just upgraded my leather HW35 to synthetic...
I should say first up, that mine had the small (2.9mm) port. If yours has the larger (3.5mm+) port, this may need sleeving.
I machined a shoulder on the piston face to form the reduced diameter section for the synth seal's shoulder to sit on. Then I machined up an ally disc approx 24.5mm dia 3mm thick to form the "mushroom button" that retains the seal. trimmed off the M8 threaded piston rod to suit, threaded on the mushroom with a bit of loctite, job done. The stroke is extended by 4mm (I could have squeezed 5 before hitting cocking slot issues, but 4 was easier). I grabbed a tatty used Hw80 seal from the parts bin and sized the rear edge down a tad (leaving the front lip untouched).
Before starting, power was bang on 11 FP with leather... with this setup, I had to remove 2 coils to get it legal, back to 11.4. Importantly, it is so much smoother - because it is less snappy - and it has a better air cushion (no doubt also due to the stroke increase). The reduction in preload also means the piston is more cushioned in it's landing. It was quite nice before, but this has transformed it.
Thoroughly recomended if you have a small port (or can sleeve it).
HTH - JB
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.