Piston to Cylinder - what's a good fit, what's bad?

New here to the forum, appreciate all the good info and experience...
I did search to see if this topic has been covered, certainly has, but must be buried inside some topics, so here's my question:

I've bought several "Thimble Drome" stamped 049, must be original Baby Bee?  All of them were stuck, but soaking in "Marvel Mystery Oil" for at least 24hr, then use of Kroil or PB Blaster penetrating oils gradually loosens them up just fine.  (careful, Marvel Mystery Oil appears to dissolve the rubber venturi gaskets...)  (also use 91% Isopropyl to dissolve out gum after free them up)

Latest motor was locked up, but appeared to have little time on it: piston-rod had very little play.  But when trying to re-assemble, the piston would not easily slide up into the cylinder (I don't force things): the top of the piston would slide up even with the top of the exhaust port slots, then super snug. I didn't see any rust or gum inside the cylinder or on the piston, they appear clean.

Not being certain if it is piston or cylinder, I moved this cylinder to an engine that had poor compression (no "pop" at all, and wouldn't complete a rotation on its own) and now this engine has great compression!  Very good smooth feel and "pop", and completes rotation!  

However the "Loose" cylinder on the "tight" piston will rotate through, but still gets very snug when the top of the piston is even with the top of the exhaust slot.  Now I can make a rotation, but it's too tight to "pop", and just drags around.  Based on the simple swap test, the piston must be the problem, as it's still too tight with the loose cylinder.

Instecting the piston, I don't see or feel scratches.  I don't know any history of this engine, but as tight as the parts are I'm surprised the motor ran... could something somehow have damaged the piston?    Is there anything I can measure or check? Anything else I can do to salvage?

Experience appreciated, and if there's already an answer please direct me to it!! Or is this where I decide if I just send Bernie $19.95?

mike in SC

Cox thin wall cylinders are easily damaged. The most common problem is someone used the wrench in the exhaust slots and deformed the cylinder in that area. This can happen either from being tight in the crankcase on removal or over-torquing on install. It could be bent from crash damage, which won't be obvious to the eye. In both cases the crank/piston will typically be free a the low end of travel near bottom dead center.

I have one engine that appears to have no time on it, but is tight when the piston is just above the exhaust port. Inspection with a loupe showed a bright spot on the bore where there is a bump. On the exterior of the cylinder in the corresponding location there is a small dent. I can only assume it was hit with something either very early in it's life or left the factory this way. One day I'll lap the bore back to life.

Below the top of exhaust port, the surface is not extremely critical and can be touched up with small files and/or some fine sand paper. Use magnification to see what you're doing it's fine work. People have straightened bent cylinders.