Air leaking past piston rings?

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N1ce2k1llu

N1ce2k1llu

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Hello, I'm installing a new cam so I have the top of my engine off. With No pushrods installed, all valves closed and spark plugs still in. When I turn the crankshaft I hear air leaking past piston rings. I did a dry compression test (engine cold) 1=112, 2=115, 3=124, 4=130, 5=115, 6=120, 7=115, 8=115. So my question is that normal for air to bypass? 1971 sb. 318LA
 
And the range on your readings looks pretty reasonable for a used engine with stock compression... .must be a stock bottom-end 318 or 360.
 
nm9stheham said:
And the range on your readings looks pretty reasonable for a used engine with stock compression... .must be a stock bottom-end 318 or 360.
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It is a stock bottom end. I did replace the piston rings but had to trim them down because they sent me rings for a bored 318. Just needed some reassuring about it leaking out. Thanks guys
 
N1ce2k1llu said:
It is a stock bottom end. I did replace the piston rings but had to trim them down because they sent me rings for a bored 318. Just needed some reassuring about it leaking out. Thanks guys
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What size rings? When file fitting rings, typically a .005 over ring is used. Might not be a good idea to file fit a .020 or .030 over ring.
 
N1ce2k1llu said:
It is a stock bottom end. I did replace the piston rings but had to trim them down because they sent me rings for a bored 318. Just needed some reassuring about it leaking out. Thanks guys
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Geez, now the truth comes out!
 
yellow rose said:
Yep. If he has a stock bore and then filed a .030 ring, the OP is in the suck.
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Oval instead of round.:D
Don't know how much real difference that much off a .030 ring would make, but it sure ain't evenly distributed pressure on the cylinder walls.
 
TrailBeast said:
Oval instead of round.:D
Don't know how much real difference that much off a .030 ring would make, but it sure ain't evenly distributed pressure on the cylinder walls.
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Not sure why it would not evenly bend in all around the ring...???? Help me understand the problem.... not challenging anyone... just never ran into or thought about that problem.
 
nm9stheham said:
Not sure why it would not evenly bend in all around the ring...???? Help me understand the problem.... not challenging anyone... just never ran into or thought about that problem.
Click to expand...

When you squeeze an open ended circle (ring gaps) beyond what it was originally sized for it tries to go out of round.
You will have varying sealing pressures between the ring and cylinder at both the end gap of the rings and 180 away from that on the other side of the ring than you have at 90 in both directions from the ring ends.
It's actually quite possible the ring ends could even try to bite into the cylinder wall.

OR, you could just leave it and run another 20 years, but I doubt it. :D
 
nm9stheham said:
Not sure why it would not evenly bend in all around the ring...???? Help me understand the problem.... not challenging anyone... just never ran into or thought about that problem.
Click to expand...
What I did was compare old ring to new ring and cut the end to match then filed my cuts. Did not file outside diameter. New rings fit around the piston but left no gap for expansion from heat. Compression test was done cold and dry so that's why test numbers were low. I'm thinking if rings were to much gapped then I would be burning oil, but I'm not a professional auto mechanic just an elevator mechanic who's a weekend Warrior.
 
TrailBeast said:
When you squeeze an open ended circle (ring gaps) beyond what it was originally sized for it tries to go out of round.
Click to expand...
But all rings get 'squeezed'.... why does the contact radius get out of round once it gets squeezed a bit more than normal? If you were just pushing or pulling on the ring ends, like with a ring expander, then I agree; in that case, the bending force along the length of the ring will be uneven and will peak at a position opposite to the gap. But that is not what is going once in a cylinder. The ring is being pressed evenly inward all around, so I can't see what force would put it out of round. It is like a circular piece of spring steel with the forces being evenly distributed so I can't see anything that will put the circle out of round. And all rings have a gap so any end effects on radius and pressure would not change much... I would think. Honestly, not being obstinate, but the 'distorted contact radius' idea still does not make sense to me.

I would not put in oversized rings myself, but I am trying to understand what problems this situation will have. Very probably the ring drag goes up well beyond what it should be. That might in turn cause an issue with putting pressure on the lands. And the rings may drag and twist up and down more than they should, possibly causing rapid ring wear and/or rapid ring groove wear. That I can see. If this is a re-ring job done without checking ring side clearances or honing, there are bigger issues IMHO.

Admittedly, this is all academic.....
 
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N1ce2k1llu said:
What I did was compare old ring to new ring and cut the end to match then filed my cuts. Did not file outside diameter. New rings fit around the piston but left no gap for expansion from heat. Compression test was done cold and dry so that's why test numbers were low. I'm thinking if rings were to much gapped then I would be burning oil, but I'm not a professional auto mechanic just an elevator mechanic who's a weekend Warrior.
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OP, the procedure you describe is worrying me. How did you cut these end gaps? After 'cutting' down the end gaps, did you then put all of the rings into their bores with an inverted piston, one at a time, using the inverted piston top to push the rings perfectly flat in the bores, and then check the end gaps with a feeler gauge? If you did not, NOW you are truly 'in the suck' as YR says. I see your comment that you ended up with no gap for expansion. If that is true, then a disaster WILL occur the first time you warm this engine up. The top 2 rings for a stock setup will typical need around .015" end gap for heat expansion, and the oil rings a bit more. Please explain further; not trying to get on your case at all, but this is very worrying and we/I are trying to prevent a disaster. (Send a PM if you prefer to discuss offline.)

And BTW, your pressures aren't going up much more than you have. It's a stock compression engine and being in the 120-140-ish range would be normal. The bigger the cam, the lower these readings will go.
 
nm9stheham said:
But all rings get 'squeezed'.... why does the contact radius get out of round once it gets squeezed a bit more than normal? If you were just pushing or pulling on the ring ends, like with a ring expander, then I agree; in that case, the bending force along the length of the ring will be uneven and will peak at a position opposite to the gap. But that is not what is going once in a cylinder. The ring is being pressed evenly inward all around, so I can't see what force would put it out of round. It is like a circular piece of spring steel with the forces being evenly distributed so I can't see anything that will put the circle out of round. And all rings have a gap so any end effects on radius and pressure would not change much... I would think. Honestly, not being obstinate, but the 'distorted contact radius' idea still does not make sense to me.

I would not put in oversized rings myself, but I am trying to understand what problems this situation will have. Very probably the ring drag goes up well beyond what it should be. That might in turn cause an issue with putting pressure on the lands. And the rings may drag and twist up and down more than they should, possibly causing rapid ring wear and/or rapid ring groove wear. That I can see. If this is a re-ring job done without checking ring side clearances or honing, there are bigger issues IMHO.

Admittedly, this is all academic.....
Click to expand...

The problem is that the pressure is not evenly distributed all the way around when you put the ring in a smaller bore.
There WILL be spots that are higher and lower pressures against the cylinder wall causing uneven sealing and wear.
You are correct that the ring will be forced into the new curve, but the pressures on the cylinder wall will vary because of forcing it into that new curve.
It would be different if the rings were made for the bore and then filed down to fit, because they were made for that curve in the first place.

I can't find a way to explain it farther than I have already because if a person doesn't understand how forcing a metal circle farther than it was intended to curve distorts the circle, I don't know what else to say.:D

I also have no idea how this will affect the engine in the long run obviously.
Like I said, it may last 20 years, or trash a cylinder wall in 20 minutes.
 
nm9stheham said:
But all rings get 'squeezed'.... why does the contact radius get out of round once it gets squeezed a bit more than normal? If you were just pushing or pulling on the ring ends, like with a ring expander, then I agree; in that case, the bending force along the length of the ring will be uneven and will peak at a position opposite to the gap. But that is not what is going once in a cylinder. The ring is being pressed evenly inward all around, so I can't see what force would put it out of round. It is like a circular piece of spring steel with the forces being evenly distributed so I can't see anything that will put the circle out of round. And all rings have a gap so any end effects on radius and pressure would not change much... I would think. Honestly, not being obstinate, but the 'distorted contact radius' idea still does not make sense to me.

I would not put in oversized rings myself, but I am trying to understand what problems this situation will have. Very probably the ring drag goes up well beyond what it should be. That might in turn cause an issue with putting pressure on the lands. And the rings may drag and twist up and down more than they should, possibly causing rapid ring wear and/or rapid ring groove wear. That I can see. If this is a re-ring job done without checking ring side clearances or honing, there are bigger issues IMHO.

Admittedly, this is all academic.....
Click to expand...


If this was the case they would just make one oversize ring for every application and then just file fit them all.

When the ring OD gets bigger, so does the ID. You can file the ends down all you want, the ID will still be wrong.

I can't tell from what the OP is saying exactly what he did. If he did use the wrong oversize ring he is in the suck.
 
N1ce2k1llu said:
What I did was compare old ring to new ring and cut the end to match then filed my cuts. Did not file outside diameter. New rings fit around the piston but left no gap for expansion from heat. Compression test was done cold and dry so that's why test numbers were low. I'm thinking if rings were to much gapped then I would be burning oil, but I'm not a professional auto mechanic just an elevator mechanic who's a weekend Warrior.
Click to expand...
I am afraid. I am very afraid!
 
TrailBeast said:
The problem is that the pressure is not evenly distributed all the way around when you put the ring in a smaller bore.
Click to expand...
I honestly don't think this is the case. Draw the ring and put force arrows all around it at 1 degree intervals (a free body diagram) to indicate forces on the ring. The forces are constant all around. There is nothing there to bend the ring differently in 1 place versus another. The end gap is the only discontinuity, and that exists on all rings and is about 0.1% of the total circumference.

I believe you are thinking of what happens if you apply the force only at the ring ends; in that case it will indeed distort the ring from a nice circle.

Now it may be that, with enough side pressure, the ring will eventually tend to buckle a bit and get 'wavy' as viewed from the side. In that, case, I am 100% with you in expecting variable ring seal.

yellow rose said:
If this was the case they would just make one oversize ring for every application and then just file fit them all.
Click to expand...
I don't think so; I thought the rings' curvature is varied to give the right wall pressure. But see below.

yellow rose said:
When the ring OD gets bigger, so does the ID. You can file the ends down all you want, the ID will still be wrong.
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But if the rings width is the same for all oversizes (which you seem to be saying) and the OD is squeezed down, then you would end up with the same ID, right?

I thought they just built them with different curvature and kept the same exact width for all bore sizes, but realize now that I really don't know that answer.
 
nm9stheham said:
I honestly don't think this is the case. Draw the ring and put force arrows all around it at 1 degree intervals (a free body diagram) to indicate forces on the ring. The forces are constant all around.
Click to expand...

Except when the ring was cast at a given curve and made to curve differently.
This isn't specifically a mechanical thing, but more of a physics thing.
The very ends of the ring (at the end gap) will resist that new curve and the pressure against the bore will be higher there than anywhere else in the circumference, and also higher 180 degrees away from the end gaps.
Yellow Rose hit on the fact that we don't see rings that are made for multiple bore sizes.
That is because ring sizes are curved to correctly fit a specific bore size and no others so that there is even pressure against the cylinder wall all the way around.

Take a piece of wire coat hanger about 3 inches long and bend it in a circle with your fingers.
Where in that circle is it hardest to get it to bend to the curve of the circle. (the end gap)
This is about the best analogy I can come up with at the moment.
 
The rings are designed to be exactly round at exactly one diameter. You can gap them to whatever you want, and in the hole it was designed for it will still be round.
The top ring has little to do with oil-burning, that is the job of the oil rings to prevent.
If you take a too-big ring, one that was designed for a bigger hole than the one you are putting it into, it will want to touch the cylinder walls in about 2 or 3 places. Just put one in a hole sometime,square it up, and make a cardboard plug a little smaller than the hole and drop it onto the ring. Weight it down lightly so it sits on the ring flat. Then shine a light up the hole from the bottom. Shazzam!
Now as the ring wears at the contact points, it will soon expand as it wears the high points off.Eventually it might wear to fit the hole, but in the meantime hot exhaust gasses are being being forced into the crankcase, and in the end it will have a huge end gap, and lotsa leakdown, which is why I recommended the LD test; to see if you even had an engine left.
 
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TrailBeast said:
The very ends of the ring (at the end gap) will resist that new curve and the pressure against the bore will be higher there than anywhere else in the circumference
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Yeah I understand the end effects.. the supposedly even pressure against the rings just stops at the ends, and there is not the pressure beyond the ends to bend that last segment to the new curvature. But I was/am thinking that the end effects don't extend very far into the ring, and those effects exist even if the ring is in its normal sized bore.

AJ/FormS said:
The rings are designed to be exactly round at exactly one diameter. You can gap them to whatever you want, and in the hole it was designed for it will still be round.

If you take a too-big ring, one that was designed for a bigger hole than the one you are putting it into, it will want to touch the cylinder walls in about 2 or 3 places. Just put one in a hole sometime,square it up, and make a cardboard plug a little smaller than the hole and drop it onto the ring. Weight it down lightly so it sits on the ring flat. Then shine a light up the hole from the bottom. Shazzam!
Click to expand...

May I be so irreverent as to ask if anyone has done this light check? I know I have not.... 'cuz I never put in the wrong sized rings.

It sounds like you are saying that the rings, when loose, out of the bore, are distorted in some fashion and not perfectly circular, and reach a true circular shape just at the right diameter. I never thought of that. If that is the case, then it seems logical if you measured the diameter out of the bore in multiple places, the loose ring would not be a perfect circle. That sounds easy to do.

Well, not sure how much this is helping the OP. I was just not sure about some of the things being said. The end gap procedures described are still of concern.
 
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