Leaky Exhaust Valve Follow Up

[RFazio]

I posted about the leakdown readings I had gotten on my O-360 A1A. It was a brand new engine with 120 hours on it after 1 year and the #1 cylinder was reading 80/68 or 70 while the rest were at 80/ 77, 78 & 79.

I first tried staking the valve as some suggested. It did not seem to do anything. Next I followed aerhed's advice and unsprung the valve dropped the exhaust pipe and brushed on some valve grinding compound around the valve seat. I hand ground it by pulling out on the stem. I cleaned it all up well and ran it. Then I rechecked it with a leakdown check. It's now at 80/74. I think I'm going to leave it there and run it for a while and see how it does. I looked at the mating surface of the valve while it was apart and it looked perfect. I though I was going to see a burn't valve, but it looked fine. I couldn't see the seat but I'm not as worried about that.

What do you think should I leave it or try a little more grinding? Thanks for the suggestions guys.

[rocketbob]

I think you should have left it alone even at 68...that's not bad.

[aerhed]

C'mon Bob, guy's gotta have somethin to do on his day off. He gained 4 lbs and got to actually see his valve. Richard, I bet most of whats left is ring leakage. That amount sounds normal normal for your time. Ring/wall fit will vary a few pounds compression on a good engine. In fact you'll read a couple pounds different every time you check. Can you hear any hiss at all out of that exhaust while your testing at TDC? That's what tells you if it's really sealing. Compare it's sound with another. On the same pipe if your exhaust is back on. Also, lift your dipstick while aired up. See how much is going past your rings. You can listen at the carb/servo for the intakes. Be careful, and don't, I repeat, don't chop your head off. I had a stubborn guy once who didn't want to do a top. I hovered a ping pong ball over the oil filler with a compression tester. Levitated like 2 inches. Guy gave in finally. Richard, brace up good and back that prop partway down the choke while testing and check a few degrees below TDC. That will vary too. Sometimes you'll go 80/80 a few thousands down the jug if your valves is good. Don't chop your head off.

[RFazio]

I'm hearing about the same hiss from the #1 cylinder exhaust valve as the other cylinders. I think I'm hearing more throught the rings, at the oil filler neck. I did try moving the prop a little before and after TDC. The best I got was the 74. I think, like I said I should let it ride for a while and keep an eye on it. Thanks again.

[MrNomad]

Check valve stem lash. It should be .028 to .080. My thanks to Rocket Bob for the education.

[rocketbob]

Any time you get a low compression number, the first thing you do is note it then let the airplane fly for a couple of hours. THEN come back and check it again, and run it hard in the mean time. Sometimes all of the ring gaps line up which will cause a low compression reading, you can have some lead deposits on the valve face or the stem, etc.

[49clipper]

You are exactely right rocketBob. Fly it first, then fix. Especially at 68#.
Jim
AP/IA/EAA tech c.

[Geico266]

Originally Posted by rocketbob

Any time you get a low compression number, the first thing you do is note it then let the airplane fly for a couple of hours. THEN come back and check it again, and run it hard in the mean time. Sometimes all of the ring gaps line up which will cause a low compression reading, you can have some lead deposits on the valve face or the stem, etc.

Yep. I had a 45/80 valve hissing like crazy. Scared the heck out of me. $$$. The A&P said fly it for a day and come back tomorrow. It was 77/80. A second test at the next oil change was 77/80.

[RFazio]

I put a few hours on it after the first 68/80 reading and when I tested it again I still had 68/80. That is what got me nervous.