I posted this question on another BSA forum because I thought it was more appropriate there but no replies so I'll post here.

This to suit an M series plunger frame.
I'm after dimentions of the upper and lower spring plunger spring collars, the plain ones not the stepped ones?
Part numbers 67-4131 and 67-4127.

Does anyone have any handy that they could measure for me?

email (option): spacemonkeym@gmail.com

I'm afraid I can't help with that one Matty...

On the subject of the plunger frames though...The last one I had I took out the spring and associated collars and replaced them with aluminium spacers which I carefully machined to be a snug fit..

I retained the spring covers so that the appearance was unchanged...

Without the springs but with the larger diameter plunger wheel spindle it handled far better than the 'sprung' set up which tends to 'wallow' on high speed bumpy bends, has a pretty limited movement and also gives the rear chain a hard life....Ian

email (option): ian@wright52.plus.com

G'day Ian, thanks for the reply.
Nice idea with the spacers. I'm kinda used to the squirrely back end these days and would like to get the plungers working properly.

I was comparing plunger suspensions with a fellow M21 owner on a club run last weekend and found his to be a lot more stiffer than mine and he never had the bottoming that I get with rough road surfaces.

Now my compression spring are probably not in the best condition but the lack of these collars wont be helping either. Could you hazard a guess at the thickness say 3/16"? 1/4"?

email (option): spacemonkeym@gmail.com

I can't recall exactly how thick those collars are...3/16"-1/4" would be about right though...

Basically, you are just looking to put some more preload on the springs..

If you think your springs are a bit worn then thicker than standard might be the way to go...I'd go with 5/16" and see how you get on with that...Ian

email (option): ian@wright52.plus.com

Thanks Ian, now where's that spring compressor?

email (option): spacemonkeym@gmail.com