the motors will get looser as they age but you need to have a relatively free turning motor to start with.
Common tightness is the side casing putting excess side load on the crank or misalignment of the white metal big end, end cap.
When assembling the engine try the rotation as you go along... # with just crank in, checks side load on main bearings and side case load when bolts torqued. # with conrod attached to crank, checks big end alignment and drag on bore by rings. # with head attached and torqued, checks for any twisting in crankcase and bore due to torquing head.
If the engine passes these simple checks then you know, with confidence that when the engine is built it will rotate easily.
The only drag left are the rings against the bore and it is this swarf that you will find in the oil at the first oil change.
Loose engines are fastest, but are also on the point of wearing out/needing a refresh, as per all good racing engines.
Older cheats included allowing the side case to wear and this indicates the benefits of a loose engine.(please don't do it!- scruts can spot such mods at ten paces)
Brian P.
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