Belt drive run off

You will probably get away with it but you will prematurely wear your belt. I have seen one of the gold end plates worn away by half its thickness as a result of a belt trying to run off.

Some say the original gearbox clamping arrangement is not rigid enough to retain correct alignment for a belt. You can overcome this by buying a gearbox top mounting bolt with adjusters on both sides (you will need to drill a hole in your drive side engine plate for the extra adjuster). I got mine from RGM. Alternatively, as a belt does not require adjusting, you can weld up the adjusting slots in your engine plates and cut holes in the appropriate places in order to provide a fixed location.

I was advised by a supplier that the double adjuster was difficult to use and not always needed. I used a 'fillet' inserted in front of the shaft on the left side of the gearbox which I adjusted to fit, by trial and error, then brought the gearbox forward against it a then pulled the gearbox round to the correct  orientation. Very cheap and seems to work!

The fillet(s) I filed up using some old alloy I had. Picture attached.

Robin is quite correct about the need for a second adjuster as the chances of the engine and gearbox shafts being parallel to each other after nearly 60 years are very slim.

But before you take the plunge and splash out a small fortune on this piece of kit please check out whether it will fit your engine plates and still be adjustable. Some of the early Dommies seem to have extra tinware under the carb and this makes any spanner work in this area difficult.

An alternative used by some of the race boys is to lock the top gearbox bolt in place on the left engine plate and then  then use the adjuster on the oil tank side to pivot the gearbox about to get a good shaft alignment.

This can be achieved by filling the space in the plate in front of and behind the top bolt with some welded metal fillets. Of course you have to get the belt tension spot-on before welding.

I did not want to mess up the plates on my bike with welding and so instead cut and shaped some small Aluminium fillets to fit in the gap in the engine plate.  I was going to peg these in place but found that this was unnecessary as the head of the top mounting bolt pinched them in place against the gearbox lug.

I then ran the engine and adjusted the gearbox position until the belt only moved in the centre of the front pulley.

One point I would add about  fitting a belt drive. All of your gearbox bearings must be as-new otherwise any play in them will allow the gearbox shaft to whip about.  This instantly takes it out of parallel with the front pulley and this in turn starts to shred the belt on the alternator mount or pulley keeper plate.