I have 2 Fox 16 engines.  I was given some combat planes drilled for foxes.  Both engines are swap meet finds one I think was nearly new in the box

Engine 1 starts from hand flip, runs for 15 seconds, and stops cold seized.  The motor has got very tight.  Since this one had a lot of tar-like oil, I crock-potted it and took it apart.  One of the circle clips was destroyed and stuck in the intake port.  A little scoring but not bad at all.  It ought to run once cleaned out.   

The Scoring is not bad, but the piston is very tight above the ports.  Hand flipping seems like a bad idea

Engine 2.  Very clean inside,  Stated on hand flip, while setting NVA it stopped sticking tight as well.  Fearing the worst, I pulled it apart.  It is very clean inside, both pin clips are in place and no scoring but the piston is very tight above the ports. 

I am tempted to lap the bores and try again.  But after running a few seconds the both got a lot tighter.  Hand-flipping seems like a bad idea.   

Any idea what is happening?  They will be sport engines on old-school combat planes so they just need to start and run smoothly/

Fox instructions for the .15 suggest not to lap the piston in any way. I had to on mine because it was tight like your mentioning. I didn't want to wipe out the rod so I gave it a bit of a polishing using the piston itself and Brasso. I then took it all apart and gave it a hot soapy water bath with a bristle brush

   No manufacturer ever recommended, or will be likely to recommend, "lapping" an engine by the end user. Having to do that is a manufacturing defect. Unfortunately, you can't get it replaced any more.

      Brett

I have run a number of Fox 15 BB's on models and my son Otto still uses a couple on combat wings but they are a bit more 'sensitive' compared to modern engines like the Fora 15's or even a OS LA 15's and do not like mufflers (which some sites require) or suction feed so much. We have found the Fox 19BB or 25BB to be a much better engine for slow combat along with the OS/Magnum FP/GP 25 but the Fox 15 will do but has to work harder.     

Regards Gerald

Sounds like the problem is with the bore or the case. Does it get better when you loosen the head bolts?

it's just as tight when the liner is out of the case,

I will try the heat gun trick.  I have done that on Novel's in the past. 

it's not ABC it's a steel Cylinder and piston

 

Hello Tight with the liner out and steel liner and piston  , I think some hand lapping is in order...
They only get tighter when hot and a really tight fit will only wear out the rod if you try running it in that way, in fact sometimes the rod is shot before you get a nice fit on the L/P . Some Fox engines especially the cheaper sports models had bad fits weather too loose or too tight. Their best engines like the ABC Deluxe 40 were usually really nice fitting.

Regards Gerald