low compression one cylinder

[sleepingsquirrel]

Hi,

 

  I have a 1961 M20B with an O-360 A1D engine.  I've already decided to fly the engine another ten hours and check compression again.  Has dropped from mid 70s to 60 (all other cylinders are mid 70s). I believe that there are only two compression rings on this engine and one oil scavenger ring. The engine is making good power with 1100 fpm climb and runs smooth. However this is the cylinder(#1) that the manifold pressure gauge is hooked to and about 6 hours ago I flew it for about an hour with the tubing broken (broken after take off) . There was no roughness or  loss of power that was noticable at the time (leaned to rough then enrichen).

  I sincerely believe that the compression ring gaps have aligned is causing the differential compression test to be low.

The starting sequence has always been like this :

                                                                                most often  the first compression causes the blade to stop , I release the starter and re-engage and prop goes through with ignition and smooth start.

 Once and only once within the last 2 hours I saw the blade go through compression on to the next cylinder and then same as before "varoom" as usual. I knew that was telling me something.

 (Mechanic and I know this is leaking into the crankcase, we can hear it at the oil filler) but the oil is still very clean.

 

What say you all?

A. Put a quart of Marvel Mystery Oil in the oil and run it for ten hours.

B. Take  the cylinder off and check the rings

C. Quit flying altogether because these things are just too damned expensive

D. Have my wife increase my life insurance  and fly like there is no tomorrow

E. Run it for ten hours and see if the compression is still 60# or has increased

[N601RX]

I would determine if it is leaking out by the exhaust valve or by the rings.

[sleepingsquirrel]

Yes we heard it past the rings.

[Riq]

On my cherokee I thought the same thing, and was told as well at annual that in the 60s was fine for compression. Well, it maybe, but you best keep a good eye on it.

 

My cherokee 320 had a low cylinder and blew out the front seal, and dirty oil all over the front. I could only see through the storm vent slipping; it was hard ifr, 1000agl.  My brother was with me and I told him to keep an eye on the temp and oil pressure as we rerouted back to the airport, ~10nm. I was lined up second behind a twin, they saw me smoking and declared an emergency. Roughly 2miles from the airport my brother alerted me that oil pressure was gone, so I killed the motor and was reasonably sure we'd glide to the closest runway. (I practiced a lot in that plane)

 

We made it, and ill always make sure compressions are good, and oil is not getting remarkably dirty in my planes from now on.

 

Your plane is talking to you, dont ignore it.

[Sabremech]

I'd fly it another 10 hours and re-check it. Marvel Mystery oil should be banned from your hangar.

 David

[Alan Fox]

Marvel mystery oil is not a mechanic in a can..... Put it on squeaky hinges...Thats about all its good for.......60 on compression is good , if its not leaking out the valves , fly it for a few hours and recheck......

[johnggreen]

S/Q,

 

Pulling the one cylinder to find out the problem is quick and relatively cheap.  I simply can't think of any reason to wait.  If it was through the valve then it could easily be a carbon issue, but you seem to have ruled that out.  Pull the cylinder without further delay.

 

Jgreen

[kerry]

Was compression check done cold or hot.  A compression check after engine as been warmed up could give you a better compression reading.

[N201MKTurbo]

You can find out a lot more with a compression tester.

Test it at a few different places on the cylinder other then TDC. You can tell if your cylinder has taper wear. Try it after moving the prop CW and CCW, this will seat the rings against the upper and lower surfaces of the lands. If it seats well in one direction and not the other your piston is warn out. The first thing to happen if you overheat a cylinder is the piston will soften and the ring slots will get misshapen and the compression will suffer.

Just make sure you are holding the prop tight when you are doing these tests so you don't get bonked in the head...

[aaronk25]

Add cam guard, and run it. Does a good job cleaning up ring deposits, maybe a ring isn't seating right or has some crap in the grooves.

[garytex]

All the above is good advise.  If you are going to Cam Guard or MMO it, consider putting the cyl just before TDC and pulling the top plug and filing the cyl. with the juice.  Put the plug back in and GENTLY pull the prop to TDC.  This will force concentrated magic juice into the ring area.  It seems to work better.

 

Back in the day, Eastern's SOP for the ( I believe it was) 1820s in all their DC3s was to run MMO.  They thought it helped sticking valves.  And they did fly around the block at least a time or two. 

Good Luck

Gary