MP7 Gearbox Blues
 

I have been building an MP7 almost literally from scratch. I have every piece to build one (except a top rail, PM if you have one) and then some.

So I got the gearbox assembled after a couple of hours of struggling, but I was getting double feeding and the trigger was not perfectly responsive, so I figured I would yank the hop-up/barrel out and clean that up and see what it looks like. While I have the gearbox out I am wondering "is it the trigger mechanism or the gearbox?" so I figure I will put the gearbox on a battery while outside the gun. If it cycles then it is the trigger that is the problem. BIG MISTAKE!!! Almost instantly (the gearbox may have cycled once or even twice) the end flies off of the gearbox, cylinder head, piston and spring go flying across the room! I still haven't found the cylinder head. SNAPPED THE FRONTEND RIGHT OFF BOTH HALVES!

Well, what a letdown. Luckily I have another gearbox. I just glued the bushings in and will get around to shimming the gears into it sometime. It really bummed me out to see all that work (so close to working well) destroyed.

Anyway, does anyone have any tips for working on what I have found to be the worst gearbox EVER! and MP7's in general?

you should have put a little pressure on the nozzle while dry firing. That way the piston has a bit of resistance and not slamming into the front of the gearbox shell.

Yeah, that gearbox is pretty flimsy.

Also round the corners of the gearbox's window if it has one (Never seen a MP7 gb) with a round file.

And I don't think you should glue the bushings in...

You haven't asembled a MP7 yet then. They have 4 gears each with a micro 4mm bushing... it takes me almost an hour to close with everything in there.

I worked on at least 3 MP7s, fired all 3 outside the mechbox for testing (their trigger is very problematic) and none assploded. One of them had all the possible Nineball parts, including stronger spring.

I found that when the nozzle or cylinder head start to wear out, the nozzle will jam easilly because the friction overcomes the little spring inside the mechbox. I removed the o-ring inside the nozzle to solve the issue on one (with 2 FPS drop, probably because the nozzle was not seating properly to start with). That will cause multi-feed and trouble.

Indeed.

Just thought it was similar to the common gearbox we have, except being smaller.

Thanks for correcting me.