Piston ring rotation

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Bruce R.

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Something I’ve noticed in my model engines on several occasions and I was wondering if anyone else has experienced this and knows the cause . After running an engine for awhile, compression seems to diminish. First I check the valves and not finding any problem I suspect a broken ring, pulling the piston I find that the rings have rotated until the gaps are aligned, I’ve also noticed this when running in an engine belted to a motor. What causes this, and is it preventable ?
 

mayhugh1

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I've gone through the math on this before, and a running gap of a couple thousandths has no measurable effect on compression. You may have a circularity issue with your cylinders and/or rings that is tending to rotate them into alignment during use. I really think it's the circularity issue rather than the gap creating your compression loss. A light test should pinpoint the problem. - Terry
 

johwen

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Something I’ve noticed in my model engines on several occasions and I was wondering if anyone else has experienced this and knows the cause . After running an engine for awhile, compression seems to diminish. First I check the valves and not finding any problem I suspect a broken ring, pulling the piston I find that the rings have rotated until the gaps are aligned, I’ve also noticed this when running in an engine belted to a motor. What causes this, and is it preventable ?
I Put two rings of half width in the same groove with the gaps at 180 degrees and never have that happen. Have good compression as I make my own rings from the getgo Johwen
 

WOB

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So what is the force causing the rings to rotate? Do they stop rotating when the gaps are aligned? Must be a ghost in that engine! Just kidding. If you have the proper gap in each ring and the bore and rings are truly round and correctly sized, I agree with Mayhugh 1, you should see no decrease in compression no matter what happens with the gaps.

The fact that you are running-in an engine with an external motor, makes me think you have a problem with the piston, ring, bore geometry or finish or both. Correctly made rings in a correctly made cylinder bore should have good initial compression and it should get even better in the first few minutes of running as they break in. Running-in should not be necessary.

WOB
 

Ken I

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Rings do rotate - I'm not sure why - but based on my experience of racing 2 stroke & 4 stroke racing motorcycles - In 2 strokes the pistons are fitted with a pin to prevent ring rotation (the split ends of the ring are profiled to nest against the pin) - particularly as you never want the open end of a ring crossing a port - as in 2 strokes.
Once established (run in) in four strokes at normal rpms - they tend to stay put.
I suspect the cross hatching of a honed cylinder has a lot to do with ring rotation during run in but stops one the rings are bedded in and a longitudinal pattern is established.
I would go along with Terry in that ovality may be biasing the rings.
I've never known rings to align by the gaps by themselves.
Whilst it is good practice deliberately place the gaps far apart - this has more to do with wear uniformity than compression.
Regards, Ken
 
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minh-thanh

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Bruce R. !
This case happened to me, I didn't know why with that engine . first - the engine ran and then reduced the compression I removed the piston and found that the gaps of the ring was aligned. After , the engine ran and reduced the compression again, I removed the piston again and saw the same situation
After a lot of checking, I realized that the reason for the loss of compression is the intake valve does not move smoothly in the valve seat when the engine is hot.
And when the engine was good, I removed the piston and realized that the ring was rotation
- but it is less
And this situation does not repeat itself with later engines
It's like having "ghosts" in that engine
The engine you lose compression: maybe the ring slot is too big and the piston is too small - my engine is still running with one ring
I thought a lot about this case, it happened only twice and disappeared so I didn't learn more, and there's no way to know the exact cause because I'm not an expert and have a machine, measurement, scientific calculations,, ... So I'm content with the answer: there is "ghost" in that engine :D:D
 
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Bruce R.

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I was leaning toward the cross hatching in the cylinder from the honing as the cause, I’m glad to hear that I’m not imagining things.
 

Ozwes007

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Rings rotate naturally in engines, there is a formula to work it out however the usual is 1 rev per 1000 rotations. Hence the reason two stroke rings have a pin to make sure they don’t rotate. Compression loss though rings is usually based on your side clearance and back clearances, or not seating rings properly. All rings rotate to some degree, multi part oil rings are made to minimise/stop this behaviour by the use of differential mass opposing each other and tight fits. The process of the ring movement is on ignition the ring back and side clearance has to be large enough to allow gas behind and on top of the ring to create bore pressure to seal. To much and the ring will flutter in the groove and eventually cut the top of your piston off. Gap less rings where developed to assist in providing the “perfect seal”.
 

Andy Munns

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There was a test which used a new 4 cylinder engine - New rings, but they ran the engine and progressively opened up the ring gaps by increments until well over recommended gap limits. Results showed no real loss of compression until ring gap was huge. Conclusion was that ring gap made less difference that once assumed, and that ring in groove axial clearance and loss of ring spring and wall pressure were the real culprits. I'll www search for the article and post if found. That said, currently overhauling a Weir steam pump with paired rings - ring gap worn + gaps not in line when pulled, but clear marks showing rings not seating well on cylinder walls after years of service. Rings professionally made and double machined to bore size. There used to be a lot of designs that engineered a no gap ring - perhaps these were a waste of time?
 

Ozwes007

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The gapless rings are used in race engines, the losses are real in HP loss at the high end of the rpm range. We can be talking no more than 1- 2%, however that 1-2% can win or lose a race.production engines would not benefit from this to any great degree except fuel economy in the long term.
 

Ozwes007

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I have seen that before, the experimentation was done on lower revving engines. Graeme Bell and Smokey Yunick both proved the rotation and disproved the gap theory findings with the use of extensive Dyno testing on Race motors in both 2 stroke and 4 stroke configurations. As the revs increase the effect increases.
 

lohring

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Two strokes use pinned rings to keep the ends from catching in the ports. A long time ago the 23 cc engines we used for racing had two rings. One standard modification was to remove the lower ring for more power. The current engines come with one ring pistons.

Lohring Miller
 

a41capt

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Two strokes use pinned rings to keep the ends from catching in the ports. A long time ago the 23 cc engines we used for racing had two rings. One standard modification was to remove the lower ring for more power. The current engines come with one ring pistons.

Lohring Miller
Yep, as an old dirt track motorcycle racer that is correct-o-mundo!

When I five ported my first three port Yamaha engine, I found out exactly how true this statement is!

John W
 

XD351

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There are some radical designs out there for two stroke pistons and how they stop the rings moving .
Have a look at a youtube channel called two stroke stuffing .
 

a41capt

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There are some radical designs out there for two stroke pistons and how they stop the rings moving .
Have a look at a youtube channel called two stroke stuffing .
Well, as the old ad used to say “you’ve come a long way baby!”

My old piston port and/or rotary valve engines can’t hold a candle to the new innovations that abound in the 2 stroke world!

John W
 

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