Because these engine have no piston ring and there is no way to get the piston on the right dimension of the sleeve you will get an oversized piston you need to break in first.
As mentioned the sleeve is a tiny bit tapered but when the engine is running the sleeve is cooled on the lower part due the mixture and heated on the upper part taking the taper away to a straight shape.

Pre-heating will expand the sleeve more than the piston because there is no combustion heat making the piston hotter so it will start easy.

Important is to take the first liter with some care, do not over rev the engine