Pull the back plate and free the rod on the wrist pin. When they are stuck they exhibit that same thing.

  You need to learn about the crock pot method! Just get an old small crock pot, put some straight antifreeze in it (newer stuff OK for aluminum) and put the whole engine in there over night at low setting. It will come out looking like new. Wash off the antifreeze with some hot water, then oil everything and reassemble. I've done it to Fox and McCoy engines and never took them apart, just flushed with old fuel when finished. This process will eat the paint off painted engines. If you have it this far apart and repetitively clean, just lube the heck out of the wrist pin and heat it with a monokote gun to loosen up old castor residue. ATF is good for this also, as it has detergents in it.
  Type at you later,
  Dan McEntee

However, how do I deal with the circlips?  I don't want to press too hard on them.  I'll try to find small enough needle nose pliers.

     I use a surgical hemostat to reduce the chance of it going "sproing" into the carpet or the grass.

     Most of the suggestions on liquids to loosen up the rod are pretty good, but by far the quickest-acting is automotive brake cleaner, that stuff gets into the tiniest gaps almost immediately. But, it works so well that you must then *disassemble* the parts completely and coat them with oil because otherwise it will likely be completely bare metal.


    Brett

Peter - I may have some circlips.
Let me know and I will try and find them.
For what it's worth, I have run Foxes without the clips.
I just did a mirror polish on the ends of the wrist pin and never saw any damage to the sleeve.

Bob Z.

It's finally back together, thanks to the tips here, the circlips from Bob Z., and some patience.

Two things were wrong:

1. The wrist pin was stuck on the rod.  The crock pot / antifreeze method worked.

2. The rod was bent. 

The bent rod symptom was that the cleaned, assembled engine was tighter from BDC on the way to TDC on one side vs. the other.  Assembled, the prop would return when bounced against compression in one direction, but not the other.  On the side that was binding, the rod would not move back and forth with the backplate removed.  On the other side, the rod would move.

Testing the rod by installing a 5/32 drill bit in the rod's small end with the big end installed on the loose, uninstalled crankshaft showed the bend, which was easy to remove by bending the rod.

Though the piston/sleeve feels a little tight near TDC, even when loose and uninstalled, I'm confident enough in this engine to try running it when the weather breaks.

Thanks again for the help and parts,

Peter