Going back to the OP question about end play in engines:

A model engine is built from different materials that all have different thermal expansion properties. Aluminum in the case and steel for the crankshaft. So the engine designer has to consider operating temperatures and the changes this causes to the tolerances.

Typically the inner races of both the front and rear bearings are tighten to the crank, which is why you want the prop on to properly assess the end play. Otherwise, the crankshaft can push back into the backplate.

When the bearings are properly fitted, the rear bearing takes the thrust load, while the balls in the front bearing are in a floating position. This bearing setup is for a engine at operating temperatures, not a cold setting. Not every engine is assembled well enough to meet these requirements. Not having any end play usually means that the bearings are under a preload condition, which causes some loss of power. But remember, check when the engine is at operating temperature.

I know this thread has so many other discussions going on, It's an interesting theory that we gotta see the engine at the operating temperature. For the time being I'm not worried about the engine, it would be a little while until I'd be able to use the engine. I'm gonna keep y'all posted regarding the end play in the engine. As majority told me that not to worry about the end play just install the engine and fly, so this is what I'm gonna do and let y'all know. Have patience, as I'm gonna take it slow