I was asked about a replacement piston head for a 140 and as in the UK we have a power limit and sadly the 140 in std form is capable of exceeding it I convert mine to a pressure blow off piston (this is for a non UK gun)
this leaves me with a few std pistons so I set about converting one to an o ring
first I removed the old seal(carefully milling the brass collar in two places to leave the old seal intact so I cam possibly replicate it at a future date) and the washer and felt wiper
I then machined the piston a bit shorter and altered the felt wiper location to take a different size piece of felt
I then threaded the pistom m8 and drilled and tapped a piece of brass m8
the two pieces were screwed together and then the final machining and o ring groove was cut to keep everything as concentric to the piston rod as possible
i could have replaced all the head and made a full brass one but this way enabled me to keep the felt wiper
https://i.imgur.com/dh9IHNC.jpg