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Too big of piston ring end gap?
Topic Started: Jun 18 2011, 06:45 AM (11,167 Views)
Norby
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I recently picked up a '92 Metro 1.0 4-door and started to take apart the engine. The prior owner had a mechanic do a rebuild on this motor, but compression was 0 on cylinders 1/3, cylinder 2 had compression of 170psi.

As I pull the pistons out I notice these block numbers:
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However, piston 1 is marked A2, piston 2 is A2, and piston 3 is A1. Did the prior mechanic mix up the piston/cylinder matchup?

Their are new piston rings on these pistons, and the pistons are fairly clean. So clearly the mechanic did the work, but take a look at the ring end gap on cylinder 3. I measure it to be well over .025", versus the other cylinders at/near .012".
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That much of ring end gap is trouble isn't it? Do I need to pick up an oversized ring set for this one cylinder? Or just get the block bored?
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snowfish
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Basic GearHead

That would bother me. :shake Whether I was fixing it to sell, or planning on driving it. I'd get some .040 over sized pistons, have a machine shop bore it to fit, polish the crank, line bore the mains if necessary, and press on the new pistons.

Then send the head to 3Tech to do his magic. Many flavors to choose from. :thumb

That's exactly what I did, minus the .040 bore. So far, Absolutely No Regrets. :nfs
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billy508
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billy508

Swap around the rings. Is the cylinder too big or the rings too small.? I doubt that a 12 thousand ring gap difference would cause 0 compression. Look at the head very close. :banana :banana I did not write this but this the method I use to get a rough idea of piston clearance,..................Fit piston in bore with ribbon type feeler guages on thrust side of piston. Try various feelers until you have slight drag on feeler guage. This will indicate your clearnace. The piston is a round peg in a round hole and you have effectively pushed it to one side of the hole and determined the clearance between the inner wall of the barrel and the outer wall of the piston. The piston however, does not operate loaded against one side of the barrel. It is more or less centered in the barrel by the rings. So if you can imagine, in you head, moving that piston back towards centre half way you have taken half the clearance you determined and placed it on the other side of the piston. This is why measurements are divided by two: 0.010 measured with a feeler equals 0.005 actual clearance on both sides of the piston.
2) same deal if you measure piston with a mike and barrel with a bore guage. You have determined the difference in piston od and barrel id. Divide by two because the piston runs in the centre of the barrel.
Edited by billy508, Jun 18 2011, 07:50 AM.
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95-3banger
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I don't know where they get that divide by 2 stuff billy? I just bored a block at work yesterday. All you're measuring is the ID and OD to get your clearance. There is no halfing it and calling that your actual clearance. ;)
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Norby
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billy508
Jun 18 2011, 07:39 AM
Swap around the rings. Is the cylinder too big or the rings too small.? I doubt that a 12 thousand ring gap difference would cause 0 compression. ...

I erred in describing my problem. I should have said cylinders 1/3 have bad exhaust valves. I can fix that later with new valves. My other concern was the ring gap in cylinder 3. I just tried a new set of standard rings in cylinder 3. The gap is now only .031", whereas before it was close to .050". I think max gap on top/middle rings should be .0276", so cylinder 3 gap is still too big. Why were the "old" rings providing a larger gap? I think someone filed them down but took too much material, hence the big gap.

I wish I could just order next size over piston rings for the cylinder 3. Then I could at least file it down to fit. Or is that sloppy?
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billy508
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billy508

If you use a feeler gauge to check it. Being the piston is pushed over in the cylinder all the clearance is on one side. Not a exact method but it will give you a good idea of what you have. :banana :banana :banana
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Deleted User
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Like snowfish suggested. If you're selling it, patch and ship. It will run a long time, no matter what you do.

Keeping it? Bore oversized. Deck the top of the block, as required. Trust me. Spending a little extra time, and few more dollars will take you tens of thousands more miles down the road than a patch/fix job.
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Norby
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Yes, good advice, thanks guys. Bruce from partsdinosaur is sending me oversize rings for the one cylinder. I'm going to file the rings to fit and get the car on the road. We may just sell the car and look for a better 5 speed model.
Edited by Norby, Jun 19 2011, 01:23 PM.
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Norby
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Got the rings in today. They look purdy, made in Japan too! Now, to figure out what file to use for the job of filing them to fit.

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starscream5000
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Got 70 MPG?

You'll want something like this:
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Norby
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I wouldn't mind that tool. I found this website and they show another way/method to file rings:
file fitting rings

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But, if I can find that one you mention for cheap, I'd grab it.
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Norby
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starscream5000
Jun 23 2011, 12:21 PM
You'll want something like this:
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Ok, you were right. Using a hand file is taking WAAAYYY too long. I just bought that tool you mention, off ebay for $43 shipped. Can't wait to use it.
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Murf 59
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I will look and see what they should be. I don't have the #s right off the top of my head. But I have a manuel that has it in it. I will see if I can get online tomorrow. I normally only have access at work.
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dayle1960
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Fastest Hampster EVER

Uh,Oh..........I've never even thought about filing the ends of the rings. I ASSUMED that the manufacturer would get it right when the rings came off the assembly line. Oh well, I guess I'll have to tear my 4 year old rebuilt engine apart to examine closely the clearance on the ring ends. Woo is me, Woo is me.............
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Norby
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Yea, I think from now I will always custom file my rings to fit.
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