Hi Barry..the magneto you need is an anticlockwise one..(direction of rotation is from the drive end).
The adv/rtd cable should enter the mag to the right hand side of the contact breaker assembly (as you look at it) so that it will be fully advanced when the cable is pulled tight at the lever.
You will also need the wartime pattern main body casting.
This can be identified by the curved portion that the dynamo sits on. On wartime types this runs along the whole length of the body casting and meets the vertical part of the body at the drive end.
Postwar types have a shorter seating for the dynamo and a gap between that and the back of the drive housing.
The pickup should attach on the opposite side to the cylinder and there should be 4 locating dowels in the base.(or 4 tapped holes to take them)
Check that the base is in good condition (flat) as there is insufficient material to machine them if the base is badly worn and it makes positioning the mag correctly more difficult...Ian
email (option): ian@wright52.plus.com
The adv/rtd cable should enter the mag to the right hand side of the contact breaker assembly (as you look at it) so that it will be fully advanced when the cable is pulled tight at the lever.
You will also need the wartime pattern main body casting.
This can be identified by the curved portion that the dynamo sits on. On wartime types this runs along the whole length of the body casting and meets the vertical part of the body at the drive end.
Postwar types have a shorter seating for the dynamo and a gap between that and the back of the drive housing.
The pickup should attach on the opposite side to the cylinder and there should be 4 locating dowels in the base.(or 4 tapped holes to take them)
Check that the base is in good condition (flat) as there is insufficient material to machine them if the base is badly worn and it makes positioning the mag correctly more difficult...Ian
Hi Ian,
Can the postwar body still wok on the WD M20? What about dyno fitment, drive? What would be any other details or differences that would NOT work for the WDM20?
Thanks, Vince
email (option): bsa3man@rocketmail.com
Hi Vince..All the essential dimensions are the same on both types so they are interchangeable. Postwar changes to the magdyno were the body casting shape, the introduction of the higher output dynamo and the change from 'tight wire' to 'slack wire' adv/rtd where the fully advanced position at the handlebar lever is reversed...so the timing became fully advanced with the cable 'slack'
This led to the fitment of a different contact breaker housing to move the adv/rtd cable entry to the opposite side of the contact breakers.
The contact breaker cam was changed to match and the contact breaker cover clip and other fasteners were also moved and altered to accomodate the other changes. The 'wrong' cover clip can be fitted in either case but it will then not pick up correctly in the 'location' recess that is pressed into the centre of the cover.
The only difference that lead to non interchangeable parts was an unrelated change made to the contact breaker cam..this was increased in thickness on later models and cannot be 'retro fitted' to the earlier housing..conversely fitting the early cam in the later housing leaves some free space between the cam and the cam retaining clip and the cam is not held tightly in position..Changes to the magdyno dynamo output led to an increase in the length of the body and armature to accomodate the longer field coil which was held by two screws, not one as on earlier types. There were also various changes to the brushes, brush housings, brush covers, armature bearings,drive gear etc. etc. over the years so many internal dynamo parts are not interchangeable.However, all types are a 'universal' fit so will fit any magdyno mag. If a change is made from the early dynamo to the later high output type and a Lucas MCR1 regulator is fitted, this should be changed for either a later MCR2, an RB 108 or an electronic regulator...Ian.
email (option): ian@wright52.plus.com