The standard synthetic sealed Oldschool HW50 piston weighs in at 200-202 grams, Tone.
Bore X stroke = 25mm X 65mm.
Transfer port = 3mm X 25mm.
My playing with HW50s, if it's any help ? :-
https://www.airgunbbs.com/showthread...16#post8129716
If I did another one I'd just increase the stroke by 5mm by machining the piston arbor to accept a TX piston seal then weight the piston back up to 200 grams.
All the best Mick