Big John wrote in message . ..
JD

I must be getting really old (

The pistons go up and down. The rings go round and round??????????

Must be one of my Irish Leprechauns at work )

Big John


Agreed. Rings should not rotate, and won't if the
piston-to-cylinder clearances are within spec. Rings and cylinders are
never perfectly round when new so we usually use straight mineral oil
to make them wear each other until they match up. At that point they
still won't be perfectly round but the highs and lows will cancel each
other out and they'll seal properly. If they rotate after that you
will have really serious leaks. I spent 12 years rebuilding over
17,000 compressors, and in no case did I dismantle an old one and find
the rings (which I had spaced properly on assembly) lined up on
disassembly, unless the thing had been sucking dust and worn itself
completely out and the piston slapping around freely.
On the other hand, some engines are designed to rotate their valves
to keep wear even. Valve leakage can be much more of a problem than
ring leakage.

Dan
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On 10 May 2004 02:11:18 GMT, (JDupre5762) wrote:

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.

True enough but at least during the differential compression check you are
checking the rate of leakage so anything that interrupts the leakage path will
slow down the rate. With the ring gaps lined up the rate will increase.

Do not forget that for the rings to work they must also rotate in the ring
grooves so at some point all the ring gaps could line up making an easier path
for combustion gases to escape.