Hi Evert.
By coincidence I looked at jumping back on this on Monday and ran the action over the chrono, it's only running at 520fps so I'll be going back in to her pretty soon so I'll measure the stroke for you.
I'm thinking now that I could have lightened the piston too much or I could need more weight and a different Spring, as luck would have it I found a couple of new Titan springs in the pile which will fit the piston at 18.35 mm OD.
All the best Mick