Minto. Draganfly's excellent online exploded diagrams and parts list would have saved the effort of searching the parts book, and in addition this very Forum has a Nut and Bolt Chart, which will identify the size and thread of most of the fasteners from their BSA Part No. Have a look in the Literature section, it's near the top entries.
This chart can be downloaded from a link, published by Musky on 5 April 2018. Good job he did, the original has gone into Cyberspace.
Some folks attach the bare gearbox casting to the motor, then reassemble the box. This means the mainshaft can be slid across with less chance of damage to the primary case oilseal behind the clutch. I would wrap the shaft splines anyway, as protection for the seal.
Why BSA used studs on the bottom is debatable.....my guess is that perhaps because a stud has less chance of being removed when routine dismantling takes place, with consequent less wear or damage to the alloy thread. These two fasteners are under tension big time when you give it some stick, and a correct size (lengthwise) stud offers the most support available from the weaker alloy thread, rather than shorter "that'll do bolts" used on the top, which don't have to deal with the same applied forces.
Swarfy.
