B400 - overdrive oil

[pnaleahy]

Indeed Derek Hughes devised a system of fitting the J Type electric overdrive to the Bristol motor and gearbox and supplied all parts required. The system relied on removing the existing gearbox extension which contained the original sliding mechanism to the tail shaft, replacing the extension with a transitioning casting and the J Type overdrive unit and replacing the tail shaft with one with a sliding yoke.
The lubricating oil recommended by Derek Hughes for the J Type Overdrive unit was Castrol Transmax F automatic transmission fluid.

I have one of these units fitted to my Bristol 400 and a Bristol 403. They both work flawlessly with the recommended lubrication to the correct level.

[pnaleahy]

I now attach drawing of the oil level and filling arrangements on the Derek Hughes supplied J Type Overdrive. As can be seen from the sketch, the oil level in the overdrive is less than half full.

[Bela]

Thank you for this useful information. My box seems different. But now the oil variety is no longer a miracle. In D Ravenol ATF Fluid Type F will be a match.

[David C]

Originally Posted by pnaleahy

I have one of these units fitted to my Bristol 400 and a Bristol 403. They both work flawlessly with the recommended lubrication to the correct level.

What solution was used for the final drive? I changed from 403 differential to 405 differential but the latter were quite hard to find. Did someone come up with new crownwheel and pinion of the correct ratio?

[pnaleahy]

The J type suited the 403 rear axle and at 100 kph reduced the engine revs by about 500 i.e from 2500 to 2000 approximately. It is only possible to engage it in top gear due to an electrical cut-out switch which was incorporated on the gearbox. On the other hand, the 405 overdrive had different gearing and required the 4.2 ratio diff to suit that gearing.
As far as I can see, the Laycock de Normanville J Type overdrive system was devised to reduce engine revs/increase speed. On the 403 a steep incline required the overdrive to be taken out of gear to increase engine revs rather than strain the engine in top.