Manu-Arm Piston Hijinks

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.

Seals…

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.

Manu-Arm Air Pistol Disassembly, Part 2

On I go…

The cocking bar, that part of the cocking linkage that pushes the piston back.

I don’t know why it has that square bit. Possibly a sort of anti-beartrap? Hmmm.

One trigger pin pushed out.

The sear is odd, there’s a ball bearing on the sear and one on the piston.

The trigger and its two pins.

I couldn’t remove the “cocking body”.

The pin just wouldn’t come out more than this and I did not want to destroy the gun.

The rear sight.

A setscrew locks the plug to a cross-pin.

I was too lazy to drag out my spring compressor so I just used a bar clamp. The spring does have a significant preload.

Pushing the cross-pin out

The end plug. Notice the integral cruciform spring guide.

The piston.

See the ball? Shiny ball

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The piston seal is a simple slug of molded elastomer. I forgot to take pics but it has a groove on the underside as it’s molded. I think an easy replacement would be an aluminum slug with an o-ring.

All the parts…

Here’s a poor shot of the two sear balls in contact, representing a cocked condition. When the trigger is pulled the one ball slowly rotates out of contact, which yields a sliding motion as the sear releases. Not crisp, but smooth.

Anyway…reassembly is exactly in reverse. I took some time to properly lube everything. The gun does shoot smoother than before.

Manu-Arm Air Pistol Disassembly, Part 1

A while back a request was made to take apart my Manu-Arm air pistol. Finally got to it.

The box.

Pistol and drawing.

Drawing.

Instructions.

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It’s not a bad looking pistol, for all the potmetal and plastic.

One screw through the grip.

And two screws on either side into the action.

The housing comes apart typically.

Notice the trigger adjustment screw.

There’s a dowel pin loosely in that grip hole to align the grips.

The action removed.

A view of the action…

And another of the trigger…

Break the barrel to relieve tension.

Remove the pivot screw. Notice the steel washers on the pivot bearing.

I punched out the cocking linkage pin. I was very cautious when knocking out pins as most of the parts are potmetal and could break.

Barrel removed. More to come…