Rapid bevel gear wear

I need a photo of the pinion too.

Keep in mind not every pinion fits every bevel. But something else is off by a lot. The tips of the bevel teeth aren't as worth. Suggesting the motor height is way too high. A pinion with fatter teeth basically needs to sit edge to edge with the bevel. Thinner teeth pinions will set higher where you can't see the edge of the pinion teeth. After than you shim the pinion bevel tight but with a .3mm to .5mm gap.

If the shell bottom is ground improperly it can seat the grip off. The grip should be sanded so it doesn't rest all its pressure on the lower receiver and sits flush on the gearbox, unless the receiver is a better fit than the gearbox, which happens. The grip hole should not touch the motor tower first, the motor tower should contact the gearbox hole first. This will screw up the motor angle checker tool.

Another thing, a ton if gearbox shell manufacturers modified molds to make their 6mm or 7mm gearboxes into 8mm gearboxes. Because of this some brands poorly centered the hole and the bushing or bearing will sit off center, making the pinion wreck the bevel as it's not lined up. Even some high end cnced gearboxes such as the beloved retro arms shells have improper 8mm bearing holes due to inability to properly design a gearbox for some reason. So if your bushing/bearing is off it would cause insane wear like this, and the gearbox/motor grip would clamp down on the angle tester giving a false impression its passing the test.


The gearbox itself could also be sitting back tilting. Either due to fitment or other issues. If the grip is resting on the lower but the gearbox is tilted back from fitment issues or tightening the stock screw first it will cause alignment issues.

Best of luck.

Oh and this wear has nothing to do with the spring, rps or anything. I have plenty of high rps builds running every weekend that have very little wear on the bevel and pinion. You will likely see a rps improvement once you fix this issue and the aeg will become super quiet.

Here are some more pictures. The grub screws on the top of the motor grip are my latest attempt to correct this issue, was shredding bevels before installing them. I noticed that the shs bevel and the no name bevel seemed to suggest the motor height was too high as well. If it would be beneficial I can take the gearbox apart and get a picture of the current bevel gear's teeth as well. Could you elaborate a bit more on what you mean by shimming to have a .3 ~.5 mm gap, my interpretation would be the play in the gear, side to side? Also, any ideas on how to check for bevel gear centering in the case? I'm a senior in mechanical engineering so feel free to get extremely technical if needed lol. Thanks again.

Something else that may be beneficial to know, measured amp draw on the gun currently is 16 amps continuous, not sure of accuracy of my tester's peak measurement however, so not included.

Well no matter what, I would say ditch the SHS bearings. They are GarboDookey... I use EZO J-caged bearings. They cost $40 though. Sometimes I put a bushing under the spur, but not always. Pack them with MolyKote33, or something similar. Don't think that is your issue, but a future issue maybe. I have a weird way I shim my motor grips to my receivers. I have like 3 different grips I use only. I took one of each and cut a hole in the side of the grip. I can then see where the gearbox shell is in relation to the motor grip. I take feeler gauges to measure the gap. I then cut the correct size feeler gauges to make a shim for the new corresponding grip. When I shim the bevel gear using the half shell method, I have the shim in the grip attached to the shell half. I bought a ton of cheap feeler gauges on amazon. I cut them on my band saw and use my drill press for the screw holes, motor tower, and notches. I got a jig made, so it is not as much work as it sounds. This way I don't have a gap between my motor grip and receiver. I also know the bevel is set correctly in relation to the pinion.

I also slide the feeler gauge left and right to make sure the gearbox is not at an angle.

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. 😁

First, I want to say thank you to all you that have taken the time to post and try to help me out with this issue. That said, there has been some great ideas about what the issue may be and I was wondering if any of you think this could be caused by PME? Previous bevel gears were ruined with the gun SSd by 2 teeth rather than the current 3 and using an sp120 rather than m130.

I have opened the case and double checked my shimming of the gears and to the absolute best of my knowledge they are as good as I can get them with this shell, about 0.1~0.15 mm vertical play in each and getting roughly a dozen revolutions spinning with finger from cylinder window, with superlube. I haven't changed any of the shimming from what it was previously.

I assembled the gb with only the wiring and another loner g&g bevel gear I don't care about and installed the motor as it would be on the rifle and but left the grip screws loose so I could play around with angles. I then trial and errored moving the motor grip around forward and backwards to find where it was the absolute quietest I could get it (basically just plain motor whiring but a little louder) and had a friend mark the perimeter of the grip with a sharpie while I held it. I then shimmed the grip to match this line when screwed in tight. I followed a similar procedure with the motor height. I then marked the teeth of both the bevel and pinion with a blue sharpie like one would when sharpening a knife to check for uneven wear, and so far, no signs of uneven wear. I will continue to monitor for any wear, but I think this may have worked for me and wanted to share. Included are pictures of the bevel and pinion after running about 100~200 cycles with everything installed.

Another thing. American Consumerism Does Not Apply in Airsoft.

There are no standards in this industry just a guide line that is loosely followed at best. A true M130 out of the package is not that common. The only way to get this is if you buy a hard spring and the rating is true, like from a Japanese supplier.

Unlike a M130 made from a soft alloy where it needs time to settle to the M130 rating. Thus you overstress your mechbox and it gives you a false positive until the springs settle. This was the issue with SystemA springs from 23 years ago. It took the spring about 2-3 weeks in a compressed state to "settle" down to their spring rate. When newly installed a M100 shot like a M120...people were like WTF? Maker never mentioned that you had to compress it to settle it. Events like Savage Garden 3 really messed users up because they thought they installed a M100, but when they chrono'd they were 40fps over..."but I bought a M100"...

Then you just have low end makers like Matrix Springs which is a Evike House Brand...they are just bad. I mean...who would buy a spring that has a FPS rating like this:

235% (M130) ~420~500 F.P.S.

Sadly...thousands...a M130 should never hit 500fps. That is a M150...but the alloy...soft...cheap...and end users not know the details of the spring...its a bad cycle that is not good for the end user...but great for the seller.

I should probably make this its own post, but it seems like it fits with this one. The way I shim is also weird. As Ben said, using the half-shell method can have its issues. I call my method the 3/4-shell method. I have collected broken gearbox shells and cut viewing holes in the broken top shells. I have different shells for different brands of gearboxes. Like G&G puts their screws on the wrong side, so I have to use a G&G top shell when shimming G&G. E&C/Arcturus have a different-shaped shell; KWA has a different shell; and so on. I find the six top shells I have modified pretty much cover all the common current V2 shells. This Dytac shell lines up perfectly with a VFC bottom shell. I check bushing holes using a rod and dial indicator to assure everything is lined up correctly when mixing shell half brands. I use this along with the grips I cut and modified to shim every possible part in a V2 AEG. I find the bevel gear to be the most important gear to shim correctly. The shims under that one gear can make or break the entire performance and durability of an AEG. (This is just a selection of random parts to show my process. This is not an actual build, just an example.) I always use one of three grips on my builds. I have a test grip with a viewing hole cut in each of the 3 grip options. I sand and stipple most grips, though not always.



The first thing I do is put the gearbox shell into the lower receiver with all pins installed so that the gearbox is as it will be when done. I then screw on the test grip to the gearbox shell. This allows me to use feeler gauges to measure the gap between the motor grip base and gearbox shell (indicated with the red line in the above picture). The issue people who are new to tech work have is that when they shim a gearbox, they don't account for the motor grip and lower receiver not aligning correctly. This causes the motor height and bevel shimming to be off. A common solution is to shave the motor grip so the gearbox can sit flush with the grip (indicated by the green line above). What happens most times is that people shave too much, causing an ugly gap between the lower receiver and motor grip, as pictured above. My method allows me to measure the gap exactly, and I then cut/make a shim using a jig. Sometimes just the top or side of the motor grip/lower receiver is off, which causes the motor angle to be off. With a shim, I can be sure the grip is perfectly flush without the ugly gap between the receiver and motor grip. So the picture is a PTS grip; I would obviously use a new PTS grip and add the shim to the final build. (((The PTS grip above was a failed stippling attempt. It was shaved to fit a receiver already. This was my concept grip to test my method. I now have an unshaven PTS grip with a viewing hole cut in it to use as one of my test grips. All my stuff is in the shop at the field where I do work. These are all just junk parts used as an example.)))



With the shim added to the grip, I no longer need to put the gearbox in the lower receiver for shimming. I can now start the process of setting the motor height and shimming the bevel gear. With the modified top shell, I have a clear view of the pinion and bevel gear alignment. Different pinions and bevel gears require different motor height positions. I have a perfect view and can set it exactly where it needs to be. From here, I start adding shims under the bevel gear until it is at the right height. From this position, I take a pick and move the bevel up and down to get a rough idea of the amount of shims I need under the bevel. It is mostly just trial and error, though. As soon as I get the bevel at the correct height in relation to the pinion, I move on to the spur and sector gears. Motor height, grip to lower receiver, and bevel height are now all set.



Now on to the spur gear. With the height of the bevel set, I can shim the spur gear to get the maximum contact area with the bevel, leaving a small gap for any expansion of the gears while under load. I have modified offset feeler gauges to measure the gap. I can fit them in easily to measure with the cutout in the top shell. I repeat the process with the sector gear. I set the gap with the feeler gauges, but it just depends; every build is different, and sometimes you don't have much of a choice with setting the gap ideally or perfectly. I can then throw a piston in and check the sector and rack alignment. The top shell in the picture was also my concept shell. On the top shells I use now, I have a hole cut where the Titan optical sensor sits. This way I can check to make sure there is no interference between the sector and top board. I can now use the actual top shell and add shims to the top of the gears. I have never had a situation where one of the gears was shimmed too high when adding the actual top shell. I have found parts that work for me, and I stick to them. Everything is now set shimming wise. This has been very successful for me. I might have made it seem complicated, but it's really easy and quick.



I can also use this to check other parts in the gearbox. As you can see above, these random junk parts would not work well. The cylinder head is sitting flush with the gearbox at the bottom but has a gap at the top. This means all the force from the piston will be directed to a smaller point instead of spreading out over a large area. It could crack the bottom of the shell. (The cylinder head is not resting on the guide pins, but that is another thing to consider as well.) You can also see the tappet plate is not in good shape. It is past 90 degrees and will push the nozzle downward. It is also very common for tappets to not fit nozzles very well. You can see the gap between the nozzle and tappet; it is not seated all the way in. This, along with a poorly fitting nozzle to cylinder head, can cause mid-cap syndrome. If you have a well-fitting nozzle on the cylinder head, this gap is not a huge deal.

I get that this is not something most people will do, but it is how I have gotten the best performance possible. Millions of combined cycles over many rentals, DSGs, customer guns, personal guns... Like I said, I have parts I like to use because they work well for me and the builds I usually work on. There are always exceptions, and it is airsoft we are talking about. If a customer brings me all his own random parts and wants me to only use his parts, this method might not work out. He might not have the same grips I use, so I won't have a test grip for shimming this method. Maybe he has a gearbox that none of my top shells will fit on. You get what I'm saying.. Just what I do, everyone has their own way of doing things, I'm OCD when it comes to tech work, so I usually take it too far, lol. Hopefully this made sense. It makes sense to me, but I'm an idiot.

Well when it comes to shimming no guide applies to any given aeg, part mix and what-not. Steps 1-10 guides for airsoft will always miss things you can clearly see with your own eyes that no one else here can. Aegs can vary batch to batch, maybe the assembly line lady for the gearbox shell sanded too much of the casting line off the bottom of the shell where the grip attaches, or the factory converted a 6mm shell mold to a 8mm shell mold with some poorly centered mold inserts to save on money. (rather than make new expensive molds) Or maybe the factory worker ripped the plastic grip out of the mold while it was still cooling to save time, warping it's fitment. Or the grip has a tail to it that with some receivers causes it not to sit flush so it ends up bending/pulling the grip back without you knowing. (Yeah ive seen all this crap) This kinda stuff screws so many new techs and players up, you cant have someone help you when things like this happen as they can predict it, even testing tools can fail to detect this stuff, but your eyes and ears sure can. Airsoft teching is like trying to mix MEGA blocks into a lego build.

So to shimming...
First you MUST find a pinion and bevel that are compatible, so you must find a pair. Same brand doesn't mean anything half the time.

To "pair" a pinion to a bevel you actually must test them by hand and roll them together. You literally take the pinion and hand roll it against the bevel, You'll find where it wants to seat in the bevel as it rolls and how smooth it feels, that might be higher up than another pinion, you then must check at that meshing point if the pinion is seated at 90ish degrees to ensure its truly meshing with all the metal of the tooth, if not find another pinion. I tend to avoid pinions that sit high, its easier to work with ones that are near the edge to edge of the gears. Keep in mind pinions and bevels mesh roughly, the tooth profiles will have high and low points as each tooth makes contact.

For my shimming I used a tiny precise drilled hole in the gearbox top shell where the bevel and pinion meet and in the right side of the grip im using to see EXACTLY where the pinion seats into the bevel. This tiny view hole lets me set the motor height perfectly to where i know it should be at. When I shim, i have the motor tower shimmed snug'ish into the gearbox hole. Like near zero slack using ptfe tape, I slide the motor in and look into the tiny gearbox hole to see when the pinion meshes with the bevel edge to edge then see if the motor free hangs at a 90 degree angle while the height is exact, I shim until its at that 90 degrees then take out .3mm of shims.

You need to make sure the motor armature isnt sloppy, and shim it if you need to as well, you also need to be sure the grip and grip endplate isnt doing something funky to the meshing angle as much as you can.

To me this is the easiest way to get a good shim job every time. It also allows you to set motor height easily on the fly as well.

Is your motor super loose inside the pistol grip by any chance?

I saw some similar wear on the bevel gear like that once a few years ago on my brother's high speed ssg that I made for him. One day at the field in the middle of the game he told me there was a grinding noise coming from the gearbox so I told him to stop using it. When I took it apart, the bevel gear teeth was worn pretty much the same as yours, and I determined with some troubleshooting that the motor was way too loose and twisting inside the pistol grip and was contacting the bevel gear at a weird angle due to this.

You could literally hear the motor twisting and knocking the sides of the pistol grip everytime the gearbox was cycled. I should have caught that before a bevel gear was sacrificed, but I was younger and didn't know or realize that much at the time. Lesson learned though.

I ended up putting layers of electrical tape on either side of the inside of the pistol grip to hold the motor tighter, although I do realize the heat will increase when doing this. Oh well. If anyone has a better way to fix that problem let me know, but it seems to have worked for now. I did the same treatment to my high speed m16 so the same thing doesn't happen.

It does seem like you have done a lot of troubleshooting already and probably don't have this problem, but I just thought I would through out a suggestion. Good luck.