I'm new to the forum but am a veteran combat flyer and model builder. I purchased four new MK 6 engines when they came out. Two of them were well broken in and flown in slow combat for 4 years or so. Very reliable and one flip or two for starting. After the demise of slow combat i removed the engines from the planes cleaned and oil them well, put them in a rag and back in the box. Several years later i decided to fly the slow planes in a Sunday flying session. Mounted the two engines on planes and put up a flight on each. When i started to fly the second flight both engines were seized up. Took them home to the shop and disassembled both. One of them had a small ledge at the bottom of the boost port and a small groove in the piston, figured that one was trash. In the second one the piston would not go past the exhaust port. After some hand lapping i reassembled the engine put it on the test stand and fired her up.No problem. My question is any thoughts on why they would seize up after years of running. I was using Power Master 10/29

                 Ray, depending on what oil you used when you stored them can do just what you stated. 3 in 1 for example will nicely lubricate the parts but when stored, it turns to a shellac like glue which when turned over feels like the cylinder is rusted. Transmission fluid or Marvel air tool oil not Mystery oil works terrific. The other thing is that it's imperative to completely burn off all traces of fuel in the engine by hooking the clip to the engine and flipping burping out any residual fuel prior to storing as this can do damage to bearings if slight traces of fuel is left behind.  The MK VI is a ABC engine. You mentioned the use of 10/29. That's really too much oil  for that engine. The MK VI is the only ABC engine out of all the Mark engines. Even the MK 3 iron piston  being the first gen MK engine only needed 20% oil.  I'm surprised it even ran without the plug cooling off. There's so much castor in that fuel that it will absolutely glue the parts to the point of seizing if left to sit. Having that much oil can even take out plugs due to over compression in that engine. The Mark VI also has a groove around the top of the piston from the factory. Is this the groove your making reference to? The ledge at the bottom of the liner is something I can't explain. You only need a oil content of at least 20% with that engine  and a 50/50 blend of castor and synthetic.

                 One thing that I always would check on these engines prior to running was the backplate. There was always a lot of flash around the top of the backplate where the backplate hugged the bottom of the cylinder liner. Mark Smith would grind out this channel which usually resulted in coming through the backside of the backplate in which he would fill that entire cavity on the outside with JB weld. If something like this possibly broke loose it could easily find it's way into the liner due to the port cut into the sleeve directly there. If you remove the backplate, the port is staring you right in the face. Ken