View Full Version : quick and dirty shimming tutorial

07-24-2005, 02:11 PM
After doing a search on LSA for how to re-shim my mechbox, I turned to airsoftplayers.com for help. I found a thread containing a link to what I think is a pretty good guide to shimming a gearbox. Since I didn't find this kind of information in my LSA searches, I thought I'd share my find with the community.

The URL is http://www.geocities.com/wajoegween/aegshim.htm

In case geocities removes this site, here's the info contained at this link.

AEG: Shimming

1) Preparation
Remove everything from the mechbox that will get in the way of getting to the gears. This includes the spring/piston/cylinder assembly, and then it depends on the gear box.

With version two mechboxes, the little safety lever needs to be removed so that the selector plate can be slide forward and off. Now there should be only the trigger assembly left in place.

With other versions, other parts may or may not need to be removed.

When checking shimming, do not install the anti-reversal latch, motor or anything else that may restrict the gears movements in either forwards or backwards.

2) Shims and Bushings
[note: I speak only of Systema shims. I am not familiar with stock or other shims. For simplicity, I always replace all shims with Systema ones.]

There are three different sizes of shims, 0.5mm, 0.3mm, and 0.2mm thickness. There are two different diameters of shims, the small and the large. Of the two smaller diameter shims, they are noticeably different in thickness. They are the .2mm and .5mm thick shims. The large diameter shims are the .3mm thick ones.

Since you're reading this guide, your are most likely replacing the bushings. If the replacements are solid metal they will be a tight fit. I first try pressuring them into place by pushing with my thumb really hard. If it does not pop into place, I take something soft and tap it into place. Most commonly I end up lightly hammering them in using the plastic handle of a screw driver. Make sure to force the bushings into place EVENLY!

If you are replacing the stock bushings with bearing bushings, they will be a loose fit, and it is easier to put them on the gears than to try to make them stay in place on the mechbox. Don't worry about how unstable the bearings feel, they will be plenty beefy when the mechbox is together.

3) Spur Gear
On the bottom(left) side of the gear, place a single .3mm shim. With the Fa-Mas, it might be necessary to have more shims than any other mechbox. With my only Fa-Mas shimming experience, I had to use two .3mm shims on that side, and generally a lot of shims all around.

Put the gear into its place, and put the top(right) half of the mechbox on, and just hold them together tightly with your hands. Push the axle with a tooth pick or something similar, and note about how much it moves. Now it's just a game of guess and check with shims on the top(right) part of the spur gear. Once you think you have it just right, with little to no movement, put in three or four screws to make sure its held tightly(more than your hand can do). It should have very little or no movement side to side, AND you should be able to reach a finger in through the empty cylinder hole and freely spin the gear. If there is too many shims on the upper axle part, it will have resistance or be completely stuck. If there are too few shims, it will turn freely but have too much side to side movement.

4) Bevel Gear
Start with the bottom(left) half of the mechbox including the shimmed spur gear. Place two .2mm shims on the bottom(left) part of the axle and put it in its place in the bottom(left) half of the mechbox. Check for clearance between the teeth of the spur gear that mesh with the normal teeth of the bevel gear and the notched part that the anti reversal latch rides on.* If they interfere with each other, then the bevel gear needs more shims on the bottom(left). If there is ample clearance (unlikely), you may want to drop it down to a single .3mm shim. The idea here is: You do not want to compromise the gears' teeth because of the bevel gear having too many bottom(left) shims leading to minimal gear tooth surface contact between gears, but at the same time you don't want the anti-reversal latch contact area to be grinding with the spur gear.

OK, enough rambling. Throw the right side on and check for movement the same way you did for the spur gear. Then guess and check, put in a few screws and do the final spin check with the two gears. Re-adjust and re-test as needed.

5) Sector Gear
Almost exactly like the bevel gear, start with the bottom(left) half of the mechbox including the shimmed spur gear and bevel gear. Place one .2mm shim and one .5mm shim on the bottom(left) part of the axle. Check for clearance.* This time you want to make sure the large diameter part of the spur gear is not going to be binding with the teeth on the sector gear that engage the spur gear on it's smaller diameter part.

*A general note about gear clearance: Remember that when the mechbox is together, the gears will not be able to twist(axels slightly off of parallel) relative to each other, so when checking the clearance, you may see interference that should not be there since you only have one half of the mechbox on. Make sure to not accidentally over shim the bottom(right) side of the bevel or sector gears because of this.

I make no guarantees of it's accuracy, so if it doesn't work, then I'll probably be the first to know. I'm about to try this out myself, so wish me luck.
If any of y'all experts out there see anything that is wrong or misleading, please let me know and I'll make the necessary changes to the posting.