Need Help with Gen 8 M4A1 Motor/Gears

Hey there, just looking for some help for my Gen 8. I just went through and upgraded my internals with metal gears, a new plunger & cylinder, a metal trigger and a 1.2 spring which all works…until I screw tighten my motor back on. Essentially what’s happening is when I hold the motor in place and fire the trigger, I get a working dry fire, however once I screw the motor on, I can feel the motor try and spin but I think it’s almost a tiny bit too far in causing it to jam and not spin the next gear.

I haven’t shimmed it at all, but I’m hoping there’s a simple solution without having to do too much else considering it’s all come together and worked until the very last step of screwing the last parts back on.

I’ve attached a couple of photos too, any help would be appreciated.

Works when held like this:

Doesn’t when screwed on like this:

You could try lowering the pinion gear on the motor shaft a bit, but if it were me, I would shim the gears properly, which includes setting the pinion to bevel gear mesh properly. Shimming a gen8 box is not very hard at all. Check out low guidos YT video on it :+1:

Another interesting thing noted from your pics…is the trigger supposed to have that angle on it? Shouldn’t it point straight down when not pressed?

Yeah I have tried pushing it as far down as it can go on the shaft but I think it’s as far as it can go. I can give it another try though because I honestly believe it’s like 1-2mm off.

Always wanted to know, how important is shimming and what’s it exactly for? Would leaving it unshimmed wear out the gears or something?

Technically the trigger should be straighter but the little wire piece that came with it didn’t leave me much room to work with when putting it in place. Despite the angle though it’s super responsive and is way better than what it was stock.

Shimming is incredibly important and is one of the, if not the most important part of building a blaster. They space the gears so that their not grinding on each other or the gearbox. On the other hand it stops gears from jumping up and down which is a one way ticket to gear strip.

I know it sucks but it sounds like you’re going to have to pull it apart. I would suggest taking everything out but the gears and close up the gearbox. Try spinning them with your fingers by sticking your finger into where the cylinder would have been. If your gears don’t budge, your bearing/bushings are too thick. Ideally you should be able to spin them with your fingers easily and with no play - as in you cant push them up or down.


Okay thanks for the advice guys, I’ve just ordered a shim set off eBay… my upgrade project deepens.

What gearset do you have in there bud? Are they generic cast metal gears or a set of SHS gears? If it’s the latter you might need some slimmer bushes. I believe whilst they fit with standard bushes/bearings, they’re tight. Without the motor screws tightened there may be enough room for them to spin but once those motor cage screws are tightened it might be clamping all sides of the box too much for them to rotate properly thus putting too much strain on your standard motor causing it to not be able to cycle.
When you pull it apart again could you post some pics of how the pinion sits in relation to the bevel as well?
Good luck mate :+1:

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From what I know, just a generic set I bought off eBay. Once my shim set arrives and I pull it apart again I’ll take a photo. I just watched Low Guidos video on shimming and from what I gather I think that motor gear (not sure of all the names yet) is just sitting to tightly onto that first gear that it hits and it just needs to be lowered a mm or 2.

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Pinion onto bevel gear.

Excellent investigating mate. Sounds like you may have sussed it out there. Apologies if I added confusion. One of the fun things about building is trying to diagnose issues that you sometimes only find when the box is together and you can’t see what’s going on. Ok. Maybe that’s actually a shit part but yeah, it’s interesting.
Anyway, spend a bit of time practising your shimming. You’ll be surprised how effective that one cheap (and not overly hard) step is. Got any other random boxes lying around? Open em up and shim them too. It’s a good skill to work on and improve…

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So after multiple attempts of shimming, I still can’t get it to work. I feel like I’ve done everything correctly but once I screw it all back together, it seems to jam. On my latest attempt, when I pulled the trigger it span the motor super quick for about a second then felt like it got jammed and wouldn’t spin anymore. After opening it up, I can see that the plunger has been pulled out the correct length but hasn’t been sprung back in.

I really can’t figure out what it is, I feel like it’s the pinion not engaging the bevel properly for whatever reason no matter how many times I alter it around with the shims, but it could be anything. I’m at a point where I’d pay someone to do it for me if I sent them all the parts and shims etc.

I know this may be an extremely silly and offensive question but did you put the spring back in if you said when you pulled it apart and plunger was pulled back to correct amount im assuming that means there was no spring in there otherwise it would be forced all the way forward?

Nah the spring was definitely in, I removed it before opening the box again. The plunger was just positioned as if the system had worked up until the point of the plunger springing back into the piston.

What size spring? Also when you shimmed the box did you screw it together without the spring in place and test to see if the gears spun freely? Something if you hold the box together and spin, it will work but once screwed the shims can be too tight

It’s a 1.2. Yeah I made sure after each gear was placed and shimmed that I tested spinning them with it screwed up each time.

And then tested with all together and still spun freely

Yep, tested with 1, then with 2, then with all 3. I’ve been super thorough because I’m so over doing it and even with my latest attempt it’s the same thing.

My only other 2 thoughts are like you said the pinion isn’t engaging. I havnt delt with a gen 8 do the have the motor height adjustment screw on the bottom of the grip? My only other suggestion could be the anti reverse hasn’t been set properly. Im reasonably new to blasters myself but have done plenty of trouble shooting over similar issues to this

After you work out how many shims each of the three gears needs individually to take up the slack, take note of the amount for each gear. Doesn’t matter at this point if they are above or below.

Once that’s done, screw the motor mount into the gearbox and work out the right height for the bevel gear so it meshes properly with the pinion on the motor by swapping the shims to below the gear 0.1 at a time.

After you’ve got the bevel gear shimmed to the right height don’t touch it any more. Moving onto the other two gears, swap the shims between top and bottom of the spur and sector gears so that they are set up correctly against the bevel gear and each other.

I don’t think there is a height adjustment screw, although I know the pinion is as close to the motor as possible,… but look there is a chance that maybe the anti-reverse isn’t set as it may spring out of place when I close the shell up. I don’t think that’s the case because I feel like I would notice, however when I give it another shot I’ll pay more attention to that part.

Yeah I did try changing the placement of the bevel by swapping around the shims from top to bottom in sequence but it either way it was ultimately the same result. Essentially if all the shims are on the bottom the motor span for a second then jammed, where as every other level was just an instant jam.

It’s also hard because the gen 8 has a black nylon shell so you can’t see through it but as I said I physically went through and changed it around multiple times with no luck.