My question is, what O-ring material stands up best for model diesel engine contra piston sealing?

Got both of my 1950 McCoy .049 diesels running last week. These are the original radial mount version, not the later beam mount type (though I have one of those as well). Both apparently had worn O-rings because the compression was leaking down. Max RPM with a Cox gray 6 X 3 prop was just under 12K on Red Max fuel of unknown type (that was recommended by Ed Carlson for the MK17 Russian engines). The MK is also plain bearing like the McCoys.

After running both of these engines up to full operating temperature, I was able to unscrew the cylinder on one of them, but could not get the head off the cylinder. The other engines head and cylinder are still frozen solidly in place. Pushed the contra piston out the bottom of the removed cylinder, and found an original white rubber O-ring worn completely flat on the outside.

Made a quick trip to ACE hardware, and found some Buna-N faucet O-rings that fit perfectly. I'm hearing that those are not the best material for contra piston sealing, and would like to know what works better? Viton?

Anyway, the engine now turns about 13.2K, and holds the compression setting nicely.

I was amazed at the sloppy fit of the contra piston on this engine. Don't remember seeing that on my original, but it could have been the same way.

Bill

Hi Orv,

Thanks for the info. 

Looking at a few web sites with O-ring comparison charts, the Viton rings do appear to have the best features for our use. I'll sure give them a try, when I run out of the Buna-N rings.

Like you, I don't use pliers or vice grips. Don't think I even own a pair of large pliers.

Normally, I try heat and penetrating oil to free up engine parts, but the McCoy seemed to need a little heat, along with vibration (running) to free the cylinder. Had to make a new cylinder gasket, because the original crumbled.

I'm going to try to free up the other cylinders, by clamping in pine or other soft wood slit rings, bored just big enough to slip the cylinders into. A hose clamp should work to compress the wood clamps, and keep even pressure around the cylinder fins.

Someone suggested using a strap wrench, but I've had no luck with those.

Bill

George,

Thanks for pointing that out. 

I stand corrected - again!   

Had to look up the fine article in MEN, and it mentions dual ball races. Could have (and obviously did) fool me with the straight crank housing on that engine.

http://modelenginenews.org/cardfile/mk17.html

Might also explain the surprising good performance of my McCoys, as that Red Max fuel (label unreadable) is probably blended for ball bearing engines, with a higher kerosene content than the equal parts blend I always mixed.

I'm really tempted to order a MK17, but they are a bit heavy for an .09, and I've never owned an engine that size.

How do you like your trio of MK17 engines?

Bill

Jim,

Those are probably Buna-N, commonly used for faucet cartridges.

I bought a few of them myself, and as little as I fly,  they should last for a while.

The chart I looked at says they are good up to 300 degrees F, so I'm not expecting them to last forever. I have no idea what head temperatures are normal for a diesel model engine, so perhaps the operating temp is low enough for these rings.

What diesels are you running?

Bill

Viton O-rings can be had from smallparts.com along with viton fuel tubing and just about any wierd s.. uhh, stuff you might imagine.  Great place.

To remove cylinders, cut a block of wood about 1/2" square bigger than the outside diameter.  Drill a hole as close to the O.D. (but just oversize) as possible in the block, then cut the block in half vertically with a bandsaw, saber saw, jig saw (you do want some gap from the cut, sand it in if you use a razor saw).  Use this as a clamp in a vice to apply whatever force needed to remove the head from the cylinder or the cylinder from the crankcase.  The fins, even aluminum ones will dig into the wood without ever being damaged (if you apply enough force to eliminate slippage).  Due to the shape, you can't distort the cylinder or chew up the outside.  This has worked for me down to dealing with my Bambi .009 (yes that is the correct number of zeroes).

Thanks for the link, Larry. 

Don't have a regular drill press, or a vice, but I know people who do. 

Bill

                     Pliers -- pipe wrench -- flat screw driver -- rusty pocket knife
                 
                                         what more could a guy wish for?

Cheap parts, to replace all the ruined ones!