I decided to try making a replacement piston seal that uses an o-ring for the Manu-Arm pistol.
The Seal seems to be almost exactly 25mm.
The underside of the seal.
The screw. It has a 90 degree head…
The piston face.
Drilling some delrin. I ended up using a #4 drill which was the closest I had to the 5.25mm shank of the screw. I turned the body .976”, later ones I turned to .984”. Greater diameter leads to more o-ring support.
Countersinking. The only 90 deg countersink I had was too small…
So I set a boring bar to 90 degrees and plunged in to make the countersink. After this I found a 90 degree spotting drill that was perfect, used it on the rest of the piston seals.
Grooving for an o-ring. I was aiming for about .005” over 5mm with the o-ring installed. But I only had a 116 o-ring and it lost diameter when stretched. Later pistons seals used #118 o-rings.
It shot about 360fps, which was slower than the 426 I got with the original seal. So as I mention above I made some more seals using 118 o-rings, and played with diameters and groove depths. Current seal I have installed is getting between 428 and 436 fps, which is slightly better than the stock seal. So the design works fine.
You can see the difference in groove width & diameter. The left one is the current successful design. Both have #118 o-rings. At some point I’ll actually follow the procedures Parker recommends and grind a proper tool, etc.