Compression checks

TexasAviation

TexasAviation

Pre-takeoff checklist
Joined
Mar 9, 2014
Messages
214
Display Name
Display name:
TexasAviation
Forgive me for the newbie question ... but what does this compression check mean? One cylinder is considerably lower than the others on a plane I'm thinking of buying. 600 hours on the Lycoming 180 hp.

#1 - 73/80
#2 - 75/80
#3 - 64/80
#4 - 72/80

This was at the last annual in July last year. Obviously I'll have updated numbers from a pre-buy, but curious how big or small a deal that No. 3 cylinder compression is.
 
Could range all the way from nothing to something. When I did my prebuy, we had a low cylinder. This was right after an annual showing normal numbers. We borescoped, and I took a chance on it being a piece of carbon or something keeping the exhaust valve from doing its job 100%. Ran it hard and next pressure check everything came out well. Or course a valve not working 100% for too long can lead to other issues. Maybe get a good mechanic to borescope and see if that helps determine the cause at all.
 
meh....we need more information. Where is it leaking? and what do the exhaust valves look like?
 
Next compression check, listen at the oil filler (rings), at the exhaust pipe (exhaust valve) and the air intake (intake valve). Easy to isolate the leak.

If the engine was last at the factory before 1999, or didn't get new cylinders with the better valve guide material after that time, it's likely that the exhaust valve is leaking. In 1999 Lycoming started using a high-chromium-content bronze valve guide and their valve issues went away. I would hope that field rebuilders are using the better stuff, too.
 
Checkout_my_Six said:
meh....we need more information. Where is it leaking? and what do the exhaust valves look like?
Click to expand...
All you need to know is,, it's #3 cylinder on a 180 horse Lycoming.
Typically the #3 runs hotter and fails first. pull it have it overhauled, before it causes you a lot of trouble.
 
Dan Thomas said:
In 1999 Lycoming started using a high-chromium-content bronze valve guide and their valve issues went away. I would hope that field rebuilders are using the better stuff, too.
Click to expand...
Why will this be an issue when the other 3 cylinders are still operating as they should yet #3 fails most often? anyone notice the IO- version does not have this problem?
 
Tom-D said:
Why will this be an issue when the other 3 cylinders are still operating as they should yet #3 fails most often? anyone notice the IO- version does not have this problem?
Click to expand...

These things don't fail nice and evenly.
 
Also, just for clarification for those who might not be familiar with this test, it is not actually a compression check (like on automotive engines) but a leakdown test.
 
Has the plane been inactive for a while? That can cause a low number which will likely return to its old value easily.

I'd go over how and when it flew

Filter cut and examination

Borescope

Oil analysis is useless here as you don't have a baseline to compare it to yet.
 
Price an overhaul for 1 or 2 cylinders in your bid...
 
BINGO! It doesn't really matter where the air is leaking. Adjust the price, then run the snot out of it with some MMO in the fuel and hope it mends itself. It it does, you bank a little maintenance money for the future. If it needs work now? You're prepared.
 
Stewartb said:
BINGO! It doesn't really matter where the air is leaking. Adjust the price, then run the snot out of it with some MMO in the fuel and hope it mends itself. It it does, you bank a little maintenance money for the future. If it needs work now? You're prepared.
Click to expand...

For sure use it to beat the seller down on the price, but I'd also want a real world idea of the condition of my engine, thus the borescope, filter cut, etc.
 
Stewartb said:
BINGO! It doesn't really matter where the air is leaking.
Click to expand...
Yer right, it should be pulled and repaired, there is no magic chemical that will put the metal back.
 
Per Lycoming, as linked in post #2 in this thread....

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. b. If the pressure reading for all cylinders is equal and above 70 psi; the engine is satisfactory; less then 65 psi indicates wear has occurred and subsequent compression checks should be made at 100 hour intervals to determine rate and amount of wear. If the pressure reading is below 60 psi or if the wear rate increases rapidly, as indicated by appreciable decrease in cylinder pressure, removal and overhaul of the cylinders should be considered.
 
You must log in or register to reply here.
Back
Top