rotating rings

[40 Deluxe]

I've been thinking about the seemingly common belief that says if the piston ring gaps are all lined up, an engine will not start/run due to no or too little compression. This myth has to assume that rings do not rotate in their grooves, or else we would have engines randomly die just driving down the road! However, rings do rotate. For evidence, just look at the cylinder walls of any worn engine. You will see the most wear at top (known as "taper") with the resulting ridge that is cut away with a ridge reamer to get the pistons out. In over 50 years of doing engine work, I have never seen an unworn spot where the top ring gap was. Rather, the cylinders always have even wear all the way around. Why? Because the ring is constantly rotating in its groove, wearing the cylinder evenly. Thus, myth busted.
A second point: The diameter of a piston in the ring land area is typically about .035" smaller than skirt diameter at room temperature, or .0175" on each side. Thus, only .0175" of the ring gap is exposed to compression/combustion pressure. Let's assume a generous ring gap of .025". This gives a ring gap area of only .0004375 square inches. Now consider how much empty space there is between the top and second rings, remembering that the piston here is .035" smaller diameter than the cylinder. This space has a cross section area of .006 sq. in., counting both sides of the piston and a total volume of approximately .037 cubic inches. Thus, compression/combustion pressure getting through the top ring gap will quickly and easily find its way to the second ring gap no matter if that gap is halfway around the cylinder or directly under the top ring gap. Again, myth busted!

[mike657894]

a second piece of evidence to support you is that many two stroke engines have a single ring.

[GPierce]

I didn't believe rings rotate. I built up my aircraft engine making sure the ring gaps were spaced 120 degrees apart. 1000 engine hours later I had to pull a cylinder for a stuck valve. The rings were lined up on the piston.

This had nothing to do with the stuck valve; too much lead in the fuel and cylinder running too cool to vaporize it.

[Art Newland]

Some two stroke rings are pinnied in place so they can't rotate. Because the ring at the gap could snag on one of the ports in the cylinder.

[C26Pinelake]

If you google the subject you sill find literally hundreds of discussions. And they all end up going nowhere.

[Y-Blockhead]

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Originally Posted by mike657894

a second piece of evidence to support you is that many two stroke engines have a single ring.

Originally Posted by Art Newland

Some two stroke rings are pinnied in place so they can't rotate. Because the ring at the gap could snag on one of the ports in the cylinder.

That's correct. If two stroke rings, be it one or two rings per cylinder, were allowed to rotate it would be disastrous when the ring gets hung up in a transfer port or the exhaust port.

[SeaSlugs]

yea ive wondered that myself of why they say rings must be clocked at X positions...

Even when rings get stuck or broken engines will usually run ok - and burn oil like crazy but thats other issues...

[Jim Brierley]

I'm wondering why anyone is concerned about this, there is little-to-nothing that can be done about it, and it is not a problem!

[Mike V. Florida]

We went through this a while back. A search will reveal both sides of the question.

[Patrick L.]

In my younger days trying to race go karts I ported a few 2 strokes. Of course everyone was right when they said to just square off the top of the port and raise it a bit. By my way of thinking if a little is good them more just had to be better. So I cut all the webbing out of the port and still raised it. That cart went like crazy, for a couple hundred feet.

Not too long ago a Cessna kept having trouble pumping oil from one cylinder. The engine kept being returned to the rebuilder. The owner got fed up and asked us to figure it out. That troubled cylinder had such a taper that every 100 or 200 hours the gaps would line up and start using more oil then it should. Another rebuilt cylinder solved the problem. That engine has gone close to TBO without more issue.

[Flathead]

Stagger them and forget about them.

[40 Deluxe]

Originally Posted by Jim Brierley

I'm wondering why anyone is concerned about this, there is little-to-nothing that can be done about it, and it is not a problem!

I agree, Jim, it's obviously not a problem. Engines run just fine with their rings rotating all over the place. I just get annoyed at these old myths that keep getting mindlessly repeated over and over.

[Art Newland]

A friend of mine had an engine rebuilt in his Cessna 172. Was a 25 mile flight back to his home airport, while on landing approach the engine seized. He had sufficient altitude to land safely, but the engine would not crank over… totally locked up. Inspection revealed no oil. It was attributed to letting an unsupervised, trainee, AP mechanic install the piston/rings/cylinders. He had aligned all the ring gaps. The engine ran fine, but lost all its oil in very short order.

[tbirdtbird]

" I have never seen an unworn spot where the top ring gap was."

BUT Tom Wesenberg has, and posted it a while back

[C26Pinelake]

Originally Posted by Jim Brierley

I'm wondering why anyone is concerned about this, there is little-to-nothing that can be done about it, and it is not a problem!

You are totally correct, it means nothing. besides most people could care less. Put it away and talk about something important like whitewalls ! Wayne

[Mitch//pa]

Originally Posted by tbirdtbird

" I have never seen an unworn spot where the top ring gap was."

BUT Tom Wesenberg has, and posted it a while back

Prob was a stuck ring from carbon

[Tom Wesenberg]

Originally Posted by tbirdtbird

" I have never seen an unworn spot where the top ring gap was."

BUT Tom Wesenberg has, and posted it a while back

That's correct. Back in the 60's when I overhauled a Chevy V8 for a customer I found one cylinder with all the rings lined up and a line of unworn metal running up and down the cylinder wall. That's the only time I've ever seen this happen, but it shows that it can happen.

I have a chainsaw engine with a pair of pinned rings, and for some dumb reason they pinned the rings in line, rather than place one gap to the other side of the port. The cylinder is aluminum with a chrome plating.