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).

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 (and he's diligent - quite unlikely he'd make a dumb mistake twice in a row), but this was new to all of us!

vbgod1

I am having a hard time with the visual of this. With only a few thousandths clearance, when it comes out, where does it go?

I guess if he could swap two Pistons, he could decide if the piston or cylinder is at fault.

69427

I agree with the visual difficulty.
Swapping two pistons is a good idea.

427Hotrod

Yep..I'm needing pics too. Was it one of the rails or the expander? Just can't see how it can go very far?

JIM

mikem350

Must be coming out thru the bottom of the bore. Weird!

centuryoldracer

Wonder what brand of ring, and is this a special low tension or something where the ring is so shallow that it is not in the groove enough to stay square, and literally folds over between the cylinder wall and the piston skirt.

Pop Chevy

How much piston to wall clearance is he running?? Sounds to me like the piston is rocking too much.

mikem350

Good answer!

76Rat

Cylinder bore to big or piston to small. Wrong oil rings. Not to many other options

davepl

Yeah, that's what makes it weird. Things we've checked:

- Piston to wall
- Ring end gaps
- Piston ring correct for piston (according to mfrs)

It's not going to be a normal blueprinting step or he'd have caught it, so it's got to be something less common like back clearance on the ring - maybe the ring groove is too deep, that sort of thing (not saying he didn't check that, I don't know, but I've not checked it before).

I don't have photos, but it's coming out when the oil ring rotates around and makes it to the pin boss relief - that's where it starts to come out and no longer do its job.

It's NOT between the piston and the wall, I don't think you could have enough gap there for that (or that'd cause a lot of rock!). So it's not coming out sideways.

The bottom rail comes out when it rotates to the pin boss relief.

Pop Chevy

That helps. Did you check side clearance on the oil ring pack? Maybe it needs a support rail ? Mine did. I was just putting the pistons in my LS-7 454 and was thinking about this. Ya gotta have excess clearance somewhere.

davepl

I asked if the pin was behind the ring pack (like when you're using a long rod that pushes the pin up close to the compression height) and I thought he said no, but then how would the pin boss relief be open?

I think I need to look at the piston again, but I'm guessing the oil ring support rail might be the ticket if it's actually open at the pin boss - and if it's not, then I'm not sure where the ring is coming out, so either way I need to have a second look.

Thanks for the leads!

lionelhutz

It sounds like he has a gap in the bottom of the oil ring groove to allow the pin to be installed. In those cases, something must be used to either keep the ends of the oil ring out of that gap or to bridge that gap. Some pistons use a pin to keep the bottom oil ring from rotating. Others use a support ring or spacer that is like an oil ring but thicker and it has dimples that are put into the gap and they keep the ring from rotating. The piston must have a wider oil ring groove to allow these support rings to be installed.

claysmoker

I just have a hunch that the piston is bad. Oil ring groove isn't concentric to the piston. I don't know how it could happen, but you see some weird stuff at times. What brand are the pistons?

MotorHead

IMHO he needs a new piston. If he has problems twice in the same hole I would measure the bore with a good bore gauge not one of those silly spring loaded things.

And you have to measure piston to wall clearance by measuring the piston and then measuring the bore. If it needed a support ring they would have come with the ring pack.

davepl

My current theory is that the engine needs an oil ring support rail, but I need to get back on Monday and get a closer look at the piston. I don't know how long the "bridge" is where the oil ring is otherwise unsupported, if at all.

I thought automatically as soon as they ring is exposed by the piston pin boss relief (which is where the oil ring is escaping) you needed a support ring, but apparently not. I need to find out more. As Lionel Hutz mentions the piston is normally designed to have or not have a support rail, and this piston was designed to NOT have one, but I think it might need one.

lionelhutz

The pistons that don't need a support rail have a locator pin. It usually sticks into the bottom side of the groove away from the wrist pin. You grind a little off the bottom oil ring so that the pin is located between the ends and then the ring can't rotate.

Otherwise, the oil ring groove has to be wider to allow the use of support rails. Something like 0.030" wider comes to mind.

If it's happened on one piston then you can assume they're all put together the same way and he'll eventually have the same issue on another piston.

davepl

I wish the problem HAD occurred the second time on a different piston, that would make it appear to be a more global issue, and given he relocated the gaps the second time around, I'm surprised it didn't.

Where is the locator pin that you speak of (pic or link would be great) - built into the bottom oil ring? Or the piston?

lionelhutz

I did a quick Google search and came up with this;

http://www.beckracing.com/page09.htm

427Hotrod

If the pin gets into the oil ring groove it has to have a spacer/support. From what you're saying now, they've been left out.

Get some pics. Got a part number for the piston? What engine, stroke, rod length?

JIM