Pelican Parts Forums - Major cilinder damage 50 miles after rebuild

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- - Major cilinder damage 50 miles after rebuild (http://forums.pelicanparts.com/911-engine-rebuilding-forum/1170667-major-cilinder-damage-50-miles-after-rebuild.html)

Must admit, after seeing that I am happy I bought Mahle P/C's, complete with rings and pins - but will still check every ring gap.

I keep on getting the idea that a little more ring gap is far better than too tight - within reason of course, as the penalty for too tight is what the OP had happen, while too loose may be just a little more blowby......

D.

Quote:

Originally Posted by stownsen914 (Post 12366055)

.3 mm gap might be a bit tight for aftermarket pistons in a boosted 911. For the CP pistons I'm putting in a 3.5L NA race motor, I believe I ended up around .018-.024", going from memory. That's on the wide end of the spec they recommended for an aircooled race application. I figured better slightly too large than slightly too narrow.

Remeasuring after damage actually has the rings at 0.6-0.7mm (0.024" - 0.027"). Obviously they took damage but I will make sure to measure the new set in full. To be clear, it's not boosted. It's a 3.2 converted to 3.7 using 80.6mm crank and 98mm pistons.

Quote:

Originally Posted by Henry Schmidt (Post 12366068)

I'm still thinking this is a ring issue.

Thanks, Henry! All was measured and was OK. We measured cause initially we wanted to use a set of Goetze or Mahle but since they did not fit we went with what was supplied with the pistons from JE. I did not order directly from JE but through a German Porsche specialist called Mittelmotor. Although one particular thing I remember is that both top rings are the same (both 1.5mm). Looking through the Mahle catalog I see this is done as well for the turbo but top of mind didn't think it was so for NA applications. Not sure why. Do you happen to know?

Quote:

Originally Posted by dannobee (Post 12366104)

The variables remaining are the "tune" and an overheat condition. Are you sure that it didn't run lean? Are you sure that there's not a problem with fan? Are all of the baffles still in the fan shroud? Are the piston squirters installed and working properly? Were they tested when the engine was apart? Is the ignition timing correct?

Many people aren't aware that pistons are not truly round, but more an oval shape and are cam ground, not just finished on a lathe, so that they're round when at operating temperature. And the crown (above the top ring) is significantly smaller than the skirt, often 0.030" or more smaller. This extra clearance is needed because the crown gets significantly hotter than the rest of the piston when running. What does all of this mean? Take a look at the piston in the pic. Galling down past the grafal coating, galling above the top ring. That thing got hot.

Thanks! To the best of my knowledge we did everything right. The engine is running on the Motronic but with a Steve Wong chip he designed to the specs of my setup. We
continuously monitored oil pressure as well as AFR. Piston squirters check, the engine bay looks like 'new' meaning fan works, all shielding is there, etc.

I didn't exceed 3500 rpm neither. The overheating is a good lead and I'll watch your video. Initial thought would be it's just a result from the friction and damage to the cilinder wall which then damaged the piston sliding past.

http://forums.pelicanparts.com/uploa...1732988250.jpg

JE states there’s chrome and nickel in their pro seal rings wonder if this attributes anything to cylinder scoring? Or if it’s a combination of chrome and recommended tight ring gaps? This has happened to more than a few on the forum while others say they had no problems. JE doesn’t seem to change anything for some reason. I’m sure this issue has been brought to their attention more than once.
It would be nice to get a definitive answer.

LT86 I did not see you note what your ring end (1st and second) gaps were set to initially. What was that? You noted .3mm, then corrected that ? You need minimum .004" per inch of Bore dia. top, and 2nd. Measuring now w the cylinders seemingly scraped away w a significant portion of the Nikasil is not useful.

What was the piston to Cylinder wall clearance set at?

Grafal is a Mahle Trade name product, JE uses something else, these are not consumable break-in expiring coatings. ALL of it being gone in no time at all makes me think you had way too little clearance here.

Lastly, and most importantly, are you certain you didn't receive a type of ring made for Ductile Iron cylinders, chromed face etc? That would eat that Nikasil coating...like shown. This happened to a buddy recently.

There have also been many reports recently, last 5-6 months, of the Nikasil type coating coming off the cylinders after being Bored and replated by the Co. that does the vast majority of this work in The USA. Several engines not making is past the first dyno or event. Were your cylinders new Mahle?
or?

Last thing; where do you think all that scraped away Nikon coating went ? Yep into the oil and through all the bearings , everywhere. It would be foolish to not split the case and check everything else out thoroughly. It takes a diamond hone the finish the cylinders, so being just a little shy of a diamond in hardness, it could heavily wear many other parts when suspended in oil. If you saved any oil send a sample to the lab to see what is in it. If not extremely clean and "normal" your report, toss the oil coolers, get new ones, and OCD clean and disassemble the entire oil system. Bad ass that sounds, you don't want to do this a 3rd time.

Cruising at 14.0 A/F plus is not a way to break-in your rings( That is an entirely different subject for another time)... Did you do any hard pulls to 3500 (your predetermined limit)? What AF were you seeing w your Wong chip(clearly not done on a dyno ) guess ?

Kevin
GAS Motorsport