What symptoms are you looking for to determine lost compression?  If everything is nicely lubricated but not tight, and with lots of compression, if you pull the propeller through the top of the stroke the engine should resist you going up, then help you going down.  If it does this -- good.  With a really good seal you can pull the thing to TDC and hold it a bit, then go the rest of the way without any 'suck back'.

And why do you think it's the piston/liner?  If your compression is leaking down you should diagnose where.  Give it a good prime, look through the exhaust port, and turn the prop through TDC.  If you see a few bubbles next to the piston that's normal.  If you see _lots_, then you have a bad piston/liner seal.  Squirt some fuel between head and case, between head and glowplug, and around the glowplug stem, then turn the prop through TDC.  Bubbles indicate leaks, and you shouldn't have _any_ leakage at any of these points.  If you do, correct them.

A nicely fitting piston with a groove will (probably) retain oil better.  A poor fitting piston with a groove will just leak with style.  Given that these were reputed to be good engines back in the day, I wouldn't mess with grooves.

I have no clue where you can go if you need this refurbished -- I haven't flown any of my 'big' engines long enough to wear them out.

. . .  Also on this engine the plug is offset to the exhaust side, is there an advantage to this or maybe someone before me put the head on wrong?

Ray --

It's been about 51 years since I ran a McCoy, but the offset plug head had a slot for the baffle, so the baffle would bang into the head if the head were backwards. Or was that baffled Foxes? Anyway, if you have a baffled piston, you can be sure the head is correct.

As to the compression, I don't remember a spunky flipping McCoy. Know that Country song, "Sound like life, to me . . ." ?

      Larry Fulwider

Thanks Jim, that makes a lot of sense. As soon as the holidays are over i will try your method. Thanks again and Merry Christmas!!!  Ray

Ray,

I'm running a McCoy 35 RH on my Skyray 35, and it starts very easily.

It likes to be wet, so I hand choke it several flips, and squirt a small prime against the side of the piston, attach the glow driver, and back flip it against compression. Usually starts on the first try.

One thing I had to do was swap out the Fox glow plug I had installed, and replace it with a hotter K&B plug, because it would cut out occasionally in flight. Still started easily, so that evidently isn't your problem.

Using Sig 10% nitro, with 25% Castor oil fuel, and a 10 X 5 APC prop. It runs great, and seems to get better every flight.

Good luck with yours.

Bill

I cut my head gaskets for the Red Heads out of .016 aluminum from K&S. I use a paper punch to punch out the inside hole just inside of the line, and then use my Dremel with a sanding drum to sand it out to the line. Then cut the outside with sissors.

McCoy Redhead 35's were low grade, low price sport engines. Tolerances were all over the map. While not as high on the frustration list as WenMac, they caused a lot guys to lose interest in "gas engines".   Sure, there were some good units and some satisfied customers, but there were also countless no-start and one-start McCoy engines.  

A product that sold for $5.95 in 1962 has probably given up whatever it was once worth.

One of my neighbors had a similar story to yours in 1961 or maybe 62. The fix was an OS Max III 35.  The price of two McCoys and infinitely more valuable.

I don't think I said anything bad about the design of the McCoy engine.  It's basically similar to the K&B Greenhead, Johnson, OS Max II-III, or any "blackfin" engine.   The differnece was, that with the others you didn't need to know when to hold 'em and when to fold 'em, like a professional gambler.

I asked for one of those for my 15th birthday, but John Kilsdonk, the pro at Royal Craft, talked my father into shooting the $7.50 and upgrading to a Fox Rocket.  Thank you, John.

Thanks for all the help and suggestions!  So far i have swelled the piston (very easy, a few light taps and it was obvious it was enlarged), honed the liner, buffed up the piston (slightly out of round according to my mic), polished any and all ports and bushings on the case, made a new head gasket and venturi. Back to the bench and same as before, hand flipped for 10 minutes and will only start with a squirt to the piston thru exhaust and electric starter, this while using a cord and glo clip to my power panel and trying 3 different type plugs. Sounds so sweet when running and always settles back to the same rpm after pinching the fuel line. My conclusion is : the scoring on the piston is deeper than i originally thought and after a few runs on the bench i could tell there is just not enough compression to start easily enough to use in competition. I really wanted to use a McCoy on this plane (Scratch build Super Firecat) but will settle for a .25FP. Thanks again for your help, i learned a lot from this exercise.

I wonder if running an all castor fuel would fill some of the scratches with varnish and give you some more compression.  I have several McCoy 19's and really like them.  I have the poorest one on a trainer which has had several hundred flights.  It has no compression.  Head prime and choke; starts in a flip or two and runs fine.