M4A1 Gen 9 Metal Gears not spinning

Hey Legends,

Ive recently gone ham and done heaps of upgrades to the gen 9 gearbox. I purchased some metal gears, and after watching LowGuido’s video on how to shim gears etc. Ive decided to install them.Everything goes perfect until i screw the side of the gearbox back on. Then the gears do not spin, at all. Ive opened it up thousands of times to see if maybe a shim was jamming it, or whether there was too many shims, But it spins freely until i put everything back together.

I’ve tried and tried and am at my wits end. Has anyone had a similar experience? Or can offer some advice? I’d greatly appreciate it!

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It sounds to me like it’s shimmed too tight or there’s something getting in the way?

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Also what have you got inside the gearbox when you put it “back together”? Fully assembled or just the gears?

Are you testing each gear individually when shimming? It’s a long and lengthy process but if your not doing each gear at a time and putting the gearbox together for each gear it’s probably not gonna be right!


spur gear on shs gears and cheaper cast gears don’t have enough room in gen9/10 gearbox. You have to space the bushing out. Covered in this video.


I enjoy Low Guido’s video’s mate and I’m super appreciative of them but I’ve found some other shimming tutorials a lot better.

Take a look at this one above and perhaps reshim your gearbox.

Another thing, maybe you could provide us with some photos and a video?
We might be able to point you in the right direction easier?

Thanks everyone for your input!

I’ll try clarify a bit better, and when I get home from work I’ll provide some pics.

I’ve actually torn all the innards out of the gearbox and completely started from scratch.

First thing I’ve found is that the Spur and Bevel Gears are way too tight in the gearbox, even without shims. There’s absolutely no play. The sector gear has a tonne of play. So when I put the 2 sides of the gearbox back together, they don’t spin.

I have tried putting the Spur and Bevel in 1 at a time, they both get really stuck in the gearbox, the sides when screwed together jam it in way too tight.

So it seems like, when I reassembled the gearbox, the pinion gear (nylon) stripped because there was too much resistance from the metal gears not being able to spin.


I had a similar issue with cheap metal gears. I had to file down the brass bush on the gear shaft and I also filed down the bush surface in the gearbox. Shit of a job, better to machine down with a small lathe but I don’t have one. Took an hour or so with plenty of trial and error. Eventually got it good.
I also filed down both sides of the gear bushes in the gearbox and got just a small amount of free play that allowed the gear to spin freely.


Sounds like you’ve pretty much answered your own question then mate!

Might be worth watching the vid that @Kamchop posted to alter the gearbox slightly or do as @Daveb has mentioned.
Once that’s done it should be sorted mate!

My SHS GEArS 18:1 for v2 gearbox gen9 m4a1 not working for me have shaved down the shim slots .5mm and shaved the second middle gear as it was to heavy still getting grinding sound like its slipping out yet the gears hanging out seem completely fine any tips

Although spinning as freely as I can

You gotta take the time to get the correct free play on each gear! Then put all 3 gears in place and spin again and see how it feels. If all good then reassemble with all internals and test.

Same issues.

I have tried using heaps of techniques in the videos, i’ve ended up almost giving up. Ive tried even countersinking the nylon so the bearings sit in lower, nothing I can do frees up the metal gears.

I’m now in the market for a new blaster :wink:

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Are they cheap gears?
Maybe try another set, but you definitely need another blaster :wink::thinking:
Buy another complete gearbox for about $50 and build on while you use one!


So is the SHS gears needing rework to fit just an issue with the Gen 9 gearbox?

Going through the process of selecting metal gears for my Gen 8, curious if anyone with a Gen 8 has had similar issues with gear clearances.

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I just recently rebuilt an gen8 and fitted SHS 18:1 gears , the only gear I had an issue with was the bevel gear, I couldn’t get the pinion mesh loose enough with the pinion at the correct height . The was plenty of shim room but even with all the shims on one side the mesh was still too tight.
I had to slightly deepen the bearing socket and move the pinion back very slightly.
Oh and I had to use bearings because bushes didn’t fit down flush


Sounds like a bit of work… but worth it. Might just preempt any bush issues and swap out for bearings first off.

Can I ask why you went with the SHS gears? I know they should be better than the cast hardened cast gears for durability, but I haven’t heard bad things about the cheaper ones. There’s a complete kit on ebay that’s pretty reasonably priced that includes the cheaper gears.

Happy with the blaster’s performance after the gearbox service, Rattler? :+1:

I’m yet to use any other gears than shs for my own personal builds.

Ive used shs and Big dragon for customers as theyve supplied.

Cant provide any feedback on the generic csst gears but Ive seen people run them without drama.:man_shrugging:

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SHS gears are good quality cut steel gears, I won’t use cast metal gears as they don’t last and lose teeth !
Yes it goes very well, But the stock wiring and switch can only take a mild setup.


I have had issue with my gen 9 shs 18:1 gears have shimmed up so it isn’t tight yet the gears aren’t moving around when tilting yet no closer to it firing correctly