Any reason why the piston and sleeve on my engine would all of a sudden tighten up when it was loose before? My engine roughly gone through around 3/4 gal of fuel and should be fully broken in. Previously when taking the head off and turning the crank over, theres little to no resistance from the piston reaching TDC. But after the session this past weekend, I turned the engine over again while the head is off and felt quite of bit of resistance when the engine reached TDC.

Not sure how something being loose before can suddenly be tight. I just hope I didnt mess anything up with the engine and warped the piston/sleeve.

Did your crank possibly move and now is rubbing on back plate? Make sure crank is all the way out after any service of engine. Good luck.

I'd be pulling it down to inspect it fully before running again. Could it be circlip that holds piston pin in for the conrod?

When the piston gets real hot the molecules inside will will slightly change the material and give a permanent grow. It is a part of the break in and why there is a heatcycle methode as one of the many break in methodes..

This was a pull start engine to begin with and I just installed a pull start delete so yes the back plate was off. There arent any visible scoring on the back plate to indicate the crank/conrod was touching. Turned the engine over with the back plate off and was still tight at TDC. Crank wasnt touching any other part of the engine either and spins smoothly
The crank spins freely and piston slides up and down smoothly. The clip is still firmly in its place and have not made contact with the sleeve.

You didn't change to a fuel with more oil? Or add more oil to your fuel. Grasping at straws here. Sounds like it's getting close to time to run it and see what happens. I've never heard of an engine successfully giving itself a pinch.

More oil lube wont' causes a engine to seize up like this? It will help. .I suspect a bad bearing?

It's not a big deal to pull the engine apart and see what the bearings feel like. But he said the crank spins freely and piston slides up and down smoothly. Who knows. I'd have the whole motor in pieces on my table, but that's just me. I like to work on stuff like that.

Even if you do not understand this you should not ignore this.

It is a process every metal knows when heated up and cooled down. Then the material does not get heated up more then it will stay the same size, but when you go to lean on an engine, making it hotter dan used before, then the next day you will feel a higher pinch again.

A had a similar problem. engine stated ran fine for a few seconds and stopped. it would stop at TDC every tine and was loose on the bottom. Turned out to be a bad bearing inside. The bearing cage shattered. into many pieces and that was the problem. No damage just replaced the bearing.

You'd feel a shattered bearing cage when you turned over the motor by hand, if it would even turn over with the pieces of cage floating around in the crankcase. and it wouldn't be smooth.

No you wouldn't. just a small piece breaks off cage and can get stuck inside at TDC but still can rotate by hand. and it will start and stop the engine.

You're reaching.. you're really reaching. You really think that might be his problem? He said it turns smooth. That doesn't describe a piece of bearing race banging around in the crankcase. You might read what Roelof wrote. It makes the most sense of anything I've read in here so far. It would be fun to take that motor apart and see just what's going on in there.

It is true that heat can cause some changes in the material properties of a piston, but whether these changes result in permanent growth or not depends on several factors such as the material composition, manufacturing process, and operating conditions.

During the break-in process, the goal is to allow the various components of the engine to gradually adapt to each other, which can involve some wearing and smoothing of the surfaces in contact. The heat cycle method involves gradually increasing the engine's operating temperature over several cycles to promote this process.

However, it's important to note that the heat cycle method is just one of many break-in methods and may not be suitable for all types of engines or materials. The best break-in method depends on the specific engine and manufacturer recommendations. It's always important to follow the manufacturer's guidelines for proper break-in procedures to ensure the engine's longevity and performance.

It's the easiest thing in the world to figure out if it's bearings or the pinch. Just take the p/s out and turn it over. Is it smooth? p/s back in. Is it sticking now? Now you know. I've had motorcycle pistons get hot and gall up on the cylinder and have kind of the piston tightening effect he's describing but I've never seen it on one of these little motors. I don't know if it would do that or not.