On Sun, 09 May 2004 16:57:05 -0700, test it
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.

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

Tom, your A&P is not telling tales. Yes because there are ring gaps,
there is a certain amount of compression loss past them but it is
minimal because during engine assembly, you align each gap such that
they are offset from each other.

Additionally, I used to align the gaps so that they were opposite the
side of the cylinder that the piston pushed against during the power
stroke. The idea being that since the piston was pushed against one
side of the cylinder during the power stroke, you don't want the ends
of the rings scraping on that side, so align the gaps on the opposite
side.

When the gaps are offset from each other, the compression has a
torturous path to follow to escape. The path is so torturous that by
the time it actually escapes, the power stroke is over and the rings
have done their job of containing the high pressure.

I had a conversation with a professional engine builder (mostly Chevy
V-8's for racing, but also built Offies) who related a story about a
worried race car owner. The owner wanted "Perfect Circle" rings
because they were gapless and always demonstrated slightly higher
compression readings during compression tests than standard rings.

The builder allowed that that was true. But in dyno tests, he could
never find any difference between the "Perfect Circle" rings and
standard rings with gaps. He said that what was happening was that
when the engine heated up and the oil was doing it's job, the dynamic
pressures within the engine were virtually identical. The only
difference between the two types (gapless vs standard gap type) showed
up during a compression test, and then it was only by a few pounds
difference.

But, to get back to the question, if all the gaps lined up, then the
compression does NOT have a torturous path to escape, it has a direct
path out. Some pistons actually have tiny pins to locate the gap so
that the rings cannot rotate and align the gaps one on top of another.
Most pistons don't have the pins, or at least I haven't seen many.

Corky Scott