I did consider the issue of whether the BSA box could stand the strain. As it's torque which is the destroyer here, I deliberately chose to use a fairly large engine sprocket in order to reduce torque to the box. The fact the engine is short stroke (92mm bore / 75mm stroke) will mean power is at fairly high revs (assuming it runs!!!) also helping ease the strain on the box.
For anyone attempting to fit a belt drive to a bolt-up A series, I've made a couple of observations (presuming of course that no kit is available).
My vee twin has an idler to adjust belt tension. This is located on the rear primary case which, in my case, is a separate casting bolted to the crankcase. As I recall, the A series in question has the rear primary case as part of the LH crankcase (and a Weller tensioner for the chain) - which would make fitting an idler extremely difficult. I would therefore suggest belt tension is adjusted by shimming the gearbox. Helpfully, belts tend not to need as much re-adjustment as chains.
Secondly, the belt I chose was the only one capable of taking the power I am hoping for. Unfortunately, the only sprockets available for this particular belt are in cast iron, which, of course, is totally unsuitable for this application. I therefore had LM25 alloy cast over a splined steel centre for the engine sprocket and then cut the teeth in a mill using a dividing head. The clutch sprocket was machined likewise, but out of EN32 steel. It was then machined out to accept a BSA clutch basket which I riveted in. To avoid some of the work involved, and with the A series output being less than half of my requirements, I’d strongly advise anyone attempting this to choose a belt for which alloy sprockets are available. Of course, that still leaves the problem of fitting an alloy sprocket to the engine mainshaft, but that should not be insurmountable.
Hope the above helps,
