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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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 Posted: Thu Apr 16, 2009 9:16 pm Post subject: strange compression test. |
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I installed a new Lycoming O360 (roller cam) into a Tiger a few months
ago. The break-in was normal with one exception.
The #2 cylinder compression was strange.
Compression was checked at 10 hrs and 25 hours since new.
Both times, the compression on #2 fell off dramatically at 20 to 15
BTDC. Compression came back up slightly to 60/80 at TDC. On the other
side, ATDC, compression fell off slightly and then went back up before
going back down.
At 25 hours, the cylinder was pulled, sent to Lycon for inspection.
Valves/seats were recut, rings were honed, cylinder was rehoned. The
plane was flown again for 10 hrs and the compression tested. The
compression fall-off wasn't as bad, but, it was still there. Plane was
flown for another 15 hours. At 50 hours, I removed the cylinder and
replaced it with another new cylinder.
The owner has about 5 hours on it and I'll be checking it again.
Note: when the cylinder was pulled, the piston was removed to the
point where I could remove the wrist pin. The top of the piston was
scuffed in a "V" shape from about 1 inch below the rings to the ring
grooves. At the grooves, the scuffing was about 1 inch wide. I did
not remove the piston and look inside. I sent the cylinder to Lycon
today.
Has anyone ever heard of this before?
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Gary
AuCountry Aviation
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flyv35b(at)minetfiber.com
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| Posted: Fri Apr 17, 2009 6:02 am Post subject: strange compression test. |
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| Quote: | Both times, the compression on #2 fell off dramatically at 20 to 15 BTDC.
Compression came back up slightly to 60/80 at TDC. On the other side,
ATDC, compression fell off slightly and then went back up before going
back down.
|
Compression is normally checked at TDC. Are you saying you held the prop
stationary at 20 and 15 BTDC or where you moving the piston upwards at the
time you noticed the drop in compression? Checking the compression while
the piston is moving upwards doesn't really tell you much. You can
"generate" higher numbers and even see 80/80 or higher depending on how fast
you are moving the piston. Also, many times compression will fall off right
after TDC and then come back up shortly later, especially if there is bore
wear from the top ring at the top of the stroke and wear in the piston ring
land.
| Quote: | At 25 hours, the cylinder was pulled, sent to Lycon for inspection.
Valves/seats were recut, rings were honed, cylinder was rehoned. The
plane was flown again for 10 hrs and the compression tested. The
compression fall-off wasn't as bad, but, it was still there. Plane was
flown for another 15 hours. At 50 hours, I removed the cylinder and
replaced it with another new cylinder.
|
How do you hone the rings?
| Quote: | The owner has about 5 hours on it and I'll be checking it again.
Note: when the cylinder was pulled, the piston was removed to the point
where I could remove the wrist pin. The top of the piston was scuffed in
a "V" shape from about 1 inch below the rings to the ring grooves. At the
grooves, the scuffing was about 1 inch wide. I did not remove the piston
and look inside. I sent the cylinder to Lycon today.
Has anyone ever heard of this before?
|
Not quite sure what the scuffed area looks like. I assume you are talking
about scuffing on the piston skirt at the 12 o'clock position as it is
installed? Does the scuffing go all the way to the bottom of the skirt and
the point of the "V" start there and get wider as it approaches the oil ring
land? Was the piston scuffed on the bottom side at the 6 o'clock position?
Why not just pull the cylinder off the piston when removing it and then
remove the pin and piston? That way you can inspect all of the cylinder
bore and the head area.
Normally, if scuffing occurs it will be on the skirt at the 12 and 6 o'clock
positions. This can be pretty severe on a new engine that has tight
clearances when takeoff power is applied without the engine being warmed up
adequately. The aluminum piston expands faster than the steel cylinder and
starts to scuff.
Since it sounds like you didn't change the piston with the new cylinder, I'd
want to check the cylinder bore diameter vertically and horizontally with a
dial bore gauge and compare that to the piston skirt diameter in both
directions. There might be a clearance issue. The scuffed area as you
described it sounds abnormal and might be due to incorrectly made parts.
Maybe the cylinder is not round near the top and the rings can't conform to
the bore, resulting in a loss of compression. Lycon should be able to take
some measurements and tell you if there is a problem.
Lastly, what about a dynamic cranking compression test. Did you do that on
this engine? I guess that wouldn't show up the problem if it only occurs at
20-15 BTDC and not at the very top of the stroke.
Cliff
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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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| Posted: Fri Apr 17, 2009 10:20 am Post subject: strange compression test. |
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Cliff Wrote:
Compression is normally checked at TDC. Are you saying you held the
prop
stationary at 20 and 15 BTDC or where you moving the piston upwards at
the
time you noticed the drop in compression? Checking the compression
while
the piston is moving upwards doesn't really tell you much. You can
"generate" higher numbers and even see 80/80 or higher depending on how
fast
you are moving the piston. Also, many times compression will fall off
right
after TDC and then come back up shortly later, especially if there is
bore
wear from the top ring at the top of the stroke and wear in the piston
ring
land.
Reply:
All of the compression measurements were made with the piston static at
the various cranking positions from 30 BTDC to 30 ATDC in 5 degree
increments. I started checking compressions at 30, 20, 10, 0, 10, 20,
30 cranking degrees many years ago to build a data base of compression
behavior based on engine time. Compression at TDC was only 60/80 (or
thereabouts. Sometimes as low as 45, sometimes as high as 65). When I
checked compressions BTDC I noticed a dramatic change in compression
that was not expected. Then I did a compression survey and noticed
that somewhere in the 20 BTDC range, there was a compression peak
(75/80) and then a dramatic fall-off to TDC. I had the owner go to
another shop and have them repeat the test to see if they
found
something weird. Their results were the same as mine.
Cliff Wrote:
How do you hone the rings?
Reply: The rings were lapped into the cylinder.
Cliff Wrote:
Not quite sure what the scuffed area looks like. I assume you are
talking
about scuffing on the piston skirt at the 12 o'clock position as it is
installed? Does the scuffing go all the way to the bottom of the skirt
and
the point of the "V" start there and get wider as it approaches the oil
ring
land? Was the piston scuffed on the bottom side at the 6 o'clock
position?
Why not just pull the cylinder off the piston when removing it and then
remove the pin and piston? That way you can inspect all of the
cylinder
bore and the head area.
Reply: I didn't remove the piston from the cylinder. I rarely do
that. Lycon does all my engine work. The scuffed area looks like a
"V" about 1 inch high and 1 inch wide at the oil ring.
Note: This scuffing was not evident when I sent the cylinder assembly
to Lycon for tear-down and inspection at 25 hours. Lycon didn't find
anything specific about the cylinder that would cause a problem. The
compressions were better after Lycon worked on the cylinder, but not
perfect.
I've removed a lot of cylinders and if there is scuffing (and there
usually is) it looks like uniform scratches from the rings to the end
of the piston
skirt. This scuffing is confined to a small area just
below the oil rings.
Cliff Wrote:
Normally, if scuffing occurs it will be on the skirt at the 12 and 6
o'clock
positions. This can be pretty severe on a new engine that has tight
clearances when takeoff power is applied without the engine being
warmed up
adequately. The aluminum piston expands faster than the steel cylinder
and
starts to scuff.
Reply: I always wait until the oil temp is above 100 degrees before
adding power. New, used, or abused.
Cliff wrote:
Since it sounds like you didn't change the piston with the new
cylinder, I'd
want to check the cylinder bore diameter vertically and horizontally
with a
dial bore gauge and compare that to the piston skirt diameter in both
directions. There might be a clearance issue. The scuffed area as you
described it sounds abnormal and might be due to incorrectly made
parts.
Maybe the cylinder is not round near the top and the rings can't
conform to
the bore, resulting in a loss of compression. Lycon should be able to
take
some measurements and tell you if there is a problem.
Reply:
It may sound like that, but, as far as I'm concerned, the pistons and
cylinders are a unit. I never mix or match. A new cylinder 'assembly'
was installed. Ken (Lycon) is going to inspect the cylinder/piston
assembly just-in-case this shows up again on an
other cylinder. The new
assembly I received from Lycon had been torn down, inspected, and
measured before they sent it to me to install.
Cliff wrote:
Lastly, what about a dynamic cranking compression test. Did you do
that on
this engine? I guess that wouldn't show up the problem if it only
occurs at
20-15 BTDC and not at the very top of the stroke.
Reply: Like I said in earlier posts, I am doing dynamic testing of
cranking pressures to build a database of new vs old, roller vs flat
tappet cams, low compression vs high compression, etc. For example:
Compressions on a Lycon overhauled O320 with high compression pistons
are in the 140-145 range. Compression on a roller cam are in the 125
range. Cranking pressure on the subject cylinder was only a point
below the other cylinders. Had the engine felt smooth in flight, and
reached red line in straight and level flight like it did with the old
engine and a bad cam, I wouldn't have thought anything about it. The
engine just didn't feel right.
If Lycon finds something I'll post it. If the new cylinder is better,
I'll post that too.
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_________________
Gary
AuCountry Aviation
Home of Team Grumman |
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budzz89
Joined: 17 Apr 2009
Posts: 1
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| Posted: Fri Apr 17, 2009 11:39 am Post subject: Re: strange compression test. |
|
|
| GrummanDude wrote: | Cliff Wrote:
Compression is normally checked at TDC. Are you saying you held the
prop
stationary at 20 and 15 BTDC or where you moving the piston upwards at
the
time you noticed the drop in compression? Checking the compression
while
the piston is moving upwards doesn't really tell you much. You can
"generate" higher numbers and even see 80/80 or higher depending on how
fast
you are moving the piston. Also, many times compression will fall off
right
after TDC and then come back up shortly later, especially if there is
bore
wear from the top ring at the top of the stroke and wear in the piston
ring
land.
Reply:
All of the compression measurements were made with the piston static at
the various cranking positions from 30 BTDC to 30 ATDC in 5 degree
increments. I started checking compressions at 30, 20, 10, 0, 10, 20,
30 cranking degrees many years ago to build a data base of compression
behavior based on engine time. Compression at TDC was only 60/80 (or
thereabouts. Sometimes as low as 45, sometimes as high as 65). When I
checked compressions BTDC I noticed a dramatic change in compression
that was not expected. Then I did a compression survey and noticed
that somewhere in the 20 BTDC range, there was a compression peak
(75/80) and then a dramatic fall-off to TDC. I had the owner go to
another shop and have them repeat the test to see if they
found
something weird. Their results were the same as mine.
Cliff Wrote:
How do you hone the rings?
Reply: The rings were lapped into the cylinder.
Cliff Wrote:
Not quite sure what the scuffed area looks like. I assume you are
talking
about scuffing on the piston skirt at the 12 o'clock position as it is
installed? Does the scuffing go all the way to the bottom of the skirt
and
the point of the "V" start there and get wider as it approaches the oil
ring
land? Was the piston scuffed on the bottom side at the 6 o'clock
position?
Why not just pull the cylinder off the piston when removing it and then
remove the pin and piston? That way you can inspect all of the
cylinder
bore and the head area.
Reply: I didn't remove the piston from the cylinder. I rarely do
that. Lycon does all my engine work. The scuffed area looks like a
"V" about 1 inch high and 1 inch wide at the oil ring.
Note: This scuffing was not evident when I sent the cylinder assembly
to Lycon for tear-down and inspection at 25 hours. Lycon didn't find
anything specific about the cylinder that would cause a problem. The
compressions were better after Lycon worked on the cylinder, but not
perfect.
I've removed a lot of cylinders and if there is scuffing (and there
usually is) it looks like uniform scratches from the rings to the end
of the piston
skirt. This scuffing is confined to a small area just
below the oil rings.
Cliff Wrote:
Normally, if scuffing occurs it will be on the skirt at the 12 and 6
o'clock
positions. This can be pretty severe on a new engine that has tight
clearances when takeoff power is applied without the engine being
warmed up
adequately. The aluminum piston expands faster than the steel cylinder
and
starts to scuff.
Reply: I always wait until the oil temp is above 100 degrees before
adding power. New, used, or abused.
Cliff wrote:
Since it sounds like you didn't change the piston with the new
cylinder, I'd
want to check the cylinder bore diameter vertically and horizontally
with a
dial bore gauge and compare that to the piston skirt diameter in both
directions. There might be a clearance issue. The scuffed area as you
described it sounds abnormal and might be due to incorrectly made
parts.
Maybe the cylinder is not round near the top and the rings can't
conform to
the bore, resulting in a loss of compression. Lycon should be able to
take
some measurements and tell you if there is a problem.
Reply:
It may sound like that, but, as far as I'm concerned, the pistons and
cylinders are a unit. I never mix or match. A new cylinder 'assembly'
was installed. Ken (Lycon) is going to inspect the cylinder/piston
assembly just-in-case this shows up again on an
other cylinder. The new
assembly I received from Lycon had been torn down, inspected, and
measured before they sent it to me to install.
Cliff wrote:
Lastly, what about a dynamic cranking compression test. Did you do
that on
this engine? I guess that wouldn't show up the problem if it only
occurs at
20-15 BTDC and not at the very top of the stroke.
Reply: Like I said in earlier posts, I am doing dynamic testing of
cranking pressures to build a database of new vs old, roller vs flat
tappet cams, low compression vs high compression, etc. For example:
Compressions on a Lycon overhauled O320 with high compression pistons
are in the 140-145 range. Compression on a roller cam are in the 125
range. Cranking pressure on the subject cylinder was only a point
below the other cylinders. Had the engine felt smooth in flight, and
reached red line in straight and level flight like it did with the old
engine and a bad cam, I wouldn't have thought anything about it. The
engine just didn't feel right.
If Lycon finds something I'll post it. If the new cylinder is better,
I'll post that too. |
yeah pretty good you have that kind of cylinder, thanks for the help
_________________
aluminum plate
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