piston ring end gap spacing ?

[40 Deluxe]

Originally Posted by BRENT in 10-uh-C

To answer your question, ...yes, but again most rings today are gapless so it is moot.

FWIW, we monitor high-performance engines closely during assembly and then in teardown. The ring gaps are noted where they were installed and where they come out. They generally do not move, and usually it is because the piston was inserted and twisted as the rod was connected to the pin.

So going the other direction, what causes the ring to rotate?? Compression pressures are not going to rotate it, and if the piston is moving in a straight line within the bore, unless the rod is rotating the piston slightly during the cycle, tell me what is moving them??

Hmmm... I think most gapless rings are only used in high performance engines, not production engines. Do you use gapless in your Model A rebuilds?
Which auto engines have the rod installed after the piston is inserted?
As far as why rings rotate, maybe temperature changes as the engine warms up? Not sure, but let me relate what I saw years ago. I was cleaning the muffler on an old chainsaw and started it with the muffler off. At BDC the ring was visible in the exhaust port so with the engine running it appeared to be motionless. Running at part throttle I could see the ring gap moving. The gap would appear at one side of the exhaust port, then move across the port and disappear. It did this several times a minute. Visible proof that rings rotate!

[40 Deluxe]

Originally Posted by Patrick L.

I have, but, always because of a broken ring.

Every broken top ring ring that I've seen did not contact the cylinder at all or
maybe half the cylinder depending on where it broke. But they never leave a tiny spot that matches a ring gap.

[Patrick L.]

The main reason I know of that some pistons pin the rings is so 2 cycle engine ring ends do not cross the port.

Some engines are taken apart very often. I find the rings as I installed them.

I can't remember one I've taken apart that didn't have the gaps spaced enough.

[katy]

Originally Posted by 40 Deluxe

Have you ever seen an engine with a tiny unworn spot at the top of the cylinder indicating the ring never rotated? I never have!

As the ring heats up it expands and closes the gap, therefore no unworn spot. Methinks it would need to have a very big gap to leave an unworn spot at the top of the cylinder.
YMMV

[40 Deluxe]

Originally Posted by katy

As the ring heats up it expands and closes the gap, therefore no unworn spot. Methinks it would need to have a very big gap to leave an unworn spot at the top of the cylinder.
YMMV

You better hope that the gap never closes! If it does, ring expansion puts a tremendous load on the cylinder wall, causing the ring to weld itself to the wall and bad things WILL happen! There has to be at least a little gap which, if the ring never rotates, will leave a tiny unworn spot on the wall.

[BRENT in 10-uh-C]

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Originally Posted by 40 Deluxe

You better hope that the gap never closes! If it does, ring expansion puts a tremendous load on the cylinder wall, causing the ring to weld itself to the wall and bad things WILL happen! There has to be at least a little gap which, if the ring never rotates, will leave a tiny unworn spot on the wall.

I am not sure that is an accurate statement regarding no gap. Several piston ring manufacturers offer a gapless styled ring that uses no gap between the two ends. These are also used quite frequently in non-high performance engine applications too.

[bbrocksr]

Originally Posted by BRENT in 10-uh-C

I am not sure that is an accurate statement regarding no gap. Several piston ring manufacturers offer a gapless styled ring that uses no gap between the two ends. These are also used quite frequently in non-high performance engine applications too.

Gapless rings do have gap for expansion, It is a stepped gap where the ends of the ring overlap.
Bill

[40 Deluxe]

Originally Posted by BRENT in 10-uh-C

I am not sure that is an accurate statement regarding no gap. Several piston ring manufacturers offer a gapless styled ring that uses no gap between the two ends. These are also used quite frequently in non-high performance engine applications too.

"Gapless" rings are not truly gapless. Otherwise, how could you get them on the piston?? They are two piece rings with a gap. One piece is thin like an oil ring rail and the main part of the ring has a groove to accommodate the thin piece. The gaps are to be installed 180 deg. apart but it's reported that the gaps are sometimes lined up on disassembly. I read of a "labyrinth" ring used on Geo Metros but whatever that is, you still have to get it over the piston to install.

[aermotor]

Apart from the wear issue, does the ring gaps aligned cause any compression loss in that cylinder? I say no, what do others say?


John

[Crankster]

The piston ring manufacturers recommend staggering them at installation. Maybe it's only an issue for a brief period during the first few minutes of operation, or during break-in, to help guard against possible cylinder glazing? No idea, but if it's all the same I still like to read and follow the instructions when I can.

[40 Deluxe]

Originally Posted by aermotor

Apart from the wear issue, does the ring gaps aligned cause any compression loss in that cylinder? I say no, what do others say?


John

You are correct. No difference in compression loss whether gaps are aligned, 180 deg. apart, or anywhere in between.