My 77 has a 25mm standard piston but the comp tube has a radius in the back of the transfer port of about 6mm wide and 4mm deep,standard port width in the front.who done the work? I don't know.
the action is .177 I'm using jsb 8.4g the spring came from a .22 'unknown origin' very slim it's level with the rear of the action so only loads when the block is fitted.
V-Mach lube on the spring and dri-slide in the tube. It's doing 11fpe with 3fps variation I was very shocked when I put this together to say the least. I just built it from parts I got here n there. It hits what I point it at lol. Love it much more than any of my PCP's.
ps,first time I built it it was doing 10.2fpe and was ok then I added a 2mm delrin power washer so it was 11fpe this made it so much smoother it's unreal. Anyone have any ideas on this?

lee.