If your compression tests low

I briefly had a scare this morning and thought I'd pass on a tip:

If your compression test looks low, make sure to check the O-Ring on the compression tester. I had a small crack developing and depending on position it would let a fair amount of air leak out.

I'm guessing that you would see low compressions on all four cylinders and that would lead you to suspect a problem with the tester???

I just had to pull a cylinder and have the exhaust valve reseated due to low compression. Cost me about $250 for the cylinder work. Glad it was only one cylinder that could be reseated rather than having to replace a couple of jugs!

I've learned over the years that if you have a cylinder that is reading abnormally low compaired to the last reading is fly it and re-test. Twice now I have had false readings that were fine after the re-test. My FBO taught me this trick, he could have covinced me to pull the cylinder, but gave me some good advice.

I'll second that!

I have found this on two different RV's that I did a condition inspection on. They both put a few more hrs on them and rechecked to find that they were checking just fine the second time.

Bobby Hester - A&P, Aircraft builder, Pilot, and all around good guy

If you go fly it off....

If you go fly it off, do you lean a little more than normal? Is the culprit build-up that can be safely burned off?

its a leak down test

semantics I know but... If you suspect a cylinder problem and perform the leak down test which we all are accustom to with the cylinder on compression stroke at TDC. This position of the piston in the cylinder barrel effectively tests piston, compression rings, head, valve seat and guides what it doesn't test is about 80 percent of the cylinder itself. Turn the air way down to keep the prop from killing you and slowly back the piston down the cylinder while maintaining valve closure. At this point look, listen and read the pressure gauge for air loss. I have found more than one crack with this procedure. just a point of reference. yours may vary.

Condition Insp.

I had 1 low cyl. (55) Ran it up, still low. Pulled the rocker box and found 1 broken intake valve spring. 6 months ago this cyl was in the 70s at pre-buy inspection. Boroscoped cyl. and all looks good. Next step will be replace spring and make a flight with lots of power, then CK cyl. 1 more time. If its still low Ill just have to pull it and reseat the valves. This is a 9a with IO-320 that had not been flown much in the last 3 yrs.

I had a low cylinder this week, 62, after borescoping it, found nothing, checked the dry tappet clearance and found it out of limits. Decided to pull the lifter from the exhaust side. It had some "gunk" in it that was keeping the valve from fully seating. Clearance range is .028-.080 and I had .012 to start, .033 after cleaning the lifter. Rechecked the compression cold and it's 68 now. Haven't run it yet to get a hot check (still on jacks doing wheel pants), but I suspect that after I fly it five hours or so, it'll be in the 70's again.