Guide to shimming a gearbox.

Masada

As per mention over in the Version 2 Gearbox will not turn over thread, I thought it would be a good idea to have a sticky guide on the subject of how exactly to shim a gearbox. So here goes...



When you shim the box you should remove the cylinder, piston, anti reversal etc and only have the spur gear in there to begin with.

You want to get as thin as possible a shim under the spur gear so as to allow it to spin without touching off the inside of the shell. This gear should be as low as possible. then you test the gear by sticking the shell back together (with only the spur gear in there) and see if theres lateral movement on it. if there is you compensate this by adding shims to the top of the gear. you want it to be as tight as possible without binding at all. (keep in mind when the gearbox is screwed together it'll be a little tighter so put some pressure on the shell as you are testing the rotation of the gear.)

Then you move onto the sector gear and this one you judge by its height over the spur gear where they meet. you want the gear teeth to mesh together but not for the flat surfaces of the gears to meet. when you have that nicely, you try the shell on there again and see if you need to add any more shims. if you do, you follow the same procedure as the previous gear.

Lastly is the bevel gear and like the sector, you judge this one off teh spur and set it so the teeth mate but not the flat faces of the gears. you dont want to have this one too low (unlike the spur) because the motor needs to mate with this one and it can be a bit whiney if its too low. again, try the shell back on there and see if you need any top shims in place and add then until theres no play.

With all that done, you can add teh grip of the gun without actually rebuilding it into teh receiver. you can test it out and get the motor allignment right here to be sure its shimed properly.

Motor Adjustment:
In most cases when you reassmeble the gearbox the gun can sound a little whiney because of the motor alignment. i usually adjust this by ear in semi, go in all the way and come out in half turns while trying to fire it and listening for the smooth sound. When thats done you switch to fully automatic and adjust the screw in more of a fine tuning fashion while listening carefully to get it right on the sweet spot.

Job done.

I'll see if I can add some pics to this tomorrow.