Compression loss on cylinder when rings line up Truth of fiction?

I had a Datsun van once that had the rings lined up when I pulled it down
due to failing on one cylinder. That piston had a great big trench burnt
down the side where the ring gap was, about 3/8 inch wide and burnt right
though the side of the piston. All the rings were lined up, including the
three part oil control ring set.

It had managed to get me home on three cylinders, once I had disconnected
the crankcase blowby pipe from the air cleaner and removed the air cleaner
(switch to hot air intake?) as the blown out oil had saturated the air
cleaner. It also left the best smoke trail you have ever seen all the way
home! (info for skywriters!)

There is a theory that the rings rotate whle the motor is running, but,
since when has the real world fooled around with theory?

I'd prefer to pull the cylinder and check what is really wrong rather than
trying to guess in the air. Remember it's a bit hard to pull over and check
under the bonnet on the side of the sky!

Hope this helps,
Peter


"test it" wrote in message
...
I have heard many people, including a couple of A&Ps, say that when
the rings on a cylinder all line up that the cylinder will loose
compression. This doesnt make sense to me because if the rings are
going to leak, then they would leak anyway. There is an air path
between each ring for the air to go.

The reason I am saying this is that my wifes RV-4 engine has a
cylinder that is reading 55 psi on the compression check. She was
advised to run it a few more hours and then recheck the compression
because the rings might all be lined up and be causing the low
compression.

What say the experts. Old wifes tale or not?

Regards,
Tom Velvick

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

"Bushy" wrote in message
...
I had a Datsun van once that had the rings lined up when I pulled it down
due to failing on one cylinder. That piston had a great big trench burnt
down the side where the ring gap was, about 3/8 inch wide and burnt right
though the side of the piston. All the rings were lined up, including the
three part oil control ring set.

It had managed to get me home on three cylinders, once I had disconnected
the crankcase blowby pipe from the air cleaner and removed the air cleaner
(switch to hot air intake?) as the blown out oil had saturated the air
cleaner. It also left the best smoke trail you have ever seen all the way
home! (info for skywriters!)

There is a theory that the rings rotate whle the motor is running, but,
since when has the real world fooled around with theory?

I'd prefer to pull the cylinder and check what is really wrong rather than
trying to guess in the air. Remember it's a bit hard to pull over and
check
under the bonnet on the side of the sky!

Hope this helps,
Peter


"test it" wrote in message
...
I have heard many people, including a couple of A&Ps, say that when
the rings on a cylinder all line up that the cylinder will loose
compression. This doesnt make sense to me because if the rings are
going to leak, then they would leak anyway. There is an air path
between each ring for the air to go.

The reason I am saying this is that my wifes RV-4 engine has a
cylinder that is reading 55 psi on the compression check. She was
advised to run it a few more hours and then recheck the compression
because the rings might all be lined up and be causing the low
compression.

What say the experts. Old wifes tale or not?

Regards,
Tom Velvick

"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