broke my engine, need .020" pistons...

L

Lael Cleland

New member
Hi! I had a ton of fun blowing it up, chacing down a camaro W/Ls1, in an ITB rabbit, he couldnt shake me.......exept on the straights.... I failed to look at my temp gauge, oil psr ect.......I ripped a motor mount, witch caused a coolent leak........

So here is the dilemma, the cyll head is 5.25in not warped....but if I punch out the cyll to .020 over, will that raise compression over 9.0:1......I can get standard pistons cheep, like $100 w/o rings, .020 overs are $400 w/rings......What do I do?

(A)I have 10-15 JH engines, should I take them all apart till I find one w/ good bore?
(B ) throw a used bottom end in it?
(C) go .020over and cc evey thing and find i have 9.3:1
(D) buy a boat and go fishing, ie. learn a new hobby.......?

Thanks
Lael
 
When oversize pistons are manufactured, the designer automatically moves the piston pin so that the compression ratio stays the same. It's an industry standard. So, unless they specifically state a different compression ratio, you can safely assume it's been adjusted back to standard.

Techtonics Tuning has historically been one of my favorite places to buy aftermarket performance VW parts. Dunno if they still supply Mk1 engine parts...

GA
 
Greg Amy said:
When oversize pistons are manufactured, the designer automatically moves the piston pin so that the compression ratio stays the same. It's an industry standard. So, unless they specifically state a different compression ratio, you can safely assume it's been adjusted back to standard.

Techtonics Tuning has historically been one of my favorite places to buy aftermarket performance VW parts. Dunno if they still supply Mk1 engine parts...

GA
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I'd always heard the opposite, when you got factory oversize pistons it raised compression. Just saying greg are you sure this is really a industry standard thing cause thats not what i've heard. no personal experience.
 
frnkhous said:
I'd always heard the opposite, when you got factory oversize pistons it raised compression. Just saying greg are you sure this is really a industry standard thing cause thats not what i've heard. no personal experience.
Click to expand...
Yup, I know that for a fact, and from experience.

Consider if the opposite were true: now you have engines out there with potentially higher-than-expected compression, potentially subject to octane (not enough octane), timing (too much timing), fuel delivery (too lean) problems. The liability for a parts manufacturer for offering something that does not comply to the original design is too large, even if we're only talking a couple of fractions of a point. And since you're redesigning a whole casting anyway, it's a simple math equation to move the piston pin a few fractions of a millimeter in the process.

But, take that for what it's worth, and what you paid for it. I would strongly suggest that you contact your supplier, and ask THEM specifically. - GA
 
Thank you very much Greg, I have know worries now. the pistons going in are the same manufacture as the ones comming out. TT has no 8.5comp pistons on there site, I will call Collin monday.
 
Lael,

Sorry to hear about your motor. If Colin can't help you out, try Dick Shine or Stu Brummer (BSI in Fla.). Stu does mostly Miatas now, but he may still have some old VW stock hanging around.
 
Lael, I have a set of 8.5 compression pistons I took out of my GTI when I converted it to to Production.

But with that many engines, it looks like you just need to dis-assembly a couple, and use what you already have.
 
The 1.8 goes best with the deck at 15 off and the PTW # @ 004. Gapless rings and it will fly. It will come out vey close to the . 5 compression add.
I have not seen many stock bore engines that are unusable. I get them setup per above #s. The required hone is usually about 1.5 on a high mile engine. If the thrust bearings, are good, the bore will be good. IMHO, of 50 or so race VW engines. MM
 
If all you can get are standard pistons, you can have the damged bore block sleeved.

I would just go with the over bore.
 
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