Piston Ring Failure Diagnosis

[Rick Finsta]

Engine is an old (12 years?) street/strip 470hp, <6500RPM engine. Found one cylinder with high leakdown (43%) and pulled things apart to take a look.

Top ring used to be barrel-faced? Total Seal Classic Race CR0190. The face is smooth, not scored, so I don't think this was abrasive wear? My guess is either light detonation or fuel washdown only because those are the most likely to have occurred in this motor. Second ring looks fine to me but the wear pattern does get a little taller right at the gap. Oil ring looks to be fine.

Light scuffing of the skirts but I run pretty big piston to wall for a 4032 alloy (like 0.007-0.008" I think). Cross hatching is still visible in the cylinder and everything looks good and smooth there.

Any ideas? The motor is not getting money poured into it so it's getting one ring and then getting beaten like it owes me money, but I was curious as to what you can tell after a failure looking at the ring. Anything else I should measure or photograph?

[JoePorting]

If the wear pattern was different at the gap, that's telling me the ring must have butted together. What's the ring gap now?

[Rick Finsta]

I'll have to pull them off the piston and check. I had them at the factory recommendations for street/strip, no nitrous so I think it was 0.0045" per inch of bore? I'll get them back in the bore this evening and get a measurement.

[Newold1]

I would guess you took time to check for any compression loss from the valves?
Did you remove another piston from an adjacent non-leaking cylinder and compare ring wear and such?
If those pistons are 4032 and you've go bore clearance at .007" to .008" for a piston that's normally set for .0025" to .004" that clearance would be expecting a little barreling of those rings with a certain amount of piston rock and cold engine slap. You seem to indicate the clearances are not an issue and obviously if this engine is twelve years old it took a while for this condition to show. If I am viewing the top ring correctly as a moly ring it looks as though there is definitely a top and bottom of ring moly containment area in the middle section of the ring. If that's the case the ring surface wear does not even look great enough to take off the moly area from wear and I don't think a Total Seal plasma moly top ring is barrel faced when new??
When you ran the leak down test did you take a moment to shoot a little oil in the cylinder and retake the test on that cylinder?
Just a few thoughts and questions to help with a diagnosis.

[fdicrasto]

I say that the 43% leakage is not attributable to the rings, at least not based on what I am seeing.

[Rick Finsta]

JoePorting, the top ring had opened up to 0.029" gap, the second ring was at 0.019" which is only opened up about 0.001" from where it was set during initial assembly. I'm not sure how much wear you should see on the rings relatively speaking so I'm not sure if that means anything in this case but that seems excessive to me. I may be all wet on that.

Newold1, I had actually been checking compression and this cylinder still checked right with all the others, BUT that is just spinning the motor over five times and getting out to read the gauge. I'm guessing that the first pump would have shown something. I did not repeat testing squirting oil in the cylinder but the boroscope showed a good bit of oil sitting on top of the piston... LOL I performed the leakdown because I was trying to fix an "oil through the breathers" issue and was seeing what looked like a ton of crankcase pressure.

There is a light "gouge" in the cylinder wall on both the thrust face and a similar one opposite that where there is some scuffing from the skirt. I measured and it is below the oil rings so based on skirt wear I'm guessing I got something abrasive in there or the cold piston slap finally caught up to me but it shouldn't affect ring seal AFAIK.

FWIW I built this engine after spinning a rod bearing and I reused the block, most of the rods, and the pistons, with an aggressive hone to bring the bores into round hence the wide clearance. I knew it wasn't ideal and didn't expect to be running it as long as I have been but it turns out that big HP builds are expensive... I'm about to drop off the block for my next build at the machine shop so this is really just academic curiosity as I said.

[Warp Speed]

If the wear pattern was different at the gap, that's telling me the ring must have butted together. What's the ring gap now?

More wear at the gaps is normal.

[F-BIRD'88]

I bet the top ring groove on the piston is now distorted. If it is, just new rings will not fix it.

That is a sign the engine was run in detonation. Cast pistons break the top ring land, (often right at the intake valve relief).
Forged pistons distort the piston ring lands, scuff the piston and cylinder and pinch the rod bearing and piston pin hole.

When the ring lands get distorted the new ring will not seal up.

[ProPower engines]

Engine is an old (12 years?) street/strip 470hp, <6500RPM engine. Found one cylinder with high leakdown (43%) and pulled things apart to take a look.

Top ring used to be barrel-faced? Total Seal Classic Race CR0190. The face is smooth, not scored, so I don't think this was abrasive wear? My guess is either light detonation or fuel washdown only because those are the most likely to have occurred in this motor. Second ring looks fine to me but the wear pattern does get a little taller right at the gap. Oil ring looks to be fine.

Light scuffing of the skirts but I run pretty big piston to wall for a 4032 alloy (like 0.007-0.008" I think). Cross hatching is still visible in the cylinder and everything looks good and smooth there.

Any ideas? The motor is not getting money poured into it so it's getting one ring and then getting beaten like it owes me money, but I was curious as to what you can tell after a failure looking at the ring. Anything else I should measure or photograph?

A pic of the top of the piston will tell if detonation was an issue also have you measured the bore to confirm it is still round or really tapered when it was torn down??
You mentioned you used an "aggressive hone " what was the finish on the bores?? Or what grit of stone or stone number did you use to begin with??
Also what part of the bore was the piston in when the leak down was done. Top or bottom??
when looking at the top ring there is a intermittent line on the ring face is that a shadow or is it there suggesting the grove is badly worn or a bore isse in that spot where the ring travels??

[Rick Finsta]

Unfortunately looking at the piston tops won't help as they were pinged pretty regularly prior to this build and had the telltale flaked look before being put back in service.
I do not have a dial bore gauge to measure the bore for roundness. I could check for taper by measuring ring end gap at several depths I suppose?
No idea on hone finish, when I said "aggressive" I simply meant they removed enough material to fix out-of-round cylinders, which ended up being around 0.002-0.003" (I think that's a lot for a hone, am I wrong?). The first bore job I has done was done without deck plates and needed to be fixed.
Pistons were at 10* BTDC during leakdown.
The line on the ring is not shadow per se, but it is a lower area - you can catch a fingernail on it and it looks to be the areas where the original finish remains. That's why I thought they were a barrel profile - it looks like a round surface worn flat in the center.

I can check the ring land for uniformity with feeler gauges. I measured it at 0.079" with calipers, and the ring was 0.075" when removed and pretty obviously polished top and bottom.

ETA: Okay I checked the grooves for uniformity and they are 0.079" with feeler gauges and feel the same all the way around. Top and Second groove are the same. 0.080" of feeler gauges won't fit.

[ProPower engines]

Unfortunately looking at the piston tops won't help as they were pinged pretty regularly prior to this build and had the telltale flaked look before being put back in service.
I do not have a dial bore gauge to measure the bore for roundness. I could check for taper by measuring ring end gap at several depths I suppose?
No idea on hone finish, when I said "aggressive" I simply meant they removed enough material to fix out-of-round cylinders, which ended up being around 0.002-0.003" (I think that's a lot for a hone, am I wrong?). The first bore job I has done was done without deck plates and needed to be fixed.
Pistons were at 10* BTDC during leakdown.
The line on the ring is not shadow per se, but it is a lower area - you can catch a fingernail on it and it looks to be the areas where the original finish remains. That's why I thought they were a barrel profile - it looks like a round surface worn flat in the center.

I can check the ring land for uniformity with feeler gauges. I measured it at 0.079" with calipers, and the ring was 0.075" when removed and pretty obviously polished top and bottom.

ETA: Okay I checked the grooves for uniformity and they are 0.079" with feeler gauges and feel the same all the way around. Top and Second groove are the same. 0.080" of feeler gauges won't fit.

It sounds like a 5/64 ring 1-2 and a 3/16 oil ring.That groove size is right for a re-ring deal not too bad for wear if measurement is accurate at the outer edge of the groove. Deeper in may be shallower.Ring depth or radial wall thickness in the groove should be checked when going back together.
.002-.003" is not a lot to round up a worn hole but on fresh holes that sucks.

the bores should be checked with a proper bore gauge to ensure the sizes are good enough to re-ring other wise you will be wasting your time.
what is the current bore size and what was it when built the 1st time. That will determine if a +.005 ring set will benefit you now./ Or just use a cheapo cast ring set to get by for a while but it will not last long.

[Rick Finsta]

To be clear, I'm throwing rings on this one piston and getting it back together - it isn't going to a machine shop and will be parted out or deep-sixed in a year or so. Even running on seven cylinders the thing runs 11s so I'll cruise it as-is while I build a new shortblock. I'm thinking the bore and skirt wear was from the side clearance. It wouldn't be too much trouble to pull the piston from the best cylinder (<4% leakdown) and compare the top ring if it would help the discussion.

I am using a file-fit ring that is a 0.005" over IIRC. Original bore was 4.125 so I'm at around 4.128 now based on my notes (not direct measurement).

The thing is so old I can't find a lot of my notes from the build. To be honest until I measured them I would have sworn to you this thing had a 1/16 1/16 3/16 ring pack!

Thanks for entertaining my curiosity, here. I figure maybe this information could help someone else come to terms with an old, worn out engine that needs a bore and new pistons if it is to continue on?

[Casper393W]

I always believe in the old trick if you have a low cylinder squirt some oil in the sparkplug hole and retest. If the test increases compression then you have a ring seal issue. A vacuum guage hooked to the intake manifold will tell you if ring seal is compromised...

[Rick Finsta]

Engine is EFI, so I have all the MAP data. Engine idled at 850rpm and 68-69kPa hot with a Jones 288/292 solid flat tappet. Anything particular you're looking for? I may be able to see individual cylinders in the MAP trace at idle if I plot versus crank position.

Next time I do a leakdown I'll make sure to retest with oil on low cylinders.

[modok]

If you can't find anything wrong with the ring or the groove.....then I'm afraid the most likely explanation is.....it's fine
You know how to use a mic and freeler gauges..........get on it

I've seen a lot of oil consumption problems be "invisible" to the naked eye and basic measuring instruments, but, not compression. If you can't measure anything OFF, then it should HOLD compression just fine. Looks like the second ring is only worn halfway across the face, that's GOOD AND FRESH imo
If the top ring WASN't sealing, then the second ring would have more wear on it. Do a leakdown test with just the second ring. What do you find. Now how do you feel about leakdown tests?

I think you had dirt on the valve. OR, maybe the ring is pinched. Find out. Can't see in a picture.