Gearbox jams every cycle
 

I have a G&P version 2 gearbox here that was jammed with the piston fully rearward when it was brought to me. I disassembled the box and everything looked fine. I lubed the grease points and reassembled it. It locked back again on first cycle (never completed a cycle)

Next I changed the motor and the piston/head combo. Still locks back on first cycle.

Any ideas as to what caused this? This was a gearbox that the owner was using up until recently. Now it cannot do a single cycle with an 11.1v lipo??

What brand of piston?

Does it move freely in the gearbox with nothing else installed and the box screwed together? I've had that problem with G&P shells before, pistons were too fat.

Funny thing is the piston was just the original piston that always worked in it. Now no pistons work. The GB must have changed shape, or something.

I will have to widen the track or something.

Take the spring, spring guide and sector gear out, screw the mechbox together, and push the piston to the back of the mechbox and see if it gets jammed.
You may need to chisel/file down the rear ways or get a new shell.
I know this was common with guarder shells many years ago

short stroke it lol

Piston o-ring too. If you have a tight seal it's hard to let "gravity" move the piston.

Could be either the piston is getting caught on the spring guide or the piston rails need to be filed down. Maybe the shimming is too tight? I've had that exact problem before too.

Easiest thing is to swap piston head o-ring to a smaller one. If you have a vented piston head, it might be that as soon as the sector gear releases, the air pressure expands the ring and provides too much friction.

Trial and error lol

how friggin good does your airseal have to be to lock the piston back at the rear of the box (where the spring pressure is the greatest) , i got the impression that it stays locked back. IMO thats not an oring~ piston head problem

In my mind's eye, it just needs to stop it from moving after the release by the sector.. or slow it down enough that the sector overspins and picks up midway through the compression stroke.. if the rack is strong enough you could jam it up that way. Have to assume it's possible since he didn't mention where the sector was when he opened up the gearbox. If you hit the arl before you open it up, you may not even see evidence of it.

I agree that can happen but if you read the OP that's not what happened this time. Just splitting hairs y/know

indeed it wasn't. Only time I've ever jammed something first cycle was motor height after changing the grip on a v2, ended up double shimming the motor, and that isn't the case either. So I'm out of ideas :3

cookie time.

Ron's helped me lots of times so i'd really like to stumble on the answer here...So lets see- The motor starts spinning turning the gears Any broken teeth, shims, pieces of bushings or bearings stuck in the gears especially where hidden by larger gears above? Is there excessive wear in the bushings or gear axles? Next the sector gear starts pulling the piston and tappet plate back the spring starts to compress and twist How are the bearings on the piston or spring guide? Sector gear keeps turning and just before releasing the piston grinds to a stop Did it stop the piston or did the piston stop the sector gear hummm When the damn thing is jammed back, is there any side to side play on the piston in the cutout of the gearbox? NOTE do not use bare fingers to check for this!

I didn't wanna say anything, but you can actually have the O-ring slip over the piston head and drag through the cylinder wall without the gun jamming in the rear position. That spring applies a LOT pressure.
And yes it happened to me lol
The piston didn't actually jam until the O-ring got caught in the cylinder port, but it fired a good 30 times before it fully jammed up

I'm having the same issue with one of my m4s. Thanks for the post and replies all ")

had the same issue before on a friend'ss gun. In my case it was the spring itself. Basically the end of the spring that touches the spring guide was bent a bit causing the spring's diameter to be just a bit bigger yhan the the piston. So we just bent the spring back into shape, or you could dremel it.