"Bill Daniels" wrote in message news:9eAnc.16015$xw3.1082972@attbi_s04...
I believe the ring gap story. I've had several engines lose compression on
one cylinder and then check just fine after an hour or so. I had a Lyc
O-360 that would go on "automatic rough" every four hours. I figure it had
to be the ring gaps lining up like clockwork.

There is another thing that can cause a temporary compression loss and that
is a chip of carbon caught between a valve and its seat. The carbon can
crush and stick in a way that will prevent the valve from fully seating for
an hour or so. It burns away and the compression comes back.

Your A&P gave good advice. Assuming you have no other reason to suspect
problems with the engine like high oil consumption, run it for an hour or so
and re-check. Owners who panic at sudden low compression on one cylinder
keep the cylinder replacement industry very happy.

Bill Daniels


Some mechanics probably aren't moving the prop a little when doing
the differential compression test. I've had lots of cylinders show low
compression when the air is initially applied, and rotating the prop a
few degrees either way will almost always stop the leak. The
compression rings aren't seated tightly against the lower lands as the
crank is rotated with no compression, and they often won't seat until
they're nudged a bit. I sometimes find the best seal 10 degrees or so
on one side or the other of top dead center. Gotta hang on tight,
though.

Dan