It is , it is, Richard! Exactly so. Mine, all those years ago (more than 30), made a hole. Not a nice mendable hole which could be passed off as a factory hole, but a jagged ragged messy embarrassing hole!
But I think we and SRM agree - this darn nut has to be done up well and truly so it does not and can not come off! I'm loath to disagree with SRM because I have huge respect and they know far more than I . . . and I don't really d(isa)gree, it's just a matter of d(e)gree. My tekkie points would be that the splines on the mainshaft do not allow the crankshaft sprocket to bear on the main bearing directly; and in no way does the s/absorber assembly control c/shaft end float contrary to occasional misinformed opinion. Therefore, although shocks on the mainshaft caused by a loose sprocket whacking the inner end of the splines on which it sits are A Bad Thing, they do not directly affect the end float of the crank nor do they directly cause a problem for the shims. The crank is positively located within the cases and the cush drive is positively and independently located between the sprocket and the Big Nut. Were it otherwise, think of the life the worm gear on the oil pump would have at the other end of the crank!
Now, the bit I agree completely on - a constant whacking caused by a loose sprocket and/or jumping cams if such is possible will obviously be transmitted through to the bottom end . . . but to wreck it the Big Nut would have to be very loose, the cush drive spring broken, a face cam shattered - or a bit missing altogether - and it would have to have been like that, and ignored, for a while. It's likely that the primary chain and the clutch drum's rollers would be knackered by mis-alignment woes before the crankshaft made a fuss, although anything is possible. If - I think - the nut were loose enough to cause all these bad things, it would probably also be touching the inner side of the outer primary case.
Ergo, in my view, a split pin in one of the holes so kindly provided by the nice man at Small Heath must be better than no split pin, as even if the thing isn't placed to give the 60/65 ft lbs required, it's far enough up the mainshaft to stop the nut coming too far undone. I don't believe the Nut could consume entirely a split pin of the dimensions we're talking about and so I do not believe it could ever back off far enough to let the face cams override one another - or to hit the primary case for that matter. One can see how hard it is to make the cams jump by doing the Big Nut up until it forces the cams to do just that. It can be done - with a T Rex tooth puller or otherwise - but it takes more leverage than the clutch and/ or the rear brake can comfortably stand. I do my Big Nut up with the bike in gear and the rear brake well applied. When it is so well done up that the brake can't hold it - or the clutch slips - I stop and call it a day. It's there or thereabouts that the split pin can be inserted outboard of the Nut. Short of doing the nut up on the bench with something wedging the pistons or rods to stop the crank turning, or a heavy clamp on the primary chain (which will stress the clutch rollers let alone the chain itself), there's no way I can think of to get it any tighter. Which one shouldn't want to do anyway - or it will be impossible to get the thing undone for routine maintenance, like sprocket renewal, gearbox work, etc. In my other, AMC, world, I am constantly amazed at how darn hard it is to get the crankshaft shock absorber assemblies off - often takes more torque than clutch and brake can stand and requires 2 pairs of T Rex tooth pullers. But they don't have a split pin (although they do have hexes) and their sprockets do abut the main bearing - so slack is a capital letter NO-NO - and I do 'em up REALLY tight!
