Shimming for dummys guide

G’day everyone,

I’m relatively new to the hobby and have been bitten by the mod bug, Just wondering if anyone has a decent beginners Shimming guide? I know there are videos like Low Guido etc but if anyone has like a document or guide to shimming that would be great.

I have written a guide, it’s long but pretty comprehensive :+1:

Gearbox Setup

  1. Pinion refers to the motor gear
  2. Bevel gear refers to the first gear that engages with the motor pinion
  3. Intermediate or spur gear refers the middle gear
  4. Sector gear is the gear that engages the piston rack.
  5. DSG = Doulble sector gear, the gear has two sectors instead of one.
    This doubles the rof but requires short stroking.

Firstly start by completely stripping the box and clean everthing including the bearings spotlessly.

  1. Add one drop of light oil to each bearing and put one at a time on the sector gear in your hand and then hold the bearing and spin the gear , if the bearing feels dead smooth Your good but if it feels rough replace the bearing.
    Now install the bearings in the cases.

  2. Fit the tappet and cylinder piston assemby with no springs and close the box up and fit all the screws.
    Use your finger and ensure the piston and tappet move perfectly smooth over the entire range of motion without any sticking.
    Once happy remove them and put aside for now.

  3. Install only the bevel gear and close up the gearbox without screws , note about how much shimming is required and add shims, at this point it doesn’t matter which side they go, try to shim it with a few different thickness shims so you have at least one or two very thin ones. When you feel your getting close install all the screws and tighten them, now check the end float on the gear by using something to push the shaft up from the bottom, keep adding shims until there is as little end float as possible but the gear still spins freely with the gearbox closed up and tightened.
    Once happy remove the bevel gear and put it aside with all the shims you used.
    Now do the exact same process with the other two gears and only ever with one gear in the box at a time .
    Now fit about half the shims you used for each gear on one side and half on the opposite side, you can use a tiny bit of grease on them so they stay in place.
    Fit all the gears/shims and close the box and tighten the screws, rotate the gear assembly and make sure everything is free and smooth.
    Once happy remove the gears and set aside each with their respective shims.
    Remember all of this is done with only a single drop of light oil in the bearings and no grease on any of the gears, we will add the grease later.

  4. This step is all done with only one half of the box.
    Sit the back of you motor in to the motor base plate and set the adjustment screw so the the base plate is keyed into the motor to stop it rotating and lock the screw in place with loctite, put the motor aside.
    Fit the grip onto one side of the box with two screws and tighten.
    Fit the bevel gear with no shims and fit the motor and base and tighten the screws .
    Now move the pinion gear on the motor shaft until the back edge of the pinion is perfectly lined up with the outer edge of the bevel gear, if you move it remember to re loctite the gear onto the shaft with 609 loctite. To move the pinion and grub screw you may need to use heat because they are sometimes glued or loc tited on.
    At this point you can check that the motor is straight and that the shaft support is a good fit in the gearbox.

  5. Now you need to use the shims you have from the previous step to shim the bevel gear to pinion gear mesh, your aiming to have the minimum amount of backlash possible but you must have some.
    Once you think it’s close , install the shims you have left on the top of the bevel gear and fit other gearbox half and install and tighten the screws, remove the motor but don’t touch the motor depth adjustment screw.
    Fit two more grip retaining screws , yeah I know most only have two screws but four is better.
    Now reinstall the motor and base the put your finger or something else in from the top or holes provided and wriggle the bevel gently back and forth feeling for backlash, if there is none or too much backlash go back and shuffle the shims from one side to the other until there is virtually no backlash but you must definitely have some backlash, if you can’t get it perfect err on the loose side not the tight side. After assembly you can use the adjustment screw for minor mesh adjustment.
    Vola you now have the perfect pinion to bevel mesh.
    To verify everything put a tiny bit of grease on the bevel gear and run the motor with only bevel gear in the box and nothing else, listen to how quiet and smooth the mesh is with the pinion, then do one last check of the backlash incase something has settled.

  6. Now we need to install the other two gears, look at what shims you put where on the bevel gear and try to match it by putting the shims for each on the sides that match where you put them on the bevel gear, this just keeps the gears running approx. in the middle of each other.
    Once you are happy close up the box with nothing but the Motor, three gears and a tiny bit of heavy grease and one more drop of light oil in each of the bearings and run the motor again listening for quiet and smooth operation.
    If you have a 2s Lipo do all the running on it instead of the 3s.
    At this stage I like to run the gears in by running them for about 5mins or so with no load.

  7. You would now normally go to AOE (Angle of Engagement)
    But I now no longer reccomend doing this.

  8. Run your setup for the first 10,000 gels with no more than a 1.3 spring or referably even smaller.

  9. Lubrication…

  10. Do not use the light white grease on metal gears or rack, use the heavy red or blue grease here.

  11. Use the light white grease on anything that is plastic plus in the bearings/bushes, nozzel and piston.

  12. Oil your motor bearings/Bushes with a drop of light oil every now and then.

  13. Relube the bearings every 10,000 gels. You don’t need to strip the box to do it, just remove the receiver and press the grease into them from the outside with you finger.

  14. Strip , Clean and relube the entire gearbox every 50,000 to100,000 gels.
    Also check for wear , clearences etc.


Rattler this is perfect, cheers mate!


So you actually still want a tiny bit of end play?
Then that prob why the gen9 box is all fked,
The spur had no play at all, prob creating too much heat from friction and cooked the box

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WOW. That’s great dude. Thanks.

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And for a shorter read try “war and Peace”… :crazy_face:

On a serious note @Rattler… why do you not AOE?

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My testing has shown no adverse wear or issues when compared to an identical set up with AOE adjustment, and the advantage is more cylinder volume.


Seconded. A poofteenth of extra smoothness you can audibly hear in some boxes but if your gear meshing is correct, meh

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In my two identical set ups which both run super quiet I can not detect any extra smoothness or untoward sounds in the gear train on the one with no AOE correction, both are running smooth as silk and show virtually no wear after countless hours of testing and continual inspections


Keep in mind this is my guide not the only guide :wink:


I had two very similar

Similar to you, a well shimmed box and zero wear

The JJ SLR which I did do an AOE on just doesn’t have a crispness to the cycling…it’s soft and muffled.

The only time I’d bother with air these days is heavy springs and ice

Ain’t nobody got time fer dat

I can see how a heavy spring makes a difference?

If there are and negatives to no AOE correction it seem to me that they are far outweighed by the positives.

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I do it with heavier springs to guarantee proper mesh

It’s a paranoia thing


Sorry dudes but what is AOE?

AOE = Angle of engagement

The Angle of Engagement abbreviated to AOE is the angle in which the first tooth on the sector gear contacts the piston pick up tooth.

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Ahh right, thanks again the one and only…RATTLER! Lol thanks man

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Just an average smo who is obsessed with his toys :blush:


@Rattler what is your thoughts on 15 tooth v 14 piston?

You can use 15t no problem but…
On a standard blaster the rof is quite slow so there is virtually zero risk of any pre engagement and in this situation a 15t rack would be perfectly fine, but the average guy wants more and as your cycle time reduces the chance of the first tooth on the sector gear touching one of the teeth after the pick up tooth on the rack increases, just as it would when you make AOE correction.
In my situation I don’t use very high rof so I actually do need to use 14t but in my tests using no correction I only had 14t piston’s to use and in testing they did not have any problems or untoward noise or wear so I stuck with the 14t and even some 14.5t racks.
The other reason is that some of my setups have angles similar but not quite the dame as having AOE correction so the 14t is the best all round option for reasonable rof without short stroking.


I am in the process of shimming a new gear set
I am using SHS bushes rather than bearings
But have a slight problem with the bevel gear
It is too tight without any shims
I have all the gears running super smooth rolling them by hand until I put the two screws that are closed to the grip into the box and then it all gets a tight moving the gears

Should I try and file the face of one of the bushes?
Or do I try to run it in and see if it loosens up