The Cessna 120-140 Association Forum

Engine has Millenium cylinders with I believe maybe 200 hours on them. Previously very high compression checks, I mean high 70’s, even 80 every now and again. Prior to storage. Compressions were not checked prior to storage though so I don’t know if they were low prior or not.
Aircraft was placed in storage for three years, engine was “pickled” by filling with preservative oil, top plugs removed and quite a lot of preservative oil placed in each cylinder and slowly turned over by hand to distribute the oil, oil of course was dripping from both the exhausts and the carburetor, and ran out of the top plug hole, carburetor filled with preservative oil, carb intake sealed with thick tin foil as was the exhausts and crankcase vent, plugs removed and desiccant plugs installed, aircraft covered with sheets and left in a T hanger.

Three years later I’m bringing her out of storage, engine has about 5 hours on it since being brought out of storage, cranks fine, idles smooth, makes 2300 RPM static with metal prop, climbs as well as it always has, about 500 rpm at 80 mph with a fat guy and full fuel. Seal level 80f.

Anyway so today I take a compression check, (cold engine) start on the right side, get 80/80 and 78/80 like always, move over to the left side, see 40 ish on the front cyl and you can hear it’s a ring blow by leak as the sound is from the crankcase vent, Go to the next cyl, get maybe 50ish.

So I close her up and perform about a 5 min ground run, with about 1 min at full power. put her back in the hanger and can get I think 64 and 66 or so compression.

No oil or oil carbon on the plugs, left side plugs look color wise good, right side looks a little rich. engine does blow oil out of the crankcase vent in flight, but it’s always done that.

I’ve not yet borescoped the cylinders.


So, what’s my best option? I’m of the fly it and watch it camp, put some hours on it and see what happens compression wise.
As a side note the cylinder head temp if the gauge is accurate seems to run right at 400F at a 2400 RPM cruise.
Oil temp isn’t excessively high runs just under 200f at cruise, and doesn’t turn black real fast either, Big Lycomings I’m more familiar with and those with excessive blow by run high oil temps and turn the oil coal black real fast.

Fly it.

Drop the mike...

If you think its rings I've heard a solvent flush in the cylinder may free things up.

The best written description I've seen for the procedure is this form post:

https://www.beechtalk.com/forums/viewto ... 2#p1997662

As for what solvent to use, I've heard most people just use MMO.

It’s definitely ring leakage, If I hadn’t been so liberal with the preservative oil and desiccant plugs etc. I’d say the cylinders got a light film of rust and the rings are going to have to reseat.
Seems odd though that it’s two cylinders and they are on the same side.
But anyway, it’s legal, just a whole lot lower compression than I was expecting.
I think I’m just going to fly it and see what happens, only takes a couple of minutes to check compression, I’ll just check it more often and see where this goes.
I know for a fact that MMO isn’t really anything but mineral spirits, red dye and something that smells nice, but I’ve also seen a couple of R-1340’s that will stick exhaust valves if it’s not in the oil, But I think I will adopt the position that as long as it runs as well as it is, there isn't anything to fix.

Just have heard that Continentals were flaky with compression checks, have heard a couple of theories about low tension rings that use combustion pressures to blow the rings out to make a seal etc., just wondered if anyone else had, had this issue and what was the outcome.
Cylinder kits aren’t that expensive, but I’d rather not spend that money if it’s not needed.

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a64pilot wrote: ↑Tue Nov 10, 2020 3:40 pm It’s definitely ring leakage, If I hadn’t been so liberal with the preservative oil and desiccant plugs etc. I’d say the cylinders got a light film of rust and the rings are going to have to reseat.
Seems odd though that it’s two cylinders and they are on the same side.
But anyway, it’s legal, just a whole lot lower compression than I was expecting.
I think I’m just going to fly it and see what happens, only takes a couple of minutes to check compression, I’ll just check it more often and see where this goes.
I know for a fact that MMO isn’t really anything but mineral spirits, red dye and something that smells nice, but I’ve also seen a couple of R-1340’s that will stick exhaust valves if it’s not in the oil, But I think I will adopt the position that as long as it runs as well as it is, there isn't anything to fix.

Just have heard that Continentals were flaky with compression checks, have heard a couple of theories about low tension rings that use combustion pressures to blow the rings out to make a seal etc., just wondered if anyone else had, had this issue and what was the outcome.
Cylinder kits aren’t that expensive, but I’d rather not spend that money if it’s not needed.

Your link wanted me to sign in

Here’s a screenshot. Continental engines tend to leak compared to Lycomings. If it’s making rated power and not showing signs of distress I would fly on.

Here is what TCM says about low compression:

https://www.aircraftspruce.com/catalog/ ... lletin.pdf

Good article about compression values.

https://www.airframesalaska.com/v/vspfi ... ession.pdf

If it aint broke don't fix it.

Fly it.

Rick

Just give it some MMO and fly it like you stole it!