Shimming
The easiest way to shim is to do one gear at a time with nothing else in the gear box. It’s very easy if you take your time and do it tediously. Fit your bearings, push them in evenly, check for a tight fit in the hole, if not tight add bearing lock, which can be got for 3 pound from any motor factors, this is important as what we need is consistency, and obviously spinning bushings will eat your casing. Wait for it to set. The bushings/bearings will now be pushed home and unable to move. Now check that the gear shafts fit in the bearing/bushings holes, This is more important on bushings but worth checking on bearings also, bushings the gear shafts need to be loose enough to spin without friction, bearings need to be loose enough to come out without pulling the bearings out but not so loose as to not grab and turn the inside of the bearing.
Now you have checked compatibility of the gears with the bushings/bearings you now shim them up one at a time. Start with the spur gear, the middle one. Shim it relatively low, you’ll find when you now add shims to the either side then place the casing on it, usual the shaft ends up about even on each side. Check for up and down movement with the casing screwed finger tight / a little extra not arnie tight! EVERYTIME you check for up and down play or freedom to spin you need to tighten the gearbox up with screws tight like you would normally on reassembly, its the only way for an accurate shim. It’s boring! After 100 times. Once any up and down play has been removed with shims, and the casings tight, remember all you have is 1 gear in the entire gearbox casing as this time, not other parts, it’s easy to poke a thin screw drive in there or finger and spin it, / get a nail under it and spin it, move it up and down and back and forth.. If you were a tiny motor trying to spin these gears.. Do they feel smooth and free… it’s just common sense and a little experience here. If it feels free and can’t move up and down the spur gears set! As a general rule like a bike which is upside down, and you spin the front tire, it should keep spinning for a few seconds.

Now remove it making sure not to lose the shims. And place it too one side.

Now move on to the sector gear and do the same on its own.
Now remove that and put it too one side keeping the shims.
Now do the bevel gear.. Different motors have different sized pinion gears for the most part this can be adjusted with motor height, but too low on the bevel and the motor will not be able to push all the way in, basically it will act as the motor is too tight all the time and not turn or drain battery. Too loose and you’ll get a screech or wear your pinion no matter how tight you do it…

Looser is best to be honest you can always move the motor in. Follow the spur gear guide, watch for a nice balance between the bearings/bushings..
Now remove all keeping note of shims.
So now we are at a stage where individually all gears work and spin perfectly free with little or no movement up and down.
Key point – With up and down play, shims can be difficult to find the 100% right one, when in doubt a “little” play is better than none at all, because when that casing closes it may move into an over shimmed status.
Let’s move on to how the gears work with each other / the other parts -
The sector gear has to interact with the piston, the tappet plate, the cut off lever and spur gear. So it’s important that after you have shimmed it up based on the previous part of the guide, you now install the spur gear and sector gear and screw tight the casing. This is a test of how these two gears spin individually, when tight, spin them and listen / feel the sound. Is it smooth and effortless? Excess noise perhaps ?
What we have here to look at is the gap between the two gears, if its grinding try moving 1 thin shim from the top of the sector gear to the underside. Now try again. This should get it smooth providing it’s not over shimmed in the first place.
Now install the cylinder head, cylinder, tappet plate and nozzle, NO SPRING. With the sector gear and spur gear. Tighten the casing; if I’ve not mentioned it 10 times already every test requires tightening the casing at least 3 – 4 screws. Move the nozzle forward and back wards, does the tappet plate move freely, completely freely. Does it move the sector gear when sliding, if so you need to lower the sector gear height. IF you’ve just raised the sector gear and cause this problem you now need to lower the sector gear AND spur gear. This is where a good deal of space for shimming can come in handy, now you have room for a wide variety of shimming. Little or no space due to oversized casing or bushings isn’t good and requires dremel, see advanced section.
Please also check how the gears interact with the piston at this stage is all of the gear teeth level with all of the piston, IF it’s a half tooth piston does the gears teeth push up on the under side of the piston, if so lower the sector gear. Notice a THEME, the lower the better for spur gear and sector gear.
Last test before moving on move the tappet plate into a forward postion, when you cycle the gear it should pull the plate backwards.
Now install all three without the anti-reverse latch and without the cylinder set. Check for free spin with your thumb on the sector gear, also you can turn it with your finger on the bevel. As with the other tests like a bike wheel we want a free spin which doesn’t abruptly stop, which is relatively quiet.
It’s easy, you’ve already checked for up and down play or “bounce” so now all you need to do is confirm your handy work and that it all spins freely! If in your mind you are a little motor and it feels effortless to turn then job done.
While doing this is good to choose full auto mode to move the cut off lever out of the way for a freer spin, don’t want you thinking the cut off lever bump is a bad thing!.

That’s it, remove gears, again, add bearing oil to bearings / bushings, I’m anal i add it to bushings since i have it and some like the gaurders have oil runners. Then grease up your gears, coat mating surfaces, anywhere that touches, not to much that when it spins its fly’s off and coats the underside of your nose, but enough to give you the sense that it’s coated. Too much could actually cause friction and sluggish results. Systema engineers actually as a rule opt for less is more approach. But don’t let that make you think there products are any good.
That’s everything to shim up a gearbox. Be sure to test and adjust motor height properly, also don’t be afraid to move the bevel up or down if you feel the motor needs to go in further or is too loose.
A good motor height test is attaching the handle and motor to the gearbox now and setting and testing motor height without the receiver on.
It’s an art. And different motor and grip and motor combos can make it a little more “work”.
Advanced section, bushing brand and bearing brand makes a difference not only in quality, but in size of fit. Certain brands are different sizes… The system bushings are a little taller than most as are the kanzen bearings. In some gearbox’s you can find in a combination with some gears, that you actually have very little shimming room, to the point where you don’t have to use any shims, this isn’t a bad thing as long as the shimming’s right and the height is right with the interaction of other parts. Also the opposite, the promy bearings and gears will need lots of shims 3 on top, 3 bellow. Some times more. Which is substantially different than example the systema bushings. Others like the KWA have been designed without shims, the gears are the right height for the casing and bearings. My personal preference is the promy bearing and gear combo simply due to the amount of choice i have for shimming it, ignoring the high build quality and cost for a second. when you’ve got room you can choose to remove it, if you haven’t to start with your forced to run something which you know not to be 100% right and cross your fingers or buy new bits, ignoring the high build quality and cost for a second.
Casing / cut off lever adjustments, sometimes the parts you fit simply don’t give you enough adjustment room, and example might be kanzen bearings in a thick src gearbox casing. In this instance depending on gearset the casing wont even close let only be a smooth running process, yet all you’ve done is buy the most expensive bearings out there… Money buys quality not a GARRENTY of fit… Its up to you the gun tech to make the parts work. In this case look at the problem, you need more room, so you’ll have to remove material from the inside of the gearbox casing… it doesn’t matter about the casing as long as at the end the bearing is in a fixed and permanent position… Same goes for the cut off lever, if you’ve lowered the sector gear because you HAD to due to early testing and now its catching on the cut off lever, don’t go oh no’ssssss, remove material from the top of the cut off lever, or lower casing to provide more room.
Gear meshing, another point worth mentioning is the cuts on gears and how the teeth mess, don’t assume they are clean or perfect. Taking a point tool, clean each tooth and in-between, look for crap, metal shavings, metal pieces, small children and ruff casting imperfections… utilising’s a piece of sand paper get in each tooth and just smooth them out.. you can even smooth out the top and bottom surfaces of the teeth. And you will have already possibly sanded the shafts as per the beginning of this guide any way. Sometimes a little bit of crap is causing your lockups..

That’s about it, install and test. TESTING IS KEY. Don’t be lazy.