Derby Dynamo

[AnthonyEssington8982]

Ladies and Gentlemen i have a Dynamo that is believed to be new.
Its Identical my 3.5 dynamos but wont fit the 4-1/4 ?
are they different size bodies?

[Ewan Gallie]

I don’t have the green Technical manual to hand at the moment, but I hope the below might help, as a very amateurish offering.

Quoting from Michael Ellman-Brown’s book Bentley – The Silent Sports Car:

Page 31 …”The gear on the nearside, also driven by the camshaft gear wheel, drove the dynamo and from the back of this was taken the rev counter cable drive. These gears were made of steel except for the large bronze camshaft gear within which was incorporated the maker’s cam balancer. This consisted of three lobed cams and spring loaded plungers which evened out the rotational forces imposed by the cams and valve springs. On the 3 ½ this was a separate unit attached to the gear wheel by six studs and nuts but for the 4 ¼ the lobes were formed within the gear wheel and the plungers were held in place by a central captive nut.

There was a further refinement to the drive for the dynamo. Due to the fact that this is sometimes charging and sometimes not, a marked difference occurs in its running characteristics and this can cause flutter in the gear train. To eliminate this, a drum on the front of the gear had two shaped pad brakes which controlled this tendency. For the 4 ¼ litre the dynamo was air-cooled and the gearing was also changed.”

Ciao, Ewan

[Christopher Carnley]

The main external difference, is that the 3 1/2 litre drive is by a a pair of 3 leg steel "dogs" and Rubberoid discs, whist the 4 1/4 litre drive is by 3 rubber plugs onto a 3 hole connector. The 3 1/2 is secured by one compression strap, and the 4 1/4 has two straps.

[AnthonyEssington8982]

cheers for that Chris, any idea if the diameter of the bodies are the same?