Compression Variances In O-320

[Geico266]

I was helping a buddy do his condition inspection on his RV-9a. It has an 0-320 with compression testing warm at #1 78, #2 70, #3 78, #4 70.

250 hours ago #1&3 cylinders were rebuilt.

When do you get concerned about replacing 1&2? What compression differential between the cylinders causes concerns?

Engine runs smooth as silk in the air. I have ridden and flown this plane. Nice smooth engine.

 

[Ted]

I wouldn't be concerned about those cylinders at all. They're plenty legal. Wait until it gets into the realm where the regs say "must do something."

 

[Geico266]

I've always been told that it's the difference pressure difference you look out for. When would you get nervous? 10 psi differential?

 

[jesse]

I've always been told that it's the difference pressure difference you look out for. When would you get nervous? 10 psi differential?

I wouldn't worry about it whatsoever. Check it the next time around. Lycoming's guidance and it is just that..guidance says:

Interpretation of the results of the test is highly depndent on the skill and judicious opinion of the tester; however, the following observations cover the principle factors to be noted:

a) Pressure readings for all cylinders should be nearly equal; a difference of 5 psi is satisfactory; a difference of 10 to 15 psi indicates an investigation should be made.

NOTE

Unless the pressure difference exceeds 15 psi the investigation should not necessarily mean removal of the cylinder; often a valve will reseat itself and result in acceptable compression during a later check which should be made within the next 10 hours of operation.

 

[Ted]

Jesse quotes the appropriate material. Also, keep in mind that the difference assumptions mostly assume all cylinders are aged equally. In this case you have two recently overhauled cylinders and two older ones. Given that, the results are exactly what I'd expect.

 

[bnt83]

I've always been told that it's the difference pressure difference you look out for. When would you get nervous? 10 psi differential?

How does the gauge reading tell you the whole story?

I've caught head cracking, that passed the gauge test, using soapy water...

 

[Geico266]

How does the gauge reading tell you the whole story?

I've caught head cracking, that passed the gauge test, using soapy water...

Well, that's a cheery thought!


The PO replaced 1& 3 before the new owner bought it. The last annual was exactly the same reading.

 

[Geico266]

I wouldn't worry about it whatsoever. Check it the next time around. Lycoming's guidance and it is just that..guidance says:

Jesse quotes the appropriate material. Also, keep in mind that the difference assumptions mostly assume all cylinders are aged equally. In this case you have two recently overhauled cylinders and two older ones. Given that, the results are exactly what I'd expect.

Thank you gentlemen! I appreciate it, and so does the owner!

 

[jesse]

The PO replaced 1& 3 before the new owner bought it. The last annual was exactly the same reading.

Things that aren't changing aren't worth messing with. A good sign really

 

[Skylane81E]

It ain't broke (assuming that it is ring leakage) so don't fix it

 

[Dan Thomas]

Depends where it's leaking. I get concerned if it's at 70 or under. Most guys won't release the engine at 60 or under.

With the pressure on, listen at the exhaust pipe. If the exhaust valve is leaking you'll hear it, and sometimes it's just a bit of carbon dislodged when the sparkplug was removed. Tapping on the valve rocker will often clear it. If not, the valve is probably eroding and it's better to fix it now before it burns badly enough to need a new valve. They're not cheap.

If it's rings you'll hear it at the oil filler opening. Not much to worry about unless the number is low. You can often get a much better reading just by working the prop back and forth a bit to encourage the rings to seat better in the piston lands. Slightly before or after TDC is usually a better bet. Hang tightly onto that prop.

If it's an intake valve leak (very rare) it will be heard at the carb inlet.

Dan

 

[Ted]

Depends where it's leaking. I get concerned if it's at 70 or under. Most guys won't release the engine at 60 or under.

At least in the US, you can't legally release a Lycoming if it's 60 or under. Continentals are another matter. I think 47 was the lowest we had on the 520s before turning them in for overhaul. Whatever the number was, it was legal per the Continental SB.

 

[Geico266]

Depends where it's leaking. I get concerned if it's at 70 or under. Most guys won't release the engine at 60 or under.

With the pressure on, listen at the exhaust pipe. If the exhaust valve is leaking you'll hear it, and sometimes it's just a bit of carbon dislodged when the sparkplug was removed. Tapping on the valve rocker will often clear it. If not, the valve is probably eroding and it's better to fix it now before it burns badly enough to need a new valve. They're not cheap.

If it's rings you'll hear it at the oil filler opening. Not much to worry about unless the number is low. You can often get a much better reading just by working the prop back and forth a bit to encourage the rings to seat better in the piston lands. Slightly before or after TDC is usually a better bet. Hang tightly onto that prop.

If it's an intake valve leak (very rare) it will be heard at the carb inlet.

Dan

Good info Dan, this baby should be a sticky!

Thank you! I'm printing this one out.

 

[bnt83]

Good info Dan, this baby should be a sticky!

Thank you! I'm printing this one out.

So how many bad jugs passed your compression inspections? I thought every builder knew this stuff

 

[Geico266]

So how many bad jugs passed your compression inspections? I thought every builder knew this stuff

None. I was making the point so younger pilot's and self-mechanics would have the information.

 

[bnt83]

Should monitor trends. I've released 65/80 with no apparent leaks conditional that we retest often and then found 45/80 around 30 hours and seven years later. (it sat in storage a while) Subsequent removal showed multiple cracks between the seats.