What does a broken ring look like?
 

As I mentioned earlier, I was a little non-plussed that the compression in the #3 cylinder had fallen to 71/80, when it was 79/80 or so in January.

I couldn't find any reason for it. I didn't hear any air escaping during test, although I don't have very good hearing and I was kind of busy holding the prop under pressure to keep it from spinning.

So I bought a boroscope, the one Kevin Horton recommended -- the Vividia VA-400 -- which I think is a pretty nice piece of equipment for $149.

Last evening I had a look. Here, go on a 12-minute tour if you can stay awake.

My pal, Mike Hilger stopped by and looked and didn't see anything funny here but then a guy on Facebook said "pull the jug and fix the broken ring."

But what broken ring?

I don't know what a broken ring looks like.

Would appreciate people having a look.

Follow the cylinder to piston gap and observe the top piston ring, count the piston ring gaps, there should be only one gap :D but you won't be able to see the second compression ring, there may or may not be cylinder bore scaring.

Are you seeing a broken ring in the video?

I didn't see one....

Can't comment on the video or broken ring theory, but I have read it is possible that occasionally the ring gaps all line up enough to result in a low compression test. After a few more hours running, the rings have rotated a bit, the gaps no longer line up, and compression is back to normal. If there are no symptoms, perhaps test again after a bit more flying?

Chris

That seems consistent with what Mike Hilger said. "Fly it for 10 more hours and check," he said, noting that it happens quite often that the lower compression, particularly when there isn't an obvious cause, I gather, disappears on the subsequent check.

Sometimes the stars line up, sometimes the rings.

In my high school days I worked at a motorcycle shop. When doing compression checks, if it were low, we would squirt some oil in the cylinder and check it again. If rings were the problem, you would get a better compression. If it were valves, you would not see a change. No idea if this would work with horizontal cylinders.

My guess is that this picture is taken 90 deg to the piston pin. It is scoring from carbon on the top land of the piston. If different from other cylinders, it could be an indication of higher than normal oil consumption for that jug and a possible broken ring.

You might also have noticed more deposits on the plugs from this jug since new if that is the case, as it would have been broken since new.

The plugs here have always been more oily than the other cylinders.

So what's the consensus here: Pull the jug?

I sure trust Mike Hilger's judgment more than that of "some guy on Facebook", since Mike was there with the engine, and the FB guy...?

71/80 is actually just fine according to Lycoming, and they will also tell you that if you have an abnormal compression reading, with no other real symptoms, button things up and fly it some more - then check again. Lacking any other symptoms, I sure wouldn't pull the jug yet if it were mine.

Paul