Gearbox jams right before piston release

[Bandeen]

Hey folks,

Im trying to find out why, a recently purchased project Galaxy MP5k, jams every time the gearbox attemps to cycle.

I have taken it apart, it looks mechanically fine and nothing obvious yet for some reason when the pistol retracts full back it will lock up and not release.

Gearbox is V3 if that makes any odds.

Cheers for any help folks

[sigee]

Re: Gearbox jams right before piston release

This usualy happens when there is too big a spring fitted or too many shims/spacers fitted at one end of the spring.
Check to see if you can push the piston back all the way by hand or if the spring gets fully compressed before the piston goes far enough back to be released.

[emergencychimp]

Re: Gearbox jams right before piston release

I had a similar problem with a V2 gearbox, it was a lack of suitable lubricant along the runners for the piston and poor closure of the gearbox.

[dadio]

Re: Gearbox jams right before piston release

i've even had piston's that were too long , i think it was a hornbil blue full metal teeth piston that was 1mm too long and locked until i filed a bit off the back end.

[Figjam]

Re: Gearbox jams right before piston release

Yup, can also be spring too powerful for the motor or the shimming doesn't have quite enough slop, or a combination of the both. Had to add quite a bit of slop to a build I was doing last week that had exactly the same problem.

Possibly controversial barn-door test of my own devising: loosen the gearbox shell by about a quarter turn on each screw and then see if it cycles. I'd imagine it would be pretty bad for your gearbox if you then left it like that, but I've done it myself a few times to narrow down what the problem is.

[Bandeen]

Re: Gearbox jams right before piston release

Slop?

Well guys cheers for the help, thought I had it fixed as managed to get it to cycle outside of the gun, I tried it with a different motor (and cage as had a spare since its a v3 box) and managed to get it to cycle on single and full auto just fine!

Awesome I thought.

BUT... when I re-fitted it into the gun and set it back up it just did the same thing again and locked up?

:/ I have given up on it for tonight, will try with a fresh head tomorrow.

Cheers for all the advice so far folks.

[Stealthkiller]

Re: Gearbox jams right before piston release

Did you use the new motor when you reassembled it or the old one ? Could be that the motor is not torquey enough to push a heavy spring . Or is just old and weak ..

[Figjam]

Re: Gearbox jams right before piston release

Slop is basically a little bit of play in the horizontal motion of the gears. When you are shimming you want to leave a little or it will increase the friction within the gearbox and make it too hard for the motor to cycle the GB completely. Its usually around 0.1mm but its a bit of an art rather than a science. Have a look at some shimming guides on youtube.

So, Cycles when its not in the gun but not when you put it back in...Something pinching the gearbox when you put it back in? Is it a tight fit?

- - - Updated - - -

Ahh, also, is it a freshly charged battery. I'd say its a slightly insulting question, but as I've done it myself I'll just mention it.

[dadio]

Re: Gearbox jams right before piston release

could easily be a motor not strong enough , to get a respectable fps from an mp5k needs a hefty spring , the TM was only about 250fps , the galexy 300+fps but cycles quite slow , if it's had a stiffer spring it may need a better motor.

[Mickydoos]

Re: Gearbox jams right before piston release

I'd check for friction between the gears, make sure all three gears spin freely with the GB tightened up (with the cylinder, spring, piston etc out the way). Gears with a little play is better than one with no play but friction. Which can happen sometimes when you shim and you can only take one of the slimmest shims out.

Also try your other motor, perhaps the first one hasn't got enough torque.

Ahh, also, is it a freshly charged battery. I'd say its a slightly insulting question, but as I've done it myself I'll just mention it.

Yes, I've done this and opened the GB three or for times before realising it. Felt like such a doughnut. Specially as TM MP5s take ages to get to the GB.