can be used on 931 and CR will not be 6:1, but rather 7.5:1.
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

[Carrera RSR]

I am not so sure about 87mm pistons being unobtanium Dan. I have been quoted for a set from Mahle at £1150 and know of someone else who has just ordered a set from Porsche at £1800!!

I don't know of the numbers in storage with Mahle but they are out there. Just have to dig dig in your pockets if you want OEM.
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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

[Raceboy]

I can't find any reason to buy a set of oversized pistons new from Porsche for £1800, beats me.

If the block is toast, one could source a 2nd hand block for like £50 and get a STD size pistons for much less.

Maybe it was a conours car that had to have matching numbers and had to reuse the same block and had very deep pockets? Oh wait, that was already one reason
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

[924RACR]

Hey, Ballston Spa! I grew up up the road in Saratoga... welcome to the board!
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Carrera RSR]