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Shimming is genuinely one of the most annoying things about gearbox work, imo. I’m still perfecting shimming after over a year of doing it, and dozens upon dozens of times shimming gearboxes. These guys have gotcha pretty well covered, but just a couple things I’d add to the “info pool”.

Does the issue persist with different motors? And pinions? If so, it’s most likely your method, and you could try changing it up a bit. 😁

Does the issue persist with different gearboxes? As Ben mentioned, gearboxes can definitely be off-spec, but the chances of two or more gearboxes all being out of spec in the same way is low.

Does the gearbox sounds good?

When the gearbox is all the way tightened down, how many rotations do you get out of the gears when spun by hand? It’s possible your gears are too tight, causing the pinion to grind harder than it needs to against the bevel.

I thought I’d throw in my shimming method, developed by me (I’m sure other people do this, I just didn’t take it from a video, despite watching a ton in my early days).

First, I take a guess how many shims I think I should put under the bevel. I’ll usually go with thin shims here for maximum adjustability.

Next I put five screws in the gearbox (you can ignore the cylinder area), and spin test the bevel. If the bevel will not move when you tilt the gearbox/feel any resistance when you turn it with an Allen key, I know it’s already too tight, and I probably need to reseat/change my bushings or bearings. You can also try doing minimal shims below the bevel gear, but that can be risky.

Assuming the bevel rotates freely, I put all the screws in my grip, and screw it tightly onto the gearbox before inserting the motor. This ensures that I’m getting a proper judgement of how the motor will actually seat with the bevel. I shine a flashlight down my motor grip to make sure it’s seating flat with the gearbox, and then I do it again when I’m assembling the gun fully. If the grip is too big, I sand it so it contacts the gearbox first.

Like Ben said, I look at the pinion beforehand and try to gauge how it will interact with the bevel. Unless I’m working with, say, an ASG pinion, I’ll adjust the motor height to be even with the slant of the bevel gear.

Next I use a tool to check how much the bevel gear can turn. About half a millimeter to 1 10th of a millimeter does it for me, depending on how I feel the bevel is interacting. I then add shims to the top of the bevel gear until it has 1/10th of a millimeter of play on bushings, and a little more for bearings, and spins and rotates freely. The bevel should rotate when you tilt your gearbox.

After that I put my spur gear in the box, flat side down, and shim it up to the bevel gear so the teeth achieve 80% contact. Add shims to the top of the spur. Spin check With the gearbox tightened all the way down, and if I’m feeling nit picky, I’ll spin check with the bevel gear installed here as well.

Rinse and repeat with the sector.

Next I install the wiring harness, all of the gears, the motor grip and motor, and I’ll fire it up in SA and FA on both an 11.1v and a 7.4v to see how it sounds. If I hear anything whiny or high pitched that isn’t coming from the motor (my brushless can sound weird sometimes), I go back in and readjust. I know it’s right when I pull the trigger in full auto and don’t hear anything but shhhhshhh and the occasional gear click (spur contacting sector or spur contacting bevel).

Hope something in tis helps. 😁
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