Thursday, April 28, 2016

Tony DeMurray’s MMM Mondial Roger BB Pistol Repair

Tony DeMurray sent me the following showing his repair of his MMM Mondial Roger BB Pistol:
MMM Mondail Roger BB Pistol
Since parts are difficult, if not impossible to come by, workarounds have to be done.
Air gun leaks in two places:
Piercing pin
Valve assembly
The reader should refer to Nicholas Carter’s blog pages as to how to disassemble and reassemble the gun. He has great pictures and good commentary.
First, let’s deal with the Piercing pin area.
Since original seals are hard to come by, I used one from Walther Makarov pistol.
The green end seal is from a Makarov.
This seal requires the center hole to be slightly enlarged to fit snuggly around piercing pin. Also, a little sanding to reduce the outer circumference is needed for a good fit. (I’ve read where some put pipe tape around the threads of the collar. I did not)
Secondly, let’s move to the valve assembly area.
The original seal was virtually gone.
The stem is pushed out.
The little O-ring in the base discarded. Its function of sealing the back stem inside the cap is not needed.
The cap is ground down to remove any lip. Again, not needed since original valve seal is unobtainable.
Clean the valve stem. Notice the vent opening.
I made a valve seal from 1/8th Urethane/Polyurethane sheet. (Red)
Cut out a 1” X 1” section from the sheet
The center hole was drilled out to fit around the large part of the valve stem. I used a #3 0.213 drill bit.
(place the collar back onto the valve stem, insert into the drilled hole and circumscribe the outside diameter using an ultra fine point Sharpie.)
This outside circumference including the black ring was trimmed using an Exacto knife.
Do not cut inside the black ring. Trim the outside only enough to allow the seal to fit into the cup cavity.
It is necessary that this seal is a flush fit to the sealing ring inside the valve body.
Assembly the valve stem, seal and cup.Notice how the seal comes up to the vent on the shaft.This is a critically important assembly step.
I drilled a 7/32nd shaft into a dowel. This tool helped to push the seal around the stem and into the cup.
Notice how the valve stem is almost flush with the end of the cup.
Disassemble one last time.
Put a small drop of metal Epoxy into the small hole. This is going to hold the valve stem in place and seal up that end of the cup. (This is why the little O-ring was no longer necessary.)
Reinsert the valve stem to the original depth. Don’t worry about a little dome of epoxy. The spring will fit over it.
Let cure the appropriate amount of time.
Insert the seal flush with the edge of the vent and level. Do not worry if cup was not ground level.
Insert valve stem assembly into air valve housing.
Try blowing into the piercing pin and moving the hammer. Even if properly sealed, a very small amount of air escape may be notice. The CO2 will supply far greater pressure to fit the seal.
If not, disassemble and reseat the seal again. It took me three tries to get it right.
Reassemble gun. (I had significant trouble getting the barrel out. I took a Dremel tool and ground off the painted surface from the tip and a little more so that the barrel would slide into the housing. The set screw is all that is needed for a firm fit.)

This is very important!
Place the gun in the Open position. This takes the pressure off the hammer.
Pierce a CO2 cylinder. A slight hissing will occur until the neck of the cylinder is sealed by the end seal. Then, the valve assembly will jolt into its sealed and ready position.
Load BBs. And remember, this is not a pure repeater. Each shot requires the gun to be pointed up to drop a BB into the loading chamber.