Thursday, June 16, 2011

Manu-Arm Piston Hijinks

I decided to try making a replacement piston seal that uses an o-ring for the Manu-Arm pistol.


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The Seal seems to be almost exactly 25mm.


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The underside of the seal.


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The screw. It has a 90 degree head…


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The piston face.


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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.


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Countersinking. The only 90 deg countersink I had was too small…


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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.


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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.


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Seals…


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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.

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