some designs DO require fore aft clearance -
This is done to prevent end loading the bearings.
The cases expand and unless the crank can move in one bearing - an end load will occur.
most engines once fully assembled, will have no detectable "end play"
However - one of the bearings is taking the end load - the other is not.
When specifying bearing-- the load ratings call out rpm-side (end) load and possible radial loads .
The worst setup is when the side (end) loads change with case expansion.
Typically- When I redo an engine to run fast -- I setup the crank to bearing clearances such that case growth never end loads the bearings .
Now - back to sleep -