The International Cessna 170 Association

gfeher wrote: now you are supposed to compare the compression readings to the reading you get through a "master orifice" (which can be in the 40's), not an arbitrary fixed number like 60 psi used in the past.

I've been doing 3-4 compression checks a week, for the last 3 years, 98% on TCM products. 42lb is the most common number. I might recall seeing 41lb once and I think I recall 44lb but day in and day out it's likely 42lb

1951 C-170A

Bruce Fenstermacher wrote:I've been doing 3-4 compression checks a week, for the last 3 years, 98% on TCM products. 42lb is the most common number. I might recall seeing 41lb once and I think I recall 44lb but day in and day out it's likely 42lb

That's been my experience as well. The master orifice reading usually has been in the lower 40's. Kinda shocking for those used to using 60.

Good advice….but, I think to get a better sense of the situation I would ask how many hours on that cylinder ?

TSO = 511 hours.

The good news is that after I flew the plane an hour the compression cam up to 68 PSI.

Thanks for all the info and advice.

Two Annuals ago my #4 Cylinder was right on the edge for my IA to sign it off. Leaking by Exhaust Valve. I told him to send it in. One new Valve Guide and Seat.

Last Annual it was leaking by Exhaust Valve again. Repaired under warranty but I had to pay for R&R.

I notice now when I first start it up it's not running "even" until it gets warm. If it's leaking by the Exhaust Valve again this Annual, I think it's getting a new cylinder.

I just received my April 2022 copy of Cessna Flyer and coincidentally it has an excellent article by Bill Ross of Superior on compression testing.

It also has a pretty good looking C-170B on the cover. Congratulations Miles!

Thank you Gene!

A little different cylinder question. My O-300 has 440 TSMO, but it sat for a number of years without being flown. All the cylinders show cylinder wall pitting. The valves look good, including the exhaust. Runs smooth. Burns a quart about every 8 hours. No leaks. The oil analysis reports are all normal. Does the pitting create any reason for alarm?

I once bought a Baron with IO470 Continentals on it which had been sitting outdoors in the grass in Smyrna Beach, FL for over 4 years. After flying it home and doing some restorative work on the interior, avionics, etc., I flew it to a “Bonanza Society Service Clinic” which was hosted by a group of engine shops and attended by the Teledyne-Continental Field Rep who boroscoped both engines/all cylinders. The left engine had chrome cyls and the right one all steel cylinders and (this is in 1996) both had been TC Smith “Re-mans” from 1973. They each had about 1200 hrs on them….but had sat outside for those 4 years in Florida humidity.
The right all-steel cylinders had heavy corrosion in the upper-ends, above where the rings stopped at the top of piston-travel… and minor pitting below…and the chrome cyls showed no corrosion. The compressions were all in the low 60’s…I expected the Continental Rep to advise me to overhaul or exchange that right engine ….(that was why he was sent to the Service Clinic by Continental…to sell factory re-mans)…. and he surprised me by asking how much oil consumption I was experiencing with that engine.
I told him it was burning/leaking about a quart every 5-8 hours…and he recommended I continue to operate the engines…. which I did for the next 4 years putting another 500 hrs on them before selling the airplane to an airline pilot who flew it in Oklahoma for another two years before “topping” both engines.
That airplane is now back in Texas, across the lake from me, owned by a maintenance shop for their own use and those 40+ year-old reman engines are still operating (last I heard) and planned for re-build.

Thanks for the background, your experience confirms what I was hoping to be true.My compressions are all very good as well.

Mark, if you end up needing a cylinder, let me know. I have a nice first overhauled cylinder from Alaska Aircraft engines with a yellow tag. It has all new moving parts and a new piston, rings and pins. I purchased it but didn't end up using it for my plane.