Damping rod/ piston dimensions on 1977 forks

[arcticXS]

Hi All!
My XS is a 1977 ("D") model, so it has 35 mm forks, but no top out springs as standard.
The "clunking" these forks make when they occasionally do top out is a little bit unnerving..... So I would like to modify my forks by installing top out springs, and at the same time I get the added benefit of a little shorter forks.

I am looking at the selection from a supplier of springs, and need to know the exact outside diameter of the damper rod (for correct spring ID), and outside diameter of the damper piston (for correct spring OD)
I am sure somebody have some damping rods handy for measuring
My plan is to install springs with a free length of around 30 mm, reducing the fork stroke down to 120-125 mm.

 

[arcticXS]

Sorry for "bumping' the thread.....I already have ONE top out spring from a later model 35 mm fork, this spring has 25 mm OD, 19 mm ID (giving 3 mm wire dia) and 21 mm free length. I was planning to use a little longer top out spring, and just add a little more preload to compensate for the slightly shorter effective fork travel.