*New* How to Perfectly Shim a Gearbox:

Discussion in 'Gun Building, Modifications & Repairs' started by TheAirsoftTech, Apr 22, 2014.

  1. TheAirsoftTech

    TheAirsoftTech New Member

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    Since I couldn't edit in pics I had to make a new one..

    *The following guide is written using a Version 2 M4 as the example.*I'm writing this to replace the out of date, flawed guide in the stickies currently.



    As the title states, this a guide on how to “perfectly” shim your gearbox: for those of you who are new to teching let’s start with some Q&A.



    Q: What is shimming?

    A: Shimming, in this context, is the art of aligning the gear height to make the most optimal mesh with the gears it contacts all while making it as easy for the motor to cycle them as possible.



    Q: Why should I shim my gearbox?

    A: Proper shimming helps the gears to last longer as they are under less stress and are making proper contact with each other. This allows the motor to use the least amount of energy to cycle the gearbox.



    Q: Will I notice any noticeable improvements from proper shimming?

    A: Your gun will most likely sound much healthier (less grinding sound and less whine.) You may also notice a slight bump in ROF and trigger response due to the less resistance.



    Q: Is shimming difficult?

    A No, not really; of course you doubt me, but it really isn’t that difficult it is just time consuming. Take your time, and you’ll be fine.



    Q: How long will it take me to perfectly shim my gearbox?

    A: For the beginner about 2 hours. For the expert it will take only 25-30 minutes. I’ve gotten to the point where I can do it in 15 minutes on a V3 and 20 minutes on a V2.



    Q: What shims are best?

    A: Not really any are “best”, but I prefer SHS shims and G&P shims in unison. SHS Shims come in different shapes and colors making them easy to differentiate; G&P are just very precise and come in different packages to separate them from each other.



    OK! Now that we have some of the basic questions answered, let’s get into what you’ll need!



    You will need:

    1) Shims (Duh)

    2). Jb weld. (I’ll tell ‘why’ you later)

    3) Sand paper and a dremel. (I’ll tell you ‘why’ later)

    4) Solid steel bushings (SHS, ZCI, DA, Modify, Lonex, etc.) or GOOD bearings (Modify Ceramics or G&P Bearings- nothing else.)

    5) A flat head screw driver.

    6) Lots of time and patience.



    Before we move onto to the actual guide, remember, TAKE YOUR TIME. You don’t want to mess up; you won’t if you follow my guide to the ‘T’. Also, you NEED to screw down the gearbox every time you test shimming… Do it or you will fail.



    Alright, first things first, take everything out of your gearbox- everything. Grab the gears, cut-off lever, bushings/bearings, piston, and gearbox shell - this is what you’ll need to shim. Take everything else and set it aside. Take the gears, gearbox, bushings, bearings, and piston and wash them in warm soapy water if they’re not fresh out of the package. If they are you can skip the cleaning part. Dry them with a hair dryer when you’re done cleaning them.



    Next, check to see if your bushings are too risen up to prevent proper shimming. How to check for this is noted here- [ame]https://www.youtube.com/watch?v=5LC7YkqQibA&list=UU2pQqREGEaZKfo2RSeJkLzA[/ame].



    If they are, then you’re going to need to sand them down. Go do that now; sand them down until they’re almost perfectly flush with the gearbox shell. Note: more than likely you’ll only need to sand the bevel’s bushings and maybe the spur’s bushings if they are risen.



    Then, take your JB weld and weld your bushings/bearings down into the gearbox. Why do we do this? Because it prevents the bushings from rotating in the gearbox shell, this can wear down the gearbox shell to where it’s useless and causes shimming to be off. Some people will argue that this part isn’t necessary, but I, and countless other techs, do think it is necessary. To each his own, but I’ve tried not welding my bushings in and it resulted in tight shimming because they rose up constantly during cycling.



    Once you’re done welding your bushings down (and they have sat and cured for 24 hours) take your pistol grip, motor, and bevel gear; put the bevel gear in the upper half of the gearbox to where the flat portion of the bevel gear is facing down. Next, install the grip onto the half of the gearbox and install the motor. This is where we set the motor height; this method is known as the half shell method: ImageUploadedByAirsoft Society1398173803.045132.jpg


    Set the motor height to where the pinion gear is making the most possible contacting while not “over contacting”, being higher than necessary. Why do we set the pinion to be contacting like this? Think of it in the way of leverage: when you hold a baseball bat close to the bottom, where you’re supposed to hold it, you have much more force than you would if you were to hold it further up it. The pinion and bevel are the same way: if the pinion is too far up the bevel it will put more stress on the motor because it has to work harder to do the same thing it would if it were properly contacting.



    Once the motor height is set, take your gearbox and put the bevel in with NO shims. Then, take the gearbox and install it into the lower receiver. Now, lock it down with the pins and buffer tube screw like so: ImageUploadedByAirsoft Society1398173866.426527.jpg


    Then install your grip and motor. Finally, take your flat head and reach down through the cylinder window of the gearbox and test the side to side movement of the bevel gear. Now, take it apart and add shims to the top of the bevel gear to “shim out” this side to side movement to its lowest possible. In other words, add shims to the top of the bevel gear until it has the smallest amount of movement possible and it spins freely when you turn it with a flat head. This is how I shim the bevel gear; it essentially results in a flawless bevel to pinion shim. ImageUploadedByAirsoft Society1398173927.671923.jpg



    Now that the hardest part is out of the way, take your gearbox and put the bevel in it. Since we’ve only added shims to the top of the bevel gear, and since we more than likely still have some up and down movement, we need to add shims to the bottom of it to “shim out” the remaining space. Add shims until the bevel gear has .05mm of up and down movement (if you’re using bushings), and .08mm of up and down movement (if you’re using bearings). DO NOT add shims to the top to tighten it, this will screw up the pinion and bevel shimming.



    Next we’re going to shim the spur gear. Take your bevel out and insert your spur gear; don’t close the gearbox yet. Press down on the spur lightly and rotate it to see if it is contacting the bevel’s lower bushing/bearing; if it is, add shims until it doesn’t. Once that is done, insert the bevel and check to see how much contact the bevel teeth and spur teeth are making: if their teeth are making 50-90% contact that’s great, because that’s what you want. ImageUploadedByAirsoft Society1398174037.122854.jpg


    Now take out the bevel and insert the spur and add shims to the top of it to “shim out” the space. *Remember to leave .05mm-.08mm of side to side movement.* Once the spur is shimmed, spin it to make sure it is not contacting anything it shouldn’t (bushings/bearings, gearbox walls/floors, etc). Once the spur is shimmed, put the bevel in, screw down the gearbox, and check them both together to make sure the .05mm-.08mm of movement in them isn’t allowing the bevel’s ARL stops on the spur’s surface. If they are, re-arrange the spur’s shimming configuration to fix this.



    Last but not least, we come to the sector gear. Take out the bevel gear since it doesn’t directly affect the sector. At this point make sure that your cut off lever is installed. Put the sector gear in and screw down the gearbox. Check to see if the sector’s lower gear surface is rubbing down on the spur’s surface. If it is, add shims to the bottom of the sector gear to prevent the sector gear from rubbing on the spur gear. *Remember you want at least 50% contact between the teeth; no less or you risk gear teeth failure.* Usually it is pretty easy to get upwards of 80% contact between the spur and sector because of the way they’re designed. I usually get around 70-75% contact. ImageUploadedByAirsoft Society1398174095.794163.jpg


    Anyway, once you have a good amount of space between the two of them check to make sure the sector gear is clearing the cut off lever; if it’s not add shims to the bottom of the sector to allow it to clear the cut off. Make sure you don’t add too many, if you do the cut off cam on sector gear will not make proper contact with the cut off lever and can throw off your semi auto consistency not to mention wear down the cut off lever more quickly. After you make sure there is sufficient space between the cut off lever and sector, add shims to the top of sector to “shim out” the rest of the space. *Remember to leave .05mm-.08mm of side to side movement.*

    Once that is done add all three gears into the gearbox and screw it down. Test them by spinning them to make sure they spin freely and don’t sound like they’re rubbing up against each other. If you hear a loud, high-pitched squeal while spinning the gears that is called “zipping”; it isn’t your fault. It’s when the gear axle is slightly smaller than the bushing hole it fits into. So therefore, when you spin it fast it the gear axles rapidly vibrate while spinning, this results in a squeal y sound. To fix this, add a small amount of grease to the gear axles. This fixes the problem 99.5% of the time. If it doesn’t, you need new bushings. ImageUploadedByAirsoft Society1398174184.431092.jpg



    You think you’re done? Nope. Open the gearbox back up and install the piston to see how well it meshes and contacts with the sector gear. If all of the surface of the sector gear teeth contact the piston teeth, you’re good to go. Very, very, very rarely will the sector gear contact the piston teeth improperly. If it doesn’t, do your best to fix it. But keep in mind you need to maintain proper spur and cut off lever ImageUploadedByAirsoft Society1398174143.971639.jpg contact.





    -Orni



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    Last edited: Apr 22, 2014
    BOA_SP3CT3R likes this.
  2. TheAirsoftTech

    TheAirsoftTech New Member

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    10k Chracacter limit reached:

    There you have it, a guide on how to shim. I use this exact process when shimming all guns and I’ve built guns up to 50RPS and guns up to 600FPS that all still function today. If you’re using a Version 3 gearbox use the same steps but without dropping the gearbox into a receiver do to the fact V3’s have motor cages.. Some people use a volt meter to monitor the amperage they’re pulling to check shimming. I don’t do this because I’ve never found it necessary. To each his own, though.

    Anyway, if you have any questions or something to add please post them below. Enjoy:)





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    Last edited: Apr 22, 2014

  3. R34P3R

    R34P3R I'M THE CONDUCTOR OF THE POOP TRAIN! Supporting Member

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    Elizabethton
    My goodness, this is an awesome guide. I always had trouble with shimming back in the day, and had to learn through trial and error.

    Great job!
     
  4. Magpulman

    Magpulman Active Member

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    Stevenson
    Better guide than the one we've got right now IMO. I even learned some stuff that I needed help on.

    Right on man!
     
  5. MaxChairSoft

    MaxChairSoft New Member

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    Texas
    Id use a dab or 2 of super glue on the bushings instead of JB weld. It will hold them solid but can still be removed with a hammer and punch. JB weld will be a lot more permanent. Also id only ever glue them in if they dont snap and stay in on their own.
     
  6. TheAirsoftTech

    TheAirsoftTech New Member

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    JB weld is more permanent, but it can be removed easily if you know what you're doing. I've done it before. Since it's slow curing it's easier to work with and harder to mess up with than super glue.

    Like I said in my guide, I've tried not locking them down and it resulted in tight shimming because they rose up during cycling.

    Edit: Thanks magpulman and r34p3r!


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    Last edited: Apr 22, 2014
  7. Magpulman

    Magpulman Active Member

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    Stevenson
    To add to this a little bit. I was never really a huge believer in locking down the bushings for a long while. But then my gearbox jammed up one day and I dove in to see what the problem was.

    Well one of the bushings came up and got stuck at a weird angle which then affected the gear and in turn, jammed the whole operation.

    I glue them down now.
     
  8. TheAirsoftTech

    TheAirsoftTech New Member

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    Yup, exactly. I just weld them down for assurance.


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  9. JamSones

    JamSones New Member

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    Richmond
    I am still unsure about my shimming. I have a few questions I cant seem to find answers to.
    How much play in the gears is ok/ideal? Is it ok if the gears dont seem to move side to side at all but spin? How many cycles of the gears should be expected with a good shim job? What is worse, too tight or too loose of a shim job?
     
  10. Squirrel-Feathers

    Squirrel-Feathers New Member

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    Mtn Home
    How about a picture or tutorial on how to properly JB Weld the bushings? I've used two-part epoxy before since it's stronger than super glue but less permanent than JB Weld, but I would likely use way too much JB so wanna see how you would do it.

    Also, you say to sand the bushings down until they're close to the same height as the inside of the GB. As long as there is ample play for the gears to get a good shim job, is that really necessary?
     
  11. TheAirsoftTech

    TheAirsoftTech New Member

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    I might make a miniature guide on that but it's pretty simple, really. Just apply JB weld to the flanges on the gearbox and push the bushing/bearing in and wipe off the excess.

    I sand them till they're flush just to be safe; though it may not be needed I do it like that anyway.


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  12. Squirrel-Feathers

    Squirrel-Feathers New Member

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    Mtn Home
    So JB is the same as I do the epoxy. Got it. Thanks!
     
  13. TheAirsoftTech

    TheAirsoftTech New Member

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    Bump.

    I want other people to see and read this!

    L3gacy, where are you? ;)


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  14. averageairsofter

    averageairsofter New Member

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    Menifee
    3-4 cycles from spinning the sector gear is usually what I get on a good shim job with bushings, I cant comment on bearings because I havent used them in a build yet but I would assume a lot more. And too tight would be worse than too loose but neither one is good. Great guide orni, this really helped me, especially with making sure my pinion to bevel shimming was ideal as the other guide didnt really explain that. This should be in the stickies imo but we will see :)
     
  15. TheAirsoftTech

    TheAirsoftTech New Member

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    There is not a number of cycles you will have with perfectly shimmed gears. Sometimes it's 2-3 other times it's 10+.

    My DSG is 10+ easy, but my G36 is about 3-4. Both are perfectly shimmed. One just spins longer because it has nice bearings and the other is slower because it has solid bushings.


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  16. dkenny45

    dkenny45 Active Member

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    Atlanta
    WHERE WAS THIS WHEN I WAS TRYING TO SHIM.

    FYI- Ace hardwares shims are WAAAAY too thick.
     
  17. TheAirsoftTech

    TheAirsoftTech New Member

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    Bump.

    Come on guys, what do you think of this guide? Is it helpful? Is it clear?


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  18. averageairsofter

    averageairsofter New Member

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    Menifee
    Yes very helpful and clear, this has helped me a lot with shimming and I have no idea why its not getting a lot more attention. STICKY THIS MODS!!!
     
  19. yattmaster

    yattmaster New Member

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    Sandy
    So I've been shimming my gearbox and the gears just inside the gearbox sound great, I also shimmed bevel to pinion but when I put the motor on it sounds like random bits of crap are flying through the gearbox and hitting all over and playing pinball in the gears, the gears and shell are perfectly clean(besides grease) but what can I do? I've tried switching out the sector gear, shimming all possible angles on the bevel to pinion(pretty tight, only a few possible) but it still gets that sound. Not a screechy sound of gears grinding but it's like I dropped some bits of metal in there.

    Edit: just put it in with just the bevel and pinion is and that is where the noise is coming from.

    Double edit: I switched the motor from an SHS to a ZCI and it sounds a lot better but the noise it still present, I can't use the ZCI motor for now because the pinion is extremely loose and I don't have an hex wrench small enough to tighten it.

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    Last edited: May 8, 2014
  20. Nyt

    Nyt New Member

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    Smiths Station (right between Columbus Georgia and Auburn Alabama)
    Subbing here so I can find this awesome guide for when I upgrade my p90. Thanks bro.


    Nyt óÔÔò /)*(\