Weird - Oil ring pops out of groove while running on dyno

[davepl]

A friend of mine ran into a weird problem and I have no good ideas, so thought I’d run it by the board to see if anyone's ever heard of anything similar.

After a rebuild they were running it on the engine dyno and it started smoking badly; it turns out the oil ring had come out of the groove on one piston. Thinking he must have done something wrong during assembly, he ordered new rings to be sure, tore it down, reassembled, and guess what – the exact same thing happened again. In the same cylinder, but with the new rings.

The wrist pin is below the ring pack on this motor, so there’s no support rail and the pin couldn’t hit it from the back (that was my only guess).

The second time he assembled it he put the gap on the thrust side, which he doesn’t like to do, to make sure it would be “held” in place, but it still popped out. Fortunately didn’t damage the cylinder wall either time. I don't know much else about the engine (Big Block Chev or Chrysler RB).

Ever heard of anything like that, or have any random guesses? He's a machinist and a retired Winston Cup engine builder, so there's not a lot he hasn't seen before, but this was new to all of us!

[swamp rat]

I hope he mic's all the bores in all the cylinders or used a boar gauge to check them before assy, then also checked all the pistons with a mic or calipers too? Yea i know if one is off it would probably knock...

Is it possible the oil rings and the width of the piston ring grove are not matching?

[Bob B.]

If the bore mic's out O.K. about all I can think of is the ring expander or the oil groove is miss-machined in the piston.

[davepl]

The bottom rail comes out when it rotates to the pin boss relief. So it's not coming out sideways like there were too much piston-to-bore, it's coming out after the ring rotates a ways and the end reaches the pin boss relief.

He would have checked all the basic clearances, so anything you'd normally mic and check with a bore gauge, etc, I can only assume was checked (I've never seen him skip a step).

[swamp rat]

Well when you figure it out (or even if ya don't) post some pictures..

[davepl]

I'm thinking the piston needs an oil ring support rail but wasn't designed for one, but I need to check more on Monday!

[Redline novdog]

That's odd. The engine builders credentials are good. I would suspect the piston also. When I build an engine I expect a piston to spec out. Shouldn't have to put each one on the marble table!!!!
Curious!

[sean1969c10]

If the bore is in spec and the clearance between the skirt and bore is right, and it only happens in this one cylinder it stands to reason that something is wrong with the way the oil ring groove was machined into the piston. Very slim chances of getting two ring sets that are bad. Let us know how this works out.

[Kennuck]

Has the engine been stroked?

[davepl]

I think it's factory stroke.

Here's a picture of the piston. You can see the pin boss relief area does NOT interfere with the ring groove at all, so how or why the ring pops out when it rotates around to this area is the question.

Checked radial depth of the ring, groove depth, amount of barrel to the piston, bore roundness, piston to bore clearance, it all checks out.

As a last ditch attempt he's going to order an oil ring with increased radial depth so it'd have an even harder time getting out of the groove, but we're all still worried since no one can explain how or why it could pop out with the old one!

[1972RedNeck]

390 Ford, right?

What brand of pistons?

[haysonj]

ring pops out when it rotates around to this area is the question
Rings don't rotate on the piston otherwise every manual out there would not tell you to put the gaps in a certain place on the piston. Those pistons do not require a support ring as the relief for the wrist pin does not go all the way up to the oil ring. I had one come out on me but as I look back I think I rolled it out when I put the piston in and it did destroy the bore

[haysonj]

After searching Google it looks like the majority of people believe the rings do rotate so I guess I have been wasting a lot of time to make sure I put the gaps where the ring makers tell me to. Once they seat I find it hard to believe they move

[saxart]

This might sound "dumb" but would it be possible for him to reassemble the engine with that piston in a different cylinder and run it? If the ring still blows out of that cylinder, then you know it's an issue with the cylinder. If the ring blowing moves with the piston, then you know it's the piston (or rings) at fault.

Just trying to help. Keep us posted...

[jdw]

How'd it pop out twice with no damage? Bad piston?

[hamjet]

It does sound impossible unless theres enough piston to bore clearence for a ring rail to slip by and even then I would think a problem with the compression rings to allow a high amount of blow by to force the rail out of the groove.

[davepl]

Here's a picture. It pops out just after it rotates around to the relief area for the pin boss. Even that would seem impossible, and it leaves a little scratch where it exits (can't see it in the photo well enough), but it does it.

He's going to try a ring that is deeper radially that should have a harder time coming out, but its still uncomfortable not knowing, even if that "fixes" it.

Everything from back spacing on the ring to the amount of barrel in the piston checks out according to spec. Checked everything we have numbers for on the piston and then some!

[jdw]

He ran the piston both ways and everything specs out, I'm going with a warp/cracking/metallurgical defect in the piston that's not showing up until it heats up and or force is applied. Doesn't make sense to keep trying if the math adds up.
I'm really curious to know.

[GRX]

Strange. Have you checked that expander ring for inconsistencies and ensured it is installed correctly? Next step would be to try mixing up parts as suggested above.

Originally Posted by haysonj

Once they seat I find it hard to believe they move

^^ Not so much when everything is new. Rings will move more once the cylinders take on a bell shape, and piston skirts wear allowing them to rock in the bore. On a few occasions I have seen a high mileage engine smoke like a freight train from one or more cylinders, then clear itself out. No doubt because the end gaps lined up for a time.

[swamp rat]

I was also gonna suggest too large of ring gap but Hamjet already did.. I agree with Jdw its got to be the piston.

[hamjet]

OK here it goes, I've been biting my tounge not to say what I'm about to get into. piston to bore clearance is lets say .003, Your oil ring rail is ,(just a guess) .015 .020 thousands. It is IMPOSSIBLE for an engine with correct clearance for this to happen, let alone make a 90 degree turn for it to happen with out tearing something up. The engine bore or piston is not in specs with each other, plain and simple. I hope you don't take offence to this Dave, but your friend didn't check something correctly. Every one can over look something, (except me, I'm perfect) LOL. don't get mad at me, but common sense tells me its impossible....OK, I'm pushing the button. Joe

[Kennuck]

You are exactly right hamjet, that's why I asked if it was stroked. It seems to me the only way this could happen is if the oil ring is coming out of the bottom of the bore on the downstroke.

[davepl]

That'd certainly account for it, I'll check into it. I do not know whether the engine has been stroked (I don't think so, but anything's possible, it's not mine). I'll check!

[swamp rat]

Originally Posted by Kennuck

You are exactly right hamjet, that's why I asked if it was stroked. It seems to me the only way this could happen is if the oil ring is coming out of the bottom of the bore on the downstroke.

I'm with you and Hamjet, i would like to see this ring after it comes out the bottom.

Also if it does come out the bottom then the cylinder should show some kind of damage, the thickness of that bottom ring is thicker than the boar to piston clearance, it should be catastrophic. Bore to spec, proper piston, it should show damage of that is infact happening.

A block clearanced for a 383 stroker would have the machining perpendicular to the wrist pin, even if it came out there it would still show some damage.

I keep thinking look inside the piston bottom and ring groove for a casting flaw? but you would think the piston would have a catastrophic failure..