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Craig 930 RS's Avatar
 
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Question New rings on old pistons and cylinders in a 930......

Scenario:

About 40k on a 930. My sense and some experience says this may not be a good idea - at all.

Anyone BTDT, and can weigh in with a 'professional' opinion and advice?
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- Craig 3.4L, SC heads, 964 cams, B&B headers, K27 HF ZC turbo, Ruf IC. WUR & RPM switch, IA fuel head, Zork, G50/50 5 speed. 438 RWHP / 413 RWTQ -
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Old 10-29-2008, 02:42 PM
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What exactly is your question?
Old 10-29-2008, 02:47 PM
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Is to help this guy:
Got the 930 Back from Shop...

Re-ringing old P&Cs......good,bad or 'doesn't matter' - opinions based on experience is wanted
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- Craig 3.4L, SC heads, 964 cams, B&B headers, K27 HF ZC turbo, Ruf IC. WUR & RPM switch, IA fuel head, Zork, G50/50 5 speed. 438 RWHP / 413 RWTQ -
"930 is the wild slut you sleep with who tries to kill you every time you "get it on" - Quote by Gabe
Movie: 930 on the dyno
Old 10-29-2008, 02:49 PM
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Craig, I'm only writing because I made the mistake...
I would say $hit-can the old pistons for new units because I reused my 40-50,000 mile pistons on my motor with new rings and it was not so great. On my cylinders, you could see at TDC and BDC where the rings had "extra"movement. There is defined lines where the rings were bouncing or getting smacked by the piston in the opposite direction. As for the cylinders, I would have someone (much better than I) measure them and re-use them if in spec. If the cyls are re-usable, just scuff them with a gray scotch pad. I would definitely make ash-trays out of the old pistons though.
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Old 10-29-2008, 06:08 PM
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Regarding re-use philosophy:
Right on, I believe this is what Engine Builders Supply would tell foks - it's a fact that cylinders are rebuildable, but pistons typically are not.

This IS what EBS told me over the phone regarding my used 3.4 P&C set from *my* 930 -
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"930 is the wild slut you sleep with who tries to kill you every time you "get it on" - Quote by Gabe
Movie: 930 on the dyno
Old 10-29-2008, 06:26 PM
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From a pure machinist point of view. Moving parts wear into each other, from the time they are fresh & new, till the time they need replacing. No matter what style of engine you have, there is always a thrust side to the pistons & cylinders. The shallower the angle between banks of cylinders the greater the side thrust load. So simply put, a V8 has greater side thrust loads on the piston & cylinders than a flat 6 has. But none the less, all have side thrust loads due to the angle of the connecting rod vs the straight line of piston travel.
Therefore, rings will wear into the piston ring lands, and to some degree alter the lands profile, but they wear as a mated pair.
Cylinders also are subject to this very same side thrust load wear pattern, coupled with the wear pattern the rings leave at TDC & BTC. Yes the cylinders can be sanded, honed, bored, blah, blah, blah, in order to remove these wear patterns, and then effectively placed back into service.
The same can not be said for pistons, once altered by wear & tear, there is no going back.
Kinda like re-using a condom..sure it's a little stretched out, sure is already been up-n-down & in-n-out, sure it might not tear or break...but is it really worth the risk?
Old 10-29-2008, 08:01 PM
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Quote:
Originally Posted by full quack View Post
Kinda like re-using a condom..sure it's a little stretched out, sure is already been up-n-down & in-n-out, sure it might not tear or break...but is it really worth the risk?
No worries there- I change tires regularly..
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Old 10-29-2008, 08:13 PM
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Quote:
Posted by full quack

The shallower the angle between banks of cylinders the greater the side thrust load. So simply put, a V8 has greater side thrust loads on the piston & cylinders than a flat 6 has.
Care to expand on this? I've thought side loading to be a function of rod length to stroke ratio.

John
Old 10-30-2008, 12:34 PM
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Jti, I completely agree, this ratio is the prime contributer to side loading. The longer the stroke & shorter the rod, the more side loading would occur.
As I was taught in school, the increased side load of a typical V8 had something to do with the operating angles between the 2 banks of cylinders, which differ from a flat opposed or in-line engine design. We were shown several V8 blocks(chev.) that had severe ring ridge wear @ TDC, and then shown several in-line 6 (chev.)and flat opposed engines(VW) showing very minimal ring ridge, all with about the same mileage on them. All us students simply nodded our heads and accepted it as gospel. Hummmmm.
But now you have brought doubt to my mind, I would really like someone to VERY clearly explain the V8 vs flat opposed/in-line to me also.
When I was taught about this 30+ years ago, it was much clearer in my mind...damn old age!
Mark
Old 10-30-2008, 01:22 PM
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FWIW, when I turbo'd my 3.2 Carrera, I bought a second hand set of low mileage 930 P&C's, got my mechanic to measure them to make sure they were within tolerances and he re-ringed them with new rings.

Haven't had any problems for years and the car is a blast to drive. At only 1.0bar boost, I've toyed with, and pulled away from a new (stock) 997TT numerous times... not bad for a 20 year old car!!! The driver of the 997TT looked pretty shocked by the look on his face, but gave me a friendly wave

If I ever do a top-end rebuild, I'd probably scrap the original pistons and get some 7.5 or 8.0:1 forged JE's custom made and slip them in for a bit better pre-boost response and still limit the boost to 1.0bar... it's plenty fast enough for street and can surprise the latest TT's out there

I just love this GT35R turbo!

Last edited by JMA-RUF; 11-06-2008 at 08:42 PM..
Old 11-06-2008, 08:31 PM
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