[YellowBird]

Hi folks,

I have a ST G-51 ringed engine. It's about 5 years old and has been lots of flying time. Lately it seems to have lost some compression. I can turn it over by hand with a little, but not a lot of resistance (plug installed). I figure it may need a new ring. I have taken the engine apart to have a look. I can push the ring entirely into the groove, so I figure its likely thinned out over the years to the point where I should replace it. Does anyone know how thick the ring should be? Just wondering before I order one up. I'd hate to get a new one just to find out its the same size as the old one.

Any thoughts?

[AJCoholic]

A ring doesnt wear in "thickness". The usuall cause of ring's loosing compression is ingestion of dirt, wear on the ring and cylinder surface. Is the cylinder clean and shiny withiut any vertical grooves? Try taking some fine (220 to 400 grit) emery, and with a little oil roll it up and insert into the sleeve and rotate while stroking the sleeve back and forth. Do NOT go up and down, just around and back and forth trying to recreate the original honing cross hatchpattern.

Reassemble and see how it is.

Also, if you flip against compression backwards, is it better? A ringed engine once broken usually feels like it has more compression flipped backwards.

I doubt the ring has worn out unless you have run many hours a summer over the last 5. Any idea of the accumulated time? How many lean, hot runs?

[YellowBird]

The inside of the cylinder is very smooth, there's little or not remnant of a crosshatch pattern on it. The piston isn't sloppy in the cylinder. I'm not sure if thickness was the right word. The ring fits neatly into the groove, perhaps width is a better word.

The engine has done time on three different planes. I can only guess at the hours of operation. I don't keep track of flights.

I can admit to probably having had a few lean runs on it. For a time I ran an 11x4 APC on it but noticed the exhaust was expelling black material from time to time. Lean runs on it are not the norm, it's usually a green mess on the plane. I have been running it on Cool Power, which I think might be a no-no for a ringed engine?

Thanks..

[AJCoholic]

Cool power is fine, as is any 100% symthetic good fuel as lon as you dont run lean.

I would try to re-pattern the sleeve and then run it a bit. How does it handle? I mean, is the engine still idling well, and accelerating OK? Top end power?

All these things can tell you what is wrong if anything...

Andrew

[Vulcan1]

Jamie,
Just to add to Andrews comment the ring should be able to slide in the groove otherwise if it doesn't that's a no no and its seized and won't do the job. Also is there a pin in the groove for keepin it from rotating. Just trying to be helpful.
John

[kersplat]

I have owned one of these powerplants and the maximum ring end gap should not exceed 10 or 12 thou. A piece of paper is @ 2-3 thou for comparison. If you add some Baker AAA castor oil @ 1-2 oz. to the fuel it will help performance if the ring is worn excessively, with the expense of a little varnish buildup. The cylinder is not chromed I believe. It is made of leaded steel and the rings are usually cast iron or chromoly steel. Cross hatch patterns will not be visible once the cylinder is worn excessively. If you have a cylinder with barely visible crosshatch marks, there is a possibility that the cylinder is slightly out of round and a new ring will not fit exact in the cylinder causing hard starting and slow break in. I have re-built many pattern engines where the cylinder was "blue" on the outside top 1/4 inch and it was out of round. Sorry for the Blah Blah I hope I might have helped.

[YellowBird]

Yup, the pin is in there. The engine seems to run just fine, it just doesn't seem to have the compression it once had when you roll it over by hand. It's currently disassembled so I haven't tested rolling it over backward to see if it has more compression.

I'm going to take the advice so far and give it a shot with the emery paper. I'll let you know what happens.

Thanks guys,

[AJCoholic]

If the engine is still performing as it was, ie, all OK, then dont worry about it, unless it becomes hard to start.

[Vulcan1]

Jamie,
Some more food for thought. Some rings are designed to seal on compression when firing. I can't remember all the types but it sounds like if it is running do like Andrew says until you have problems.
John

[YellowBird]

I just finished cleaning and reassembling the engine. I used some 150 grit emery cloth to restore a cross hatch pattern to the inside of the sleeve as Andrew suggested. The engine has been totally disassembled, various parts boiled in antifreeze in my "high tech deep fryer/engine cleaner device" and cleaned of all gum and varnish. I pre-lubed everything with lots of machine oil while reassembling it. I'll run it up in the morning and see how things seem from there.

I noticed that it does seem to have more compression turning it backward than forward. That's interesting...

We'll see what the morning brings....