Dynamo Drive Belt Kit



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Description and features

DRL also offers a better dynamo belt drive kit option for BSA A7 and A10 pre-unit twins. Our kit employs the widest possible 15 mm belt, 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. 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 cork seal is included to stop 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 the early (to 1950) A7 long-stroke engines are different in a number of ways which impact on the dynamo drive. Firstly the distance between drive and dynamo shafts is smaller, and a shorter belt is required. We keep some of these in stock. The large pulley is a close fit in the early casing. Castings vary in detail dimensions and may well need to be slightly relieved (e.g. with a Dremel burr) to ensure clearance. The dynamo pulley may not readily align, which can be overcome by removing a small amount from the dynamo mounting plate face. Alternatively the inboard flange of the small pulley may be removed (pushed off) for correct alignment without detriment. Finally the gasket we supply is not suitable for the different shape of early cases. Other than that the kit will fit the long stroke engine straight from the box! Just mention you are buying for a long stoke engine when ordering.

View Belt Drive Kit data sheet (Adobe Acrobat PDF format)

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

View Improved Charging BSA A7-A10 (Adobe Acrobat PDF format)

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.


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