air hammer design question

I've been given an old CP No 40 air rivetting hammer, the snag being it's minus the piston I've asked CP in the UK, they say it's completely obsolete, no parts
or drawings remain. I have a CP no.2, which is a shorter stroke version of the same thing, with the same bore (1 1/16"). I hoped the piston would be common to a range of stroke lengths, but no, the piston from the No 2 won't work in the No 40, it just blows out without any indication of being blown back. I would like to get the No 40 working - does anyone out there know enough about air hammer design to advise on how? The piston from the smaller gun is just a hardened solid steel slug, maybe 3 1/2" long, with a waisted section towards one end.
Thanks Tim Tim Leech Dutton Dry-Dock
Traditional & Modern canal craft repairs
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