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

[test it]

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

[Bushy]

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]

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
> >
>
>

[Orval Fairbairn]

In article >,
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
>


Your best bet is to use your ears! If you hear air escaping (hissing)
at the exhaust pipe, your exhaust valve is leaking; if you hear it in
the intake manifold, intake valves are leaking; hearing it at the oil
filler indicates ring leakage.

A cracked cylinder head will sometimes have a draft at the crack, which
you can feel with your fingers.

[JDupre5762]

>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.

[Big John]

JD

I must be getting really old :o(

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

Must be one of my Irish Leprechauns at work :o)

Big John
`````````````````````````````````````````````````` ``````````````````````````

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.
>

[[email protected]]

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

[Dan Thomas]

Big John > wrote in message >...
> JD
>
> I must be getting really old :o(
>
> The pistons go up and down. The rings go round and round??????????
>
> Must be one of my Irish Leprechauns at work :o)
>
> 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
> `````````````````````````````````````````````````` ``````````````````````````
>
> 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.
> >

[Cam]

Most two strokes have ring locator pins to prevent rotating.

Cam

[Big John]

Cam

That's right. Keeps the end of the rings from catching on the intake
and/or exhaust ports.

Big John


On Tue, 11 May 2004 10:31:06 +1200, "Cam" > wrote:

>Most two strokes have ring locator pins to prevent rotating.
>
>Cam
>

[Dan Thomas]

"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