If you've played airsoft for very long you have probably seen this phenomenon. One of the more vulnerable parts in the gearbox is the Tappet plate that moves the nozzle back and forth to load each BB and seal off the barrel during firing. The plate gets very thin where it needs to get past the Gears and often breaks right at the start of the tab that contacts the sector gear. This is a common failure in all AEG's but seems more common in Thompsons and P90's than others.
You can buy reinforced (carbon fiber) Tappet plates but in my experience these don't last any longer. Below is a guide on how to repair a broken tappet plate or reinforce one so it doesn't break. I have done this trick more than 20 times (many for other people) and to my knowledge one that I have repaired has never broken again (not a guarantee). It is also handy to be able to repair rather than replace so you don't miss out on games while waiting for parts in the post.
The first picture shows the tools you will need. At the bottom is a Ver VI tappet plate with the typical break. You will need a dremel type tool with a reinforced cut off wheel (you could do it without but would take much longer) if you haven't got a Dremel tool get one, you'll never regret it!. You will also need A drill chuck for the dremel or small drill, small wire available at hobby shops and some hardware stores commonly called music wire, not the soft type that comes on a spool or coiled up but rather the type that comes in lengths of 12" or 36" and is stiff. and some super glue (any brand will do). For this example I am using 1/16th wire (.062)but smaller sizes are easier to work with .035-.045 is best but you will need a drill bit matching the size of wire you use.
If you are dealing with a broken tappet plate first thing is to super glue it back into one piece. make sure you get the pieces lined up straight and NO it won't hold together in use with just glue. Next you will drill two holes all the way through from the top as show in picture 2. Most tappet plates have two short holes already in the top. You can use the one closest to the back as your rear hole. Be careful when drilling. Go slow and keep the drill straight up and down so as not too have the drill burst out the side of the tab that contacts the sector gear. The front hole needs to be ahead of the thin part of the tappet plate (see picture) doesn't matter exactly how far. Note that from above the holes look off center but are exactly centered with the tab underneath.
Next you need to make a groove in the top of the tappet plate from one hole to the other. This is best done with the cut off wheel. Take your time and go slowly. It doesn't have to be perfect but be carefull not too burst out the side or bottom. Make a pass with the cut off wheel then test fit your straight wire in the groove. You want it to sit all the way down in the groove with no wire sitting above the tappet plate when viewed from the side. You can also use the drill (same size as wire) as a clean up tool to help create the slot or use a small file, sand paper etc. Note that the slot extends beyond the holes at each end. it goes all the way off the back end. This is only done to simplify cutting the slot.
Next you will need to make the metal staple. First make a 90 degree bend in the wire 1/2 an inch from the end. Insert it in one hole and mark where to make the second bend so it will fit in the other hole. I usually bend the wire in a vise and pound it into a right angle with a hammer. It needs to be a sharp bend, you can use a file or the cut off wheel to make sure the inside curve of the wire is a nice square 90 degree bend. It is a little tough to get the second bend in exactly the right spot. If you don't get it right the first time (holes don't line up) don't worry the wire is cheap and you've probably got plenty left.
Test fit the wire into the plate. If it doesn't sit all the way down. file or cut as needed until it does. When satisfied with the fit. Rough up the surface of the wire with the dremel, file or sandpaper (this is to get the glue to stick better) Fill the slot with super glue. Install your wire. Hold it upside down against a flat surface (picture 5) while the glue cures (helps keep everything lined up), Make sure everything looks straight from every angle. When dry you will need to clean up all the excess glue. Using a file or sandpaper make sure that each side of the tappet plate is square, flat and no bigger in any dimension than it was originally. Cut off the wire that protrudes below the tappet plate with the cut off wheel so it's flush with the bottom (note the rear one is at an angle).
Test fit into the right side of your gearbox. be sure it fits all the way into the groove and moves fore and aft easily, if not just file some more until it does. I often have to file the top to get them flat again, this is best done in a vise with a flat file but can easily be done in your hands too.
Be sure the tappet plate grooves in both gearbox halves are well greased and re-assemble your gearbox. When the gears are stopped with the nozzle fully extended push in on the nozzle it should push in and spring all the way back easily, and that's it!!
It sounds long and complicated but is actually pretty easy to do (takes me about 30-45 minutes.)