Sure the cush nut has to be tight, but this is one answer to the question "Why"
The inner race of the main bearing is a tight fit on the crank, that is why shimming the bearing between the race and the crank cheek is such a pain, pulling your new bearing off the crank without damage, trying shims, tightening down, off again, you know the score.
The cush nut tightens down onto the drive sleeve, which in turn pushes the inner race and shims hard against the crank.
Now unless these parts are nicely clamped together and despite the inner race being tight when cold, there is evidence that in use the inner race can move, rotating on the crank and gradually wearing away the shims, bits of which end up in the sump, the crank then develops the sideways movement that spells a strip down.
So to answer Mik's conundrum, the big nut must do the two job's of tightening the drive sleeve against the bearing, shims, crank, plus act as a retainer for the cush spring.
BSA Service Sheet 208 (Engine Re-Assembly recommends a C Spanner, and to tighten the nut "as securely as possible"
In practice folks tightened the nut with a hammer and drift just enough to get the split pin to fit. By which time the original nut was looking somewhat sad. This is in any event not tight enough, as certainly the nut is bearing on the spring, but not entirely on the drive sleeve enough to lock the bearing inner race in place by friction against the sleeve, shims and crank cheek.
That was how it was done by the average backyard maestro, me included. In these more enlightened times make a peg spanner or buy a Peugeot ball joint tool and give that nut some real oomph.
All these cush foibles are well covered in previous Forum Posts along with the possible incorrect cush drive sleeves some fool has fitted in a previous top class rebuild.
If buying a used crank, wear to the shim position is an additional awkward repair, and easily overlooked, more so if displayed with a bearing in place.
Swarfy.
