Major cilinder damage 50 miles after rebuild

[LT86]

Hi everyone,

Looking for some guidance in trying to find the root cause here.

My engine has been fully rebuild from the ground up and had perfect compression ratio, oil pressure and overall drivability during the first miles. However, after only 50 miles a knocking sound appeared which had me worried so it was torn apart again only to find major damage to the brand new JE pistons and Mahle cilinders. Only 1 cilinder suffered from compression loss but I'm amazed the other ones had not already as well...

The JE piston set included a special order compression ring set which came in the package. I'm thinking along the lines of compression rings delivered for cast iron blocks instead of Nicasil coated cilinders but again... puzzled by this.

Anyone has an idea? I'm looking at buying a full new set of pistons + having to re-do all cilinders.







Kind regards

[gled49]

Judging from the striping, I’d say the oil rings are the culprit. I stoped using aftermarket pistons but when I did, I always had the ring groves cut for factory cast rings. I think more responses are to come.

[Henry Schmidt]

Having seen this type of damage before I would suspect that the rings were gapped incorrectly. IE: not enough end gap.

[snbush67]

Can you post a picture of the piston with the oil ring installed that caused that damage?

….total seal ring?

[LT86]

Originally Posted by gled49

Judging from the striping, I’d say the oil rings are the culprit. I stoped using aftermarket pistons but when I did, I always had the ring groves cut for factory cast rings. I think more responses are to come.

I considered this initially but we were not able to use the factory spec rings as the groves were indeed too small (top of mind around 1.5mm /0.06inch) and the machinist was reluctant to make the cut as the surface is so small as well as damaging the pistons while locking them in the lathe.

I can take better pictures if you'd like to see a specific part of the piston/ring combination.

As you can see, the piston has side damage all the way to the top and the black coating has already rubbed off on the side. I've also added a picture of the pistons new.

[Dpmulvan]

What did you set gap at? I just installed a set of JE pistons and one cylinder is already screwed up just from setting timing. After clearance checks I tore it down and noticed some scratches. And I used large gaps and carefully measured and remeasured. I won’t be using them again for sure.

[gled49]

That type of 3piece oil ring, especially if they have chrome edge, will ruin a Mahle cylinder. I had cylinder striping with other than cast top rings also.

[snbush67]

Piston is too big. Installed within the manufacturer clearance specs?

[Dpmulvan]

JE knows of this issue and has done zero to correct it. Still selling piston and ring sets for air cooled 911’s. I read all the threads and was careful to set larger gaps et. Now I’m out at least a couple of grand if not more. Buy mahle and goetze.

[Dpmulvan]

Originally Posted by gled49

That type of 3piece oil ring, especially if they have chrome edge, will ruin a Mahle cylinder. I had cylinder striping with other than cast top rings also.

Glen what did you end up using? I’m assuming I’ll have to put these on the lathe?

[LT86]

Originally Posted by Dpmulvan

What did you set gap at? I just installed a set of JE pistons and one cylinder is already screwed up just from setting timing. After clearance checks I tore it down and noticed some scratches. And I used large gaps and carefully measured and remeasured. I won’t be using them again for sure.

Set to 0.3 which should be ideal. Thanks for the insight!

Originally Posted by snbush67

Piston is too big. Installed within the manufacturer clearance specs?

I disagree the pistons are too big. Special order 98mm JE in combination with new Mahles. One cilinder had a little mishap during installation so was measured again by specialist company for the piston and received new nicasil coating.

I suppose I have to re-order the same JE pistons which match my 3.7 setup but have the ring surface set to have clearance for Mahle rings which should be a bit more than 4mm instead of 3.X which it has now.

[gled49]

I only used goetze rings and stuck to piston diameters that they made rings for. No fancy diameters. Ordered rings first then pistons, not any more, just use Mahle now, I’m old, tired and need to keep it simple.

[Dpmulvan]

Well I solved my problem, what I thought were scratches in the nikasil turned out to be raised areas like smears instead of scratches. Never used these pistons or rings before and was certainly paranoid after all the failures I read about. Now just wondering how the smears got there, maybe oil combined with ring coating I dunno never seen it before to this extent.

[Dpmulvan]

Originally Posted by LT86

Set to 0.3 which should be ideal. Thanks for the insight!



I disagree the pistons are too big. Special order 98mm JE in combination with new Mahles. One cilinder had a little mishap during installation so was measured again by specialist company for the piston and received new nicasil coating.

I suppose I have to re-order the same JE pistons which match my 3.7 setup but have the ring surface set to have clearance for Mahle rings which should be a bit more than 4mm instead of 3.X which it has now.

No don’t set at .3 I never said that.

[stownsen914]

You're getting advice from some pretty knowledgeable folks here, so hopefully the second go 'round will be more successful.

I didn't see you mention the piston to cylinder clearance. Some run it pretty tight on normally aspirated 911s. I left it a little larger like .002" for a race application. CP pistons in my case. For a boosted application you'll want larger clearances all around.

[LT86]

Originally Posted by Dpmulvan

No don’t set at .3 I never said that.

No, I'm not implying that you said 0.3 but the gap is important! I set at 0.3mm which is within Porsche specifications.

Originally Posted by stownsen914

You're getting advice from some pretty knowledgeable folks here, so hopefully the second go 'round will be more successful.

I didn't see you mention the piston to cylinder clearance. Some run it pretty tight on normally aspirated 911s. I left it a little larger like .0025" for a race application. CP pistons in my case. For a boosted application you'll want larger clearances all around.

Thanks! Will report out when everything is back together again. Gameplan is to modify the pistons for Goetze rings, re-do the nicasil coating for the cilinders and put the engine back together again.

[stownsen914]

.3 mm gap might be a bit tight for aftermarket pistons in a boosted 911 (never mind the boosted part, my mistake). 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.

[Henry Schmidt]

I'm still thinking this is a ring issue. Before installing the rings, did you verify the radial depth. I recently got a set of after-market pistons (highly respected) and upon inspection noted a radial depth inconsistent with our ring selection.
This company generally uses NPR rings and we indicated on our purchase order that we were using Goetze rings. The PO indicated ring width, radial depth and stack positioning (I have some old school loyalties) but I guess the design engineer missed my notation. No real harm because we caught the issue before assembly.

Good rule of thumb: Sometimes new parts don't work!

[dannobee]

While some of the cylinders show no wear above the top ring, the piston in the pic shows otherwise. The grafal coating on the piston skirt is all gone, but more troubling is the piston galling above the ring package. The grafal coating is there as sort of a safety measure; it will rub off as needed to optimize the piston to wall clearance, but should never be completely worn off, even on very high mileage engines.

Let's say for the sake of argument that you and your machinist/engine builder did everything right; ring gaps were measured and are at or above specs, piston to wall clearance is at or above specs, and, as Henry pointed out, the rings are correct for the pistons.

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.

https://www.youtube.com/watch?v=aCr9iltAx2w

[stownsen914]

Henry makes a good point about measuring the ring grooves, including the depth. I had a similar issue on a set of custom pistons. The manufacturer had made a mistake. A set of custom top rings was the fix.