BSA Mercury piston head
I am fettling (or have fettled) a MKII .22 Mercury ZA... 1973/74. (Batfink posted about it a few days ago as it has a chromed barrel). It has a piston head I have not seen before, most likely an after-market type, but interesting anyway.
It is plastic (whitish nylon?), buffer washer present, O ring present. But about 4mm forward of the O ring there is a groove with a wire spring clip in it, the wire fitting flush with the outer edge on the piston head, almost like a scraper ring but round section wire (c. 0.5mm diameter?). The piston head ends about 5mm beyond this wire clip and is thus longer than I remember 'old' aluminium heads being. I have not inspected it any further.
I suspect that the extra length head results in some lost volume ... not that it is very important as the rifle is quite sweet as it is at around 10ft lbs..
Out of interest though: Does anyone know anything about such a head?
These were standard BSA ones at the time.........they have a habit of splitting and losing compression as they get more brittle with age.
TR Robb does PTFE replacements (about the only decent item he ever sold apart from the hilarious shakey hand, 'heres one someone else did for me off camera' how to guides) which are quite good once they have bedded in.
I have stripped 2 Airsporter Mark V which had these piston heads