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I need a little advice on my engine

RFazio

Well Known Member
I have a 6 with a Lycoming O-360 A1A. I just performed my first leakdown test at 120 hours on my engine. The engine was a new one. I found three cylinders were at 80/77, 80/78, 80/79. The number 1 cylinder was lower though, at about 80/70. It's got me a little nervous as I can hear the leak at the exhaust valve out the exhaust pipe. I know I'm supposed to run it for another 10 or 20 hours or so and then check it again. Problem is I was supposed to fly down to Sun & Fun in April. I'm having second thoughts now. Anyone have any suggestions or ideas? The Sun & Fun trip is 6 hours each way for me from New York. I feel like pulling the cylinder and checking out the valve. I don't mind flying a few hours near my airport but I think a trip is pushing it. Is 80/70 bad? Should I worry?
 
70/80 is not a bad reading. I understand your concern that it is different from the other 3 cylinders. My guess is that you had a small piece of carbon holding the valve open.
I would run the engine and not worry.

When you get a reading like this, try removing the valve cover and striking the valve with a "soft face" hammer. Many times this will dislodge any particles.
 
If that don't work.

That means you've got an exhaust seat not fitted perfectly and it will slowly get worse until the exhaust cuts a track in the seat. Then it'll dump to near zero compression, maybe 30 or 40. New cylinders are rarely lapped anymore. "Blued to fit" is what you see. Not as good. I know some piston heli operators that tear-down all their new cylinders for fitting before install. Really ups their reliability. You've probably got more than enough time. It takes quite awhile for new valves to dump even when not perfect. Trick: Unspring the valve in place using rope trick, Drop exhaust tube, smear toothpaste on valve face with pinky and lap by hand by pulling/twirling valve stem. On new valves/seats this will often give you a perfect fit. Won't work on worn rounded interface so much.
 
Thanks For The Advice

I heard about staking the valve but wasn't too sure about trying it. I guess that is the easy first step. If that doesn't work I'll try aerhed's advice. I thought of trying to lap the valve in but figured I'd have to remove the cylinder. I like the pinky idea. I'll let you know how it turns out. Thanks again guys.
 
Make sure that...

...your compression numbers were obtained with a "Lycoming recommended" 0.040 orifice in the compression tool, and not a 0.060 orifice.

If the larger one was used, then you have really bad cylinders....:(
 
If the engine runs great i wouldn't do anything. If you like you can repeat the test 100 hours or so later and maybe the numbers are vice versa. But in the end the numbers coming from a leak test don't tell you anything. Looking for the flame suit.
 
Leaky valve....

I agree with the Go Fly crowd. I found a leaking exhaust valve on the Cub at the last annual and, with it being a hand-started engine, can feel it as I pull it through when starting it. It was 72/80 which is the lowest of the four and, ironically, the newest cylinder but still within Continental tolerance of 68/80 :)eek:) That was over 150 hours ago and, at last check, hadn't changed.

I like the toothpaste fix!:D I may have to try that. Staking the valve can help. Use a rubber or leather mallet.

The referral to the old thread about this in post #9 included someone, if I read the post right, doing the rope trick and using valve-grinding compound! :eek::eek::eek: That is VERY abrasive stuff (it grinds off METAL, after all) and there is no way to get it all out of the cylinder without pulling it. Then you would have a leaky cylinder as the walls and rings would be scratched..... Toothpaste is pretty benign stuff and would likely burn off before any damage to the cylinder.....

If you haven't done the Rope Trick, it is a very handy skill to have! And a valve spring compression tool belongs in your tool box anyway. I have a small box with my "cylinder rope" tucked away in it...beats with a large stick pulling the cylinder!:rolleyes:
 
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