I may well regret posting this, opening a can of worms, but faint heart never - well you know the rest.
For practical purposes, I think of momentum as the flywheel effect of energy stored in the flywheel, crank etc, and torque is the turning force of the crankshaft caused by the engine being produced at that time.
F1 engines have almost no flywheel effect, kill them at 10,000 rpm and the stop almost instantly, while your old Lister which runs at only 400 rpm with huge flywheels takes about 3 days to stop. For our old beezers, we need somewhere in between.
In theory, stored energy in the crank helps when starting off or hitting a hill, but that's partially used up and then its over to the engine producing power and that's where you feel its torque.
I've never ridden an A10 with one of these super heavy cranks, but I would imagine its quite smooth. Although, BSA must have had a reason to make the lighter one we all know.
Anyhow, they weren't made for sidecar use.
BTW, that seller is ok, hes got a breakers in Bromsgrove, and hes ok to get random bits from.
I just find it frustrating that his adds are always only one pic, and hardly any description. Just been reground- Yeah, what to ??