I knew this would happen. I can’t remember which cush drive spring goes with the M20. I seem to have accumulated a variety. Can anyone tell be the correct one for the M20, length, number of coils or whatever. I’ve seen info somewhere but can’t find it now. 
Cheers
Pete
email (option): petercomley@web.de
The correct length for the cush drive spring is1 5/8". It is of the 'flat' wire type. Number of coils is not specified but a pitch of 5/8" (measured from the center line of one coil to the next.)is given.
The key thing is that when assembled the shock absorber cams should not 'rise' too far when the kickstart has some weight gradually applied with the engine on compression.
If the cams rise more than a 1/4 to a 1/3 of the way up before the engine turns over the spring is too weak.
This can be resolved either by fitting a longer spring or fitting a spacer over the shoulder of the spring retaining nut to increase preload....Ian
email (option): julie@wright52.plus.com
Thanks Ian!
I knew you would come up with what I needed.
Now what the hell are the other springs for??..who cares
Cheers
Pete
email (option): petercomley@web.de
Hi Pete, it's a puzzle...there seem to be numerous variations in the length of the springs that turn up.
I can only assume that the shock absorber assemblies fitted to the multitude of different BSA models thus equipped varied in detail across the models and years.
All I do is apply the operating criteria to determine if any given spring is suitable and have no idea which spring fits which bike.
The availability of the Army 'standards' book at least allows us the luxury of determining what we SHOULD have fitted to our M20s!!...
...Ian
email (option): julie@wright52.plus.com
Hi Robb, The 'Army Standards Book' was a Ministry publication to guide workshops during M20 overhauls and to assist inspectors in assessing the finished job.
As such it is full of specifications, tolerances etc. etc.
As I am an 'Anorack' for the technical/engineering side I find it a fascinating publication.
The only examples I have seen to date are both post war versions and I don't know if there was a wartime one...probably.
I have one copy which was part of a small run produced by Steve Madden for general distribution but I don't think he has produced any more.
Also I have an original of another version produced by the 'Ministry of Supply, Inspectorate of Fighting Vehicles' dated 1951 for 'Inspection and Instruction...reconditioning data and standards (motor cycle BSA Model M20).
I don't think either publication is currently in print.
I also fairly recently acquired (via e mail)a similar document relating to the G3L Matchless but haven't gone through that in any detail as yet....Ian
email (option): julie@wright52.plus.com
Hi Henrie...I guess they all follow the same lines...the M20 version is also pretty comprehensive....Ian
email (option): julie@wright52.plus.com
...and is that cush spring the same as the single spring in the WD clutch? [I also have a large box of them!]
Hi LJ, The clutch spring is a different diameter as well as a different length...Ian
email (option): julie@wright52.plus.com
Ah right, thanks Ian.