** NOTE THIS IS A GENERALIZED INSTRUCTION **

Some gears will not work with other gears and some gear will not work in some mech boxes. Other people will have other ways of doing this, but this is how I have been doing it and I have not had a single ‘come back’ yet.
The mech box, bushings, gears, shim sizes and shim placements are for illustration purposes ONLY and the actual parts may vary depending on you set up.

Step 1: You will need a gear box.

Step 2: You will need you shims, don’t be cheap on these. Buy the good ones, they’re not expensive.

Step 3: There is a size chart on the shim package. Verify that you have the correct sizes and familiarize yourself with them.

Step 4: Open up your gear box, remove the old gears and clean up the box. Replace your bushings at this time (if needed). You don’t need any special tools, just a small tac hammer will work and a ¼” punch is all you need.

Step 5: The first gear that we will be installing is the spur gear.

Step 6: I always start out with a 0.3mm shim. Place that on the bottom of the gear like this.

Step 7: Place the spur gear into the mech box and make sure that it isn’t ‘rubbing’ on anything.

Step 8: Pressing downward carefully but firmly, rotate the spur gear, again making sure that it is not rubbing on anything. Also be sure that it is not ‘laying flat’ on the mech box.

Step 9: What I like to do, is without a shim on the top of the gear, I ‘test fit’ it first.

Step 10: Place the top of the mech box on making sure that the spur gear is seated correctly in the bushing.

Step 11: Take 5 of the screws that hold the mech box together. The ones that surround the gears are the one you want, as pictured here. Tighten them down as you would if you were ‘finishing’ the assembly.

Step 12: Make sure that there is no ‘gap’ between the two covers.

Step 13: With the screws in place, lift up the box and you should be able to see the spur gear, through the area where the cylinder would normally be.

Step 14: With a small flat head precision screw driver, place the tip of it under the spur gear.

Step 15: Gently ‘twist’ the screw drive to check for ‘slop’. If you keep an eye on the outside of the mech box at the bushing, you will see ‘how much’ slop it has. You can use this information to get an ‘idea’ of what size shim you will need to use.

Step 16: If you have slop, pull out the screws and remove the cover, place on a shim (probably start with the smallest). DO NOT ADD shims to the bottom of the gear.. only the top.

Step 17: Repeat steps 14 & 15. AT THE SAME TIME.. attempt to rotate the spur gear by pushing on it gently with the screw driver, you should be able to rotate the gear with little to no effort. Once all of the ‘slop’ is gone AND the gear move freely, that gear is done and you can move on.

Step 18: the next gear is the sector gear.

Step 19: Personally, I like to check it starting with no shims.

Step 20: Place the sector gear in place making sure that the correct side is ‘UP’. Take notice to see if the spur gear and the sector gear are touching as they are in this picture.

Step 21: If they are touching, you will need to install a shim. A good starting point would be the same size shim that you installed in step 6.

Step 22: Repeat step 20 and check to see if they are touching. If they are.. add another shim.

Step 23: Reinstall and check the ‘gap’ between the two gears. There should be a small gap between them, such as shown in this picture.

Step 24: Reinstall the top cover of the mech box as you did in step 11. IF there is a gap such as in this picture.. STOP HERE!! You will need to start all over from step 6 using a smaller ‘beginning’ shim. If there ‘is’ a gap, then you have too many shims and you will have to ‘reconfigure’ the shims. DO NOT attempt to go on, or you may end up damaging your mech box.

Step 25: If there is a gap, pull out the gears and inspect. Make sure that they are not incorrectly matched or damaged.

Step 26: Recheck your bushings and make sure that they are seated completely and correctly.

Step 27: Once you have figured out the problem or re-shimmed it, recheck the slop and the rotation. AGAIN, you should be able to rotate the gears (both of them) with little to no effort.

Step 28: The next gear is the bevel gear. Do the same with this gear as you did with the other two. NOTE.. I have noticed that this gear does not need to start with such a high size of the shim, if you can try the 0.2mm first or even no shim as in step 19. You’d be surprised.

Step 29: Recheck for slop and rotation as you did in steps 11-17. If there is no slop, and ALL THREE GEARS rotate with little to no effort, you are done.


**NOTE** This process is a technique. You may find that this works for you, you may find it too difficult. Either way, if you are unsure of ANY part of the shimming process, it is highly recommended that you take it to someone with experience. This is a generalized explanation and it is subject to interpretation.

What I can tell you is that using this process will significantly decrease the amount of time it takes to shim a gear box. I have left out some of my ‘tricks’ (on purpose) but for the most part, this is what needs to be done. The first time... and I MEAN THE FIRST TIME.. I shimmed my mech box was this way and it took me 1-1/2 hours. I have been playing with the same mech box (stock) with my M120 spring for over 8 months and don’t have ANY problems. I have done MANY mech boxes including stock AND reinforced and this process will work for both.

Once you get this process down, you will be able to successfully open, strip, clean and re-shim a mech box in 20 minutes or less.. and it will be correct… THE FIRST TIME!!

Also, note that if the gears are shimmed incorrectly, the box is mis aligned, the spring is mis aligned or any other part of the mech box is installed incorrectly, it could be dangerous. So, if you are not mechanically inclined, I would not suggest that you attempt this. But if you do, it is NOT as hard as people make it out to be.

Have fun