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These are Deore cranks with a Shimano BB:
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I'm not sure that that gap should be there. On some cranks there is a thin washer, usually on the drive side - am I missing that? The end cap tensioner thingy would not tighten up any further.
Also, the single BB spacer is on the drive side - I thought it should be on the non drive side?
Single BB spacer on the drive side for a 73mm shell, 2 drive side and 1 non drive side for 68mm. They always left space for a BB mount tensioner etc
Can't see the photo. (Edit: ah, now I can.)
I'm not sure what washer you think you're missing. There's nothing like that with Shimano. FSA use a wavy washer so maybe it's that you think you need?
If there's a need for a single spacer then it should be on the drive side. In effect, it's taking the place of a BB mounted mech.
I'd measure the BB shell and work from there.
What model of BB is it?
If there's a gap that shouldn't be there (i.e. bearings not preloaded) then you'd be able to move the cranks slightly (gentle tap with a mallet would tell you. Perhaps your BB shell could be a touch undersized.
Is the pegged spacer thingie between the pinch bolts fitted properly? Could be stopping the crank arm going onto the axle.
How is the crank arm going to preload the bearings with that gap?
That gap ought not be there.
What is the BB width and what is the arrangement of spacers between BB cup and BB shell on either side?
Edit >> That would be a huuuuuuuge amount for the BB shell to be "out" by.
Just reading the Shimano workshop manual for these cranks (and others), there is a variety of spacer options for the spindle. Supposedly for these cranks, M6100 it's "part of the chainring" which must be another way of saying everything's the right size and it's not needed.
I went back for a closer look, and there a slight raised part on the crank arm that you can't see in my photo, and I think that's making the gap look worse. If I loosen the tensioner bolt the crank does move sideways, and if I tighten it up it stops, apart from a barely detectable bit of squish which I guess is the point of those plastic top hat washer thingies on the bearings.
Re-looking at that picture it's much better now - I refitted the whole lot including the safety catch, so perhaps it was just not tightened up properly or something.
So I think it's probably ok now. Alert over, stand down troops.
Unless they've changed the design, there should be a small o-ring on the inside face of the crank and that should butt up against the top hat. If that's not happening then there's something wrong somewhere.