924Board.org

[Sharky]

Hi All,
I have a 81 931 Carrera GT Replica (It was like that when I bought it from Sydney and drove it to QLD)
After some 2 years of fun, and eventually some serious smoke from oil leaking out of the turbo feed and burning off on the turbo, I decided it was high time to rebuild the engine.

After pulling a piston out to check the numbers, it read:
87.6I
500
0286
STD

When I had a look at Ideola's Garage for some new Diamond pistons, the sizes are:
87.00mm
87.50mm
88.00mm

Which of those sizes is right for my 931 bore??

Forgive me if this sounds like a stupid question, but I'd rather ask a stupid question, than make a stupid mistake.
I am a radio and telecommunications technician, not a mechanic, so when it comes to spending $600+ on some new pistons, id rather get the right ones.

Many thanks for the time taken to read my post.

Sharky.

[WEASEL149]

Bore the block for 88mm pistons.
Make sure you discuss details with Ideola first though. He will point you in the right direction.
_________________
1979 UK 932

[Carrera RSR]

JE make a range of turbo, GT and GTS pistons. I know owners already using them. They (and Diamond) will make a piston to your compression ratio and bore size, Dans/ideola garage suggestions for 87, 87.5, 88mm are a guide. You bore the block to match the pistons, not buy pistons to match the block (except that the new piston must be larger than the worn bore to re machine). Given you have 87.6?mm pistons already (are you sure about that?) you will need larger diameter. The block will need to be bored to match your choice of piston/piston ring. The machine shop should only bore once they have the pistons in hand to avoid errors. 88mm is roughly the maximum you can go in the block, hence making sure the 87.6mm is correct on your current pistons.

Those pistons, if in reuseable shape, could be sold on or used in another block which needs boring from the stock 86.5mm to the 87.6mm

I am sure Diamond pistons are being used in 931's (Dans's Clubsport) but as yet I have seen no feedback good or bad regarding their performance. I do know of good feedback for several GT and GTS builds where power is strong. Dan - you may want to enlighten us of your current engine in the Clubsport, any improved performance with your Diamonds? Any dyno results?

Alternative supply would be Mittelmotor in Germany who supply 931, GT and GTS pistons http://www.ebay.co.uk/itm/Satz-Rennsport-Kolben-Porsche-924-Carrera-GT-GTS-GTR-Turbo-/181256542446?pt=DE_Autoteile&hash=item2a33bb58ee contact directly as their ebay price includes ebay and paypal fees.

You also need new bearings, very hard to find, although there is a delayed group buy on here. And you need new rod bolts, again Dan has had previous group buys for ARP fixings here.
_________________
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

[Sharky]

Hi Guys,
Many thanks for the replies.
I got my mechanic to double check the pistons and it looks like I was wrong, they actually say 87.0 not 87.6....

If I change the bore side to 88.0mm, does that mean I have to get new conrods as well as the new bearings?

Cheers again.

[!tom]

[Sharky]

Thanks for clarifying

[WEASEL149]

[ideola]

I have Diamond pistons in both the Club Sport and the 941. Neither has been dyno'd, so for the sake of objectivity, I'm going to stay silent on the performance increase because I can't back it up with numbers. Also, both of those motors have significant additional power adders, so there's no way to say how much the pistons themselves affect the overall package.

Just bear in mind, they are a more modern design, they are forged so ~40% lighter than the stock cast pistons, they have coatings for improved detonation resistance, better ring design than OEM, better dish design, better quench, and on and on. In my mind, a no-brainer when you consider the price includes new rings, new wrist pins, and new wrist pin retainers.

Agree with the above, if the current pistons are 87.00, have your machinist check the bores against the wear limit specs. If you send me an email (dan DOT beckett AT ideola DOT com), I will forward you a PDF version of the 924 tech specs booklet which has all of the specs your machinist will need.

We can get the Diamond piston in ANY bore size you want...so if you wanted 87.25mm, we can do that, no problem. My counsel has typically been to punch out as far as you can to gain displacement, so 87.5mm or 88.0mm is what most customers have selected. I've sold probably 30 sets of these pistons since I became a specialist for Diamond, and have not had any complaints or reported issues. I know of at least three guys on this forum that are using Diamond pistons in their ITB track cars.

You will not need new rods, as we spec these out with the same size wrist pins, and we spec out the location of the wrist pin to work with the OEM length rod.
_________________
erstwhile owner of just about every 924 variant ever made

[Sharky]

Thank you guys,
Weasel, I was hoping to avoid re-using the originals. I figure if we have the motor out now, I might as well go ahead and replace them because I don't want to have to pull it all down again just to do the pistons. I am going the whole hog on this one.

Dan, I don't exactly have a machinist/engineer... I live in the desolate inland area of central QLD. I Am guessing that I might have to send the block away, but I'll see if I can find a reputable mob to assess the block for me.
Also, You have my email man (psynewaves) If you can send me the tech specs, that would be very helpful to my endeavors
As a side note, I'll find out what I can with the Turbos as mentioned in my other thread, and once I can get the bore inspected/machined I'll email you whatever information I have got and we'll talk about getting some of those ceramic/teflon coated Diamond pistons.