40mm Grenade Maintenance Part 4

In this fourth and final installment, Coop takes apart some fill valves and shows how to properly maintain them.

To read part 3, click here.

Sometimes, you’ve done everything in your power to repair/improve the internals of the grenade, but you still have a leak.  The quick troubleshooting tip is, if gas is coming out of the barrels, the problem is typically on the inside.  If gas is coming back out of the fill port, then the problem is the fill port or fill valve.

These fill valve tips also work on your favorite GBB magazine fill valves, and in a pinch, you can even replace your 40mm shell fill valves with GBB magazine fill valves, depending on manufacturer.

I recommend picking up a valve tool, or permanently modifying your least favorite flathead screwdriver into a valve tool.  I am using an Element valve key here, but I am not a fan of it.  I already damaged one of the ends working on another project.


This is what a fully assembled valve looks like.  Madbull valves look slightly different (wider), but for the most part, work the same.

Fill valves can be twisted apart to access their internals.

This is what a disassembled valve looks like.

From left to right:

The main body, which houses most of the parts.
An o-ring.  This inner o-ring prevents gas from flowing back out while filling the shell.
The valve pin.  This gets pushed down when you put an air tank up to the valve to fill it.
The spring returns the valve pin to the “closed” position when you’re done filling.
The base threads into the main body and houses the spring.

Here is where that tiny o-ring goes in relation to the whole setup.  You can buy replacement o-rings online, or pick up replacement valves if for some reason it still does not work after cleaning and lubricating everything.

The valve pin drops into the main body.  The spring drops into the base.  They screw together.

So, with a tiny amount of grease inside the valve pin compartment, and some grease on the threads/o-ring, screw the fill valve back into the shell using your valve key or modified flathead screwdriver.  Excess grease will be pushed out due to tight tolerances.

Not related to fill valves, I did snap this picture while working through the first half of my grenades, showing  off a “same but different” example.  These are the internals of a Madbull PB-4.  It has a beefier dual valve o-ring (bottom left).  The “barrel holes” in the front of the shell (top left) actually store gas… that’s a lot of room, all things considered!  The fill valve is in the bottom of the shell (top right), which is handy for things like the RPG-7 warhead, so you don’t have to disassemble the shell from the warhead.  The button (center right) is shorter, looking more like an original Moscart button.  I also found an o-ring the same size as the base of the valve (bottom center) that I have added to my shells at Exarach’s recommendation as a  “shock buffer” to prevent damage to the valve.  In my testing, the o-ring stayed in place without adhesives, and successfully cycled 20 out of 20 times, much to my wife’s displeasure.


Did not get photos of the PB-4 re-assembly, but it works the same.  Make sure the button slips inside the valve.  Grease both o-rings and the 4 bearing holes.  Install the bearings.  Put the valve into the front of the shell, add the spring and button.  Rest the buffer o-ring on the valve.  Screw the rear half of the shell onto the front half.  Press the button, ensure it’s reset, gas, and enjoy!

Did about 20 more shells today.  All of Tascabe’s are operational except 1 Moscart.  I did not have any 3mm bearings on hand, but I will once McMaster-Carr delivers them.  Did both of my PB-4’s and 14 of my 922A1’s.  I have 16 more to go.  I also have a madbull foam slug shell, and a madbull foam rocket shell that I may take apart to show off, but that’s for another day, possibly tomorrow!

The last day of maintenance saw me finishing up the shells that normally live with the mortar.  Nothing terribly exciting, but I grabbed a few pics of interest:

#40mmProblems  Not bad for servicing 64 grenades… this is all the damaged stuff… and I want to take another look at that valve… I swear I can save it, lol.  I did replace about 14 ball bearings throughout the whole set, and still need to order one more 3mm bearing, because Moscarts use 8 smaller bearings instead of 4 larger ones.

This concludes Coop’s guide to modifying and maintaining 40mm shells. As always, many thanks to his time and effort in this project!