If you have zero clearance there the retainer plate is probably worn a bit from the bearing spinning against it in the past. Try another retainer... or get the retainer flange face machined a few thou.
As for the lock tabs in the gearbox, I run 'em if I've got 'em. if not, I use Loctite 242 or 262.
I no longer use the flywheel or crank pulley lock tabs though, they are soft crap these days- I find Loctite 262 more secure.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
