Luycoming 0-235 Question

[Ventucky Red]

Hi,

Would help is I knew how to spell Lycoming

Looking at a plane equipped with a Lycoming O-235 with the following compressions

#1 62/80 #2 75/80 #3 71/80 #4 72/80

The plane has not been run consistently in the past two years and I am wondering if after a few hours of x-country if the #1 compression could come up.... or are looking at a new jug?

There is no oil around the base of cylinder the heads, or the push rode tubes..

Thoughts?

Thanks

Ventucky

 

[Clip4]

It might come up, may go down, may stay the same, but my best guess is it would increase.

 

[Doug Reid]

Go fly a few hours at cruise power...check again.

 

[G-Man]

Been meaning to clarify this for a while. When we look at these numbers:
#1 62/80 #2 75/80 #3 71/80 #4 72/80

What do those represent? Obviously, cylinder numbers. But then compression with/without oil in the cylinder? Hot/cold?
Thanks for the tutoring.

 

[GRG55]

Been meaning to clarify this for a while. When we look at these numbers:
#1 62/80 #2 75/80 #3 71/80 #4 72/80

What do those represent? Obviously, cylinder numbers. But then compression with/without oil in the cylinder? Hot/cold?
Thanks for the tutoring.

Differential compression test results. The 80 is the regulated pressure measurement (80 psi) and the numerator is the actual measured pressure in the cylinder.

https://www.aviationpros.com/engine...640/engine-compression-testing-back-to-basics

https://www.aviationpros.com/engine...compression-testing-subject-to-interpretation

 

[G-Man]

Thank you!

 

[Omalley1537]

I’m sure you’ll get many more posts about this, but I’ll just add that you need to look at a lot more than just compression numbers. Borescope, oil filter, etc etc. On a Lycoming, typically a far greater (and potentially more expensive) concern is the camshaft, since it’s up high in the engine and subject to oil draining away with infrequent use. This leads to corrosion, followed by cam spalling/lifter damage, followed by an unpleasant conversation with an A&P...

 

[Dan Thomas]

The O-235 tends to run fairly cool, so condensation in the crankcase (from combustion blowby, NOT environmental factors) causes corrosion, particularly between the cylinder walls and piston skirts. #1 and #3 are front cylinders, which gets too much cooling air in some installations, and also show the lowest compression numbers, so borescoping is in order to get a look at the condition of the cylinder walls.

The corrosion, when it gets bad enough, results in the rings eating away cylinder metal and increasing the diameter of the bore just a little. This leaves a ridge at the bottom of the oil ring's travel, and as the piston pin's aluminum end plugs pass over this they get shaved and aluminum chips start showing up in the oil filter. If the wear isn't too bad the cylinders can be lightly honed and put back into service. In the case of a Citabria 7ECA we had that used an O-235K2C, we would run into this problem at around mid-time. It happened on at least two separate engines in that airplane, both Lyc factory engines. I did not see any corrosion on the camshaft when we removed the cylinders for cleanup. I did find water droplets between the piston and cylinder when we pulled those cylinders, water remaining from the runup before the inspection. The engine had sat at least overnight, so was cold, the conditions perfect for combustion blowby.

 

[chemgeek]

I would be relatively unconcerned about a single compression measurement of 62/80 unless accompanied by other symptoms like oily plugs or metal in the filter. Fly an reevaluate after 10 hours or so. If it doesn't improve, figure that cylinder is showing more advanced wear and if it starts pumping oil, resulting in increasingly fouled plugs, it might need replacement.

The O-235-C2C in my AA1A was a pretty robust engine. I did replace one cylinder after 400 hr of flying on a mid time engine. It started pumping oil although compressions weren't bad. (Compressions aren't everything, and oil in the cylinder can prop up compression measurements). No biggie to swap a cylinder.