Shimming Your Gearbox

Shimming Your Gearbox

Shims are little metal washers that come in different sizes. They are meant to apply the necessary space in between the gears. The goal is to space the gears as close together as possible without their faces actually touching; only teeth on teeth contact should be made. In addition to that there should be very little vertical play in between the gears. You do not want to shim them too tight or they will not spin efficiently and it will end up putting more stress on the motor creating unnecessary heat.

Common shim sizes: 0.1mm, 0.2mm, 0.3mm, 0.4mm, 0.5mm.
The first step before you begin is to make sure the gearbox is free of any parts that would get in the way of the gears or any necessary parts spring, guide, piston and head, cylinder and head, tappet plate, anti-reversal latch, trigger. Make sure to remove any stock shims from your gears.

There are several methods of shimming a common method is shimming from the spur gear.

From the Spur Gear:
This is the easiest method, but not the best. It is usually done by throwing a small shim on the bottom of the spur gear to rise it up off of the mechbox shell (usually a .1mm shim), then move onto the Sector gear, applying the necessary shims to clear the spur gear and getting it as close the spur gear as possible without the faces touching. Moving onto the next gear, the bevel gear and shim that in conjunction with the spur gear. Again like the shimming the sector, do not shim too high or too low. There should be about .1 mm of play between the gears.
Once you have this done to the best of your ability, put the other half of the mechbox back on. Throw some screws in and screw the two shell halves together.  Look for any play between the gears. For the sector gear, stick a knife or a thin small object and push on either side of the sector gear looking for play. The same with the bevel gear try and fit an object in the hole at the bottom of the gearbox where the motor goes in. If you cannot do this, use a small pin and push on either side of the gear axels in the center of the bushings. Notice any play? Estimate the amount and thickness of the shims you’ll need to make it so there is very little play in between the gears. Continue to do this until you have it right. Spin the sector gear from the now unoccupied space of the cylinder; see if the gears spin freely, they should continue to spin after you have spun the sector gear with some force.


For this method, you would be done. Reassemble your gearbox and adjust the motor based off noise only.

From the Bevel Gear:
This is a more tedious and more time consuming method. What is going to be done here is inserting the bevel gear and shim the bevel gear in accordance with the properly meshed pinion gear attached to the motor.
There is a few different ways to do this, however all of which have the exact same goal.

Here is the easy part. Find out exactly how many shims you need for each individual gear. This is done by placing in one gear at a time and applying the necessary shims to the gear so that it spins freely but there is barely any play. To make it more precise apply several screws on the gear box after you have it where you think is right. Now you know exactly how many shims you have to work with, take the gears out and set them aside. Do not lose the shims for each of the gears; you will need them later.

Now let’s start with the bevel gear in relation to the pinion gear. Place the pinion gear in the gearbox with no shims. You will need to see that the bottom of the pinion gear is flush with the bottom of the bevel gear. Now in order to see this you can either. Drill holes in the sides of your gearbox using an 8mm or 5/16” drill bit. Mark the two halves of the gearbox at the bottom of the bevel gear where the pinion gear should be. The entire pinion gear should be in view through this hole. As mentioned earlier you want the pinion gear to engage the bevel gear at the correct spot. It is wise to drill holes through both sides of the gearbox shell. So you can see how many shims are needed to shim the bevel to the pinion. Put your motor plate on and adjust it until the pinion gear matches up with the bevel gear, remember the bottom of the pinion gear should be flush with the bottom of the bevel gear.

If you are using a gearbox in which the motor is attached by a motor cage you can see this easily. If you are a M4/M16 (AR) user than you will need to have a modified pistol grip handy, the modified pistol grip, should be cut on the top so that it is not obstructing the view of the holes drilled earlier.

The second in checking pinion gear position is to insert the bevel gear and do not put the other half of the gearbox shell on, take your pistol grip (non-modified) and place it in the one half of the gearbox shell. Since you are missing a side of the mechbox shell, you can fully view the bevel gear’s interaction with the pinion gear.
Now you can apply the necessary shims on the bevel gear. With your motor inserted (which ever method you choose. See how much play there is between the pinion gear and the bevel gear.

This part is difficult to explain so I will try and explain this step the best I can without making it too complicated.
Ok to start; throw the necessary shims on the bevel gear without the pinion gear installed. Remember the gear should spin freely with very little play. Now that you know how many shims are necessary, take off the shims and set them in a safe place you will need them later. Now when you insert the pinion you’ll notice the bevel gear rising up. Apply the necessary shims so the bevel gear no longer rises up, but is not too low (you will be applying the shims you originally set aside and putting the amount of shims on the TOP of the bevel gear and putting the rest of the shims you set aside, on the bottom. Now you have the bevel gear and the pinion gear matching up near perfectly. You are ready to shim the rest of the gears based off of the position of the bevel gear. 

If you are unsure on what to do from here refer back to the instructions for shimming “From the Spur Gear:”


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