I am missing something like "now it runs lie crap" ?

By taking it apart and put it back together you have to be sure the sleeve is located in the right way. Sometimes the center pin is lost or what I have seen with non listed Novarossi products they have put 2 slots for the center pin into the sleeve. Also be sure the piston/rod is placed in the right way.

And if you did remove the main bearing putting it backwards in you also will face some binding issues.

Yes made sure the cylinder was placed correctly, only goes in one way really. Same goes for piston, I identified the exhaust port side and installed it as such. I did not remove the bearings as I do not have the proper tools to press them back in.

Check the head shims? A lost shimm can cause issues.

Maybe remount the head in small turns of the screws in a cross-side way (sometimes people tighten the 1st screw that hard so the burnroom is not straight)

Sorry to hijack this thread but what does everyone do to clean the engine and how often?

I only use a clean cloth and wipe the internals, nothing else. If I have a bare block in my hands (for example, when I change the bearings) I'll flush the block with brake cleaner and again clean with a fresh cloth. That's all there is to it, and make sure to lube it very well when assembling.

I never clean it, If I open the motor it is only to check the color of the piston and to replace the rod. When replacing the rod I sometimes clean the top of the piston and the burnroom but it is not needed.

I think we have a mis-communication here lol, the engine runs great! I was just letting everyone know that a little play in the bearing is ok.

I use nitro fuel to clean it. Shouldn't be too dirty though. Mainly I clean the piston top and the area around the carb. I also polish the crank when I have time. Be careful with the silicone on the crank though. And also lube it with after run oil lightly when rebuilding. I do that after every race weekend and have never had an issue.