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O-290 Broken Rings

Tom Jones

FRIEND
Crestline, CA
Okay guys, here?s one for you engine gurus. I have a ?51 L21-A with an O-290-11 (military D) I feed 100LL. I?ve used TCP for the last year. Can?t use mogas due to the alcohol here in California. That and I?ve always tried to support my local FBO.

Just finished an annual where I sprayed water through the intake for a while to try blow out the excess carbon and lead to get the compression up. It was just enough water to make it start to lay down a bit, then back off and repeat when the rpm picked up. (Yeah, I know I probably have too many old hot rodder friends). Silly as the procedure sounds, I get 50 to 75 static rpm more after the water. This is the second time I've used the water, the first was last year.

At the suggestion of my old AI, I also lapped the valves without removing the cylinders, using fine valve grinding compound. Blew all of the compound back out the intake and exhaust ports - the airplane was tilted so it would run out - with solvent and a sprayer. I had done the same thing two years ago with really good results. Seemed to work fine this time, too.

The compression tests were good (three in the high sixties and one in the low seventies) after all this and the engine ran great. I took it to Lompoc for the Cub flyin. Ran great up and back, and for a few hours more. After a little over ten hours post annual, at the end of two 30 minute flights I noticed a lot of my nice clean oil lubing up the belly instead of the engine. Not down much on the dipstick, but the belly was soaked. Everything had been fine prior to this.

I did a warm compression check and ALL the cylinders had gone south to one degree or another. Yesterday I pulled all four and found broken compression rings in 2, 3 and 4. Cylinder #1 looked okay, even though the compression had dropped by about 6 lbs. No apparent cylinder wall scoring or damage to the pistons in any cylinder.

Because this seems to have happened more or less all at once, I?m convinced I killed it with the grinding compound even though I used almost a gallon of solvent to clean it out. Still, it seems a bit unlikely that a little remaining grit (if there was any) would have caused broken rings almost simultaneously in 3 of the 4 cylinders. I can?t believe I did that poor a job flushing the stuff back out! Of course, if everything I believed to be true was?

Anyone ever heard of something like that happening? Was I as big an idiot as it appears?? The engine is a little over 6000 TT, 875 SMOH, about 775 since top, and about 300 since I put rings in #4.

As a second question, should I top the O-290 or stick an O-320 on the front?

The Myers STC from Wag-Aero is two part, the first is the 320 and the second the gross weight increase. I don?t want to uncover the fuselage right now (I already have one project in the fire), and none of the fabric guys I?ve talked to think it?s even close to reasonable to weld in the extra tubes without a recover job. They think any attempt to do patching that extensively is insane! So, if I go the 320 route it's probably without increasing the gross weight.

So what say you guys??

Tom
 
My opinion.......

Tom,
I don't believe you did any harm lapping your valves. I think the harm came when you added water to a hot engine, it was too much expanding/contracting/expanding for the rings.
An old trick the WWII military used for radials using lots of oil was to let bon-ami be sucked up into the intake, run like heck for a while, then drain the oil. Abrasives cause wear, not breakage. Now the cold water is quite another story, in my humble opinion.

Mike
 
The water is basically a vapor when it goes in. Like water injection on the radials. I have seen a lot of cylinders that were logged as topped where all they did was put rings in. Did they check ring end gap or mic the cylinders? I would be afraid the grinding compound would go up the guides and cause them to go south. I would go with the 320 if the pocket book allows. Old cylindrs tend to cause problems down the road. I don't mind the O-290 engines but I do favor new cylinders at overhaul and as far as I know there are no O-290 cylinders being manufactured. If your pocket book doesn't allow the O-320 get those cylinders checked for size and roundness along with the valves and heads for cracks.
 
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