Dynamo Belt Drive Kit for BSA A7 & A10

Now discontinued - no further stock available

You can still purchase this kit from Steve McFarlane
See Start Your BSA for details (near bottom of linked page).
Spare Belts are still available (see below)

Description and features

DRL offers a better dynamo belt drive kit option for BSA A7 and A10 pre-unit twins. Our kit employs a 15 mm wide belt (the widest it's possible to fit), and gears the dynamo speed up by 20%. Low maintenance, quiet, strong and non-slip. A 'fit and forget' solution to an old problem area. We have stock ready for rapid dispatch.

The kit employs a superior quality high temperature toothed timing belt drive which uses a wide 15 mm belt for long life and low maintenance. The speed ratio is 20% up on the standard A10 ratio so electrical loads are balanced at modest road speeds; especially useful with 12V operation of a '6V' standard dynamo. Incidentally this makes the dynamo: crank speed ratio the same as on a magdyno equipped machine. Unlike any variety of 'V' belt the drive is tolerant of oil and cannot slip, and it can even be run in grease. The small pulley has an internal thread for easy extraction with a magneto puller. This thread is 11/16", 20 TPI, BSC. A special nut is supplied to secure the pulley to the dynamo shaft, threaded 3/8" x 20, BSF. The pulley's flanges are small enough to pass through the inner timing cover when fitted to the dynamo. The large pulley has 2 holes with a 1/4" Whitworth thread to aid removal (use a timing cover screw). A felt seal is included to help prevent the passage of oil from the inner timing side case into the drive partition, although oil will not affect the belt in use. To complete our kit a timing cover gasket is included.

Note that experience has shown that the large pulley can be a close fit in the earlier A7/10 casings. Castings vary in detail dimensions and may need to be slightly relieved (e.g. with a Dremel burr) to ensure clearance. Note also that the early (to 1950) A7 long-stroke engines are different in that the distance between drive and dynamo shafts is smaller, and the belt we stock will not fit.

View Belt Drive Kit data PDF

Read an article which appeared in the BSAOC Star magazine and describes the performance of this kit in practical terms:

View Improved Charging for your BSA PDF

Spare Drive Belt

The 15 mm wide belt supplied with our kit has the highest known specification available for the task. For example it is rated for 100°C continuous operation, and is far stronger than previous generations of belts. Life expectancy is estimated at 10 years or 30,000 miles so the requirement to replace it will be rare. But because the belt is a special item we do offer spares, as much as an insurance for carrying when riding far from home as anything.