My first experience with shimming is tied indelibly to my first time taking apart a gearbox years ago. Like a lot of first-timers, I unknowingly unleashed a storm of flying parts when I foolishly tried to open the gearbox halves without tripping the anti-reversal latch first or attempting to hold the spring down. And like a lot of first-timers I had little clue what the little round shims (then incorrectly presumed by me as “washers”) were for. So once I figured out which gear went where, and how to squeeze the spring into the one half, I slapped the whole apparatus together and upon hooking up a battery was dismayed to find my gun would not turn over at all.
The above experience is a pretty common one for a lot of guys who have never taken apart a gearbox and have little understanding of the importance of shims. You will often hear in Airsoft circles that “shimming is an art form.” I don’t know that it is an art form so much as it takes an intuitive hand and a careful eye for it – plus a good helping of patience. There are good techniques and bad techniques. Quality shimming can greatly improve the efficiency of your gearbox, as well as rectify a lot of common problems.
Nowadays, there are a good many guides available on the internet showing you how to start about conducting a proper shimming job on your gearbox. The scope of this article is thus not to show you how to shim your gearbox, but to give you a few tips and tricks I’ve learned over the years from shimming hundreds of these things now.
Before commencing shimming, start with the bushings or bearings….
This is basically the foundation of your shimming job. The quality and fitment of the bushings will actually dictate a lot about how many or how few shims you will need to use. I greatly prefer bushings or bearings that fit flush with the inside of the gearbox shell. The reason for this is some manufacturers will have tighter internal tolerances than others. On really tight setups, you will sometimes find yourself hard pressed to get any shims in there at all. Raised bushings or bearings will of course take up more room between the gears, and thus require less shims – which is probably the intention of the manufacturer who produces them. Shimming is a bit of a time intensive process, and is the one aspect of gearbox tuning that most manufacturers spend the least amount of time on.
Solid bushings that are raised can generally be sanded down to where they fit flush. Take care that you do not sand too much as to weaken or eradicate the shoulder of the bushing, otherwise it may pass through the gearbox half. Some gearboxes have really shallow shoulders for bushings or bearings, and some have deep ones.
I do not recommend sanding down bearings because you will likely contaminate the bearings, or take down the outer ring too much to where the inner ring no longer locks in place.
It should not be assumed that all raised bushings need to be sanded down either. The biggest dictate is whether or not you can get any shims in there and still have freely spinning gears.
Unless the gearbox has a really tight fit with the bushings or bearings, I prefer to glue them in before starting any shimming job. I use super-glue for this purpose as it keeps a fairly decent grip on the bushing / bearing, but not too strong that it can’t be tapped out with the correct bushing punch. Any residual dried glue can be removed with a razor blade. Take special care not to get any into the bushing holes, or the bearings themselves. Just a tiny dab on the shoulders of the bushing or bearing is all that is needed typically. Never use epoxies, JB Weld, RTV, etc. because you will want to retain the ability to remove the bushing / bearing at a future date.
The main reason I will glue a bushing in is because some gearboxes can have a really sloppy fit and the bushing or bearing will fall out frequently. This can be a real pain if you are trying to shim your gearbox and every time you remove the shell the bushings / bearings fall out and dump your shims with it. Additionally, you may observe that a non-glued bushing / bearing will sometimes wander back and forth or get inadvertently depressed by your fingers during testing, giving you a “false reading” so to speak.
Throw out bad bushings and bearings….
A bad bushing or bearing will usually manifest itself in fairly obvious ways. Bushings, for example, can sometimes have off-center holes, whether through manufacturing defect or from a gear auguring it out. Bearings can become deformed, seized, or fall apart (if you see little ball bearings floating around inside your gearbox, take a look at your bearings.) The quality of the bushings or bearings will also dictate the likelihood of failure too. Bushings that are made of soft grade metals such as brass or bronze, or plastics tend to go bad more than those made of solid steel or higher grade metals. You get what you pay for, in other words.
Buy the highest quality shims you can….
There are good shims, and there are bad shims. I try to aim for the highest quality shims I can find. I do not personally like the “oversized” diameter shims myself as I have found they can interfere with cutoff levers or tappet plates depending on the variances within the gearbox. They are fine to use on the spur gear, but that is as far as I would use them. My advice is to stick with the standard diameter shims. The stiffer and stronger the shim the better too. Avoid shims that easily bend or deform.
Throw out bad shims….
As John Goodman said in Arachnophobia: “Take out bad wood, put in good wood.”
You may be tempted to recycle your old shims when reshimming your gearbox to save some time and money. Most of the time I don’t recommend it unless they are demonstrated to be flawless. Cheap guns in particular tend to have poor quality shims. You can often run across shims that have off-center holes punched in them, or deformed concave surfaces, or rough burred edges and the like. Shims – even high end shims – are fairly cheap. Buy a fresh set and start with those as your base. The idea is to eliminate as many possible outside variances as you can by starting with a known quantity.
Clean your gears….
The main aspect of reshimming is to ensure your gears spin freely and smoothly without rubbing on things they shouldn’t. Dirty, overly greasy gears will cause you to have a harder time determining whether or not you’ve done a good shim job. Your significant other may not appreciate it, but I like to wash my gears off in the sink with soap and water, taking care to thoroughly dry them afterward.
One of the big reasons to clean your gears is if you have ever had a stripped piston. Often times a bit of the piston will embed itself or get stuck in the teeth of the gears, and if you are paying attention while shimming, you may feel the gears bind up or get tight on this spot. Sometimes the only way to get it out of there is with a dental pick. Cleaning your gears also reveals damage, such as hairline cracks, that may be otherwise hidden beneath a layer of old gunk. Again, the idea is to start with a known quantity first.
Adjusting the bevel gear to motor pinion gear is important….
This will partially dictate (in conjunction with motor height) how much whine your gearbox will make. Additionally, if you noticed that the otherwise normally sharp edges of your bevel gear or motor pinion gear are rolled, jagged, or a little bunged up, this is an indication that you have bad meshing going on. The incorrect amount of shims on the flat side of the bevel gear can also cause you to have issues with your motor height adjustment; the motor pinion gear may bind too much against it.
As is the sector gear….
On a standard center-rack mounted piston, I try to get the sector gear to sit as much as possible in the middle of the gearbox to ensure proper meshing with the piston rack. For piston designs that use off-set racks, you would set the sector gear to best mesh with those setups. Obviously it’s not always possible to attain perfectly centered sector gears to the piston rack. What you don’t want is the sector gear to be sitting partially off the rack of the piston. Granted, this only happens in extreme cases though.
The “right” amount of shims can vary….
No two gearboxes are alike. Some will require many shims, while some very few. The trick is to find the golden zone for the correct amount of shims. Too many shims is bad, as is too few. Over time you will develop a feel for which shims you should start off with and apply where. You also need to pay attention to the sound the gears are making. If you hear what seems to be audible dragging or grinding noises when you spin the gears, it probably is. Add shims as necessary until you get a smooth and freely spinning set of gears. I like to see at least 10 complete rotations out of the sector gear before it slows down or stops when I spin it with my finger. Bushings will have more drag than bearings will, so you generally experience more rotations out of bearings when testing.
Modifying the sector and spur gear spacer sleeves….
In extreme cases where you may have trouble mounting a shim, sometimes you have to shave down the length of the sleeves (usually a brass tube, but sometimes a steel one) that rides the center shaft on the sector or spur gears. Some folks have argued that it is not necessary to do this – rather go without any shims on the tube side and let it rest against the bushing / bearing. While you can do this, I still prefer to get a shim in there for a few reasons:
- If you have soft bushings, such as bronze, brass, or plastic, the effect of the tube riding against the bushing will eventually wear a groove in it (especially if the tube is made of steel). Over extensive periods of time, this will cause your once good shimming to become loose. A shim will act as a ‘cushion’ of sorts, and generally prevent this.
- There is speculation with regard to bearings that a shim should always be placed against them to transfer energy and heat across the shim rather than directly into the bearing. I’m no scientist, so I can only speculate if this is true or not. Personal experience has shown that bearings (in the case of open style with visible ball bearings) seem to last longer if they have a shim between them and the gear though.
A little grease goes a long way….
There is nothing more frustrating than getting a perfect shim job, only to pull your gears out to grease them and have all your shims fall off and scatter. To prevent this, put a little grease on the shims themselves to help them stick to the gears. Greasing the gear teeth typically comes last of all – after you have finished shimming.
Always, ALWAYS, screw your gearbox together and check your shimming….
This is probably the biggest mistake most who do shimming the first few times make. You may find that you have freely spinning and awesome sounding gears when you have the gearbox halves held together with your hands. But once you apply actual tension with the gearbox screws, you can often times have too many shims installed, and thus have a gearbox setup that’s working harder than it needs to or will whine worse than a two-year old kid who broke his candy cane at Christmas.
Cheap guns often times take longer to shim than higher quality guns….
By now you will have noticed a theme: cheap is bad. On an average, it takes me longer to reshim a cheaper gun than a higher quality gun. Most of this is due to fitment issues, tolerances, and whether it was Friday at the Factory.
As an example: I recently worked on a JG S-System here. The gears were almost on the verge of seizure, but still turned, albeit very tightly. My first assumption was “too many shims.” Upon opening the gearbox, I discovered there wasn’t a single shim at all on any of the gears! The bushings were flush with the inner gearbox halves, so that wasn’t the problem. After some inspection and measurements, I came to the conclusion that the internal specs of the gearbox were slightly undersized compared to the “norm” (whatever that is, lol.)
I could safely add shims and attain free spinning to the spur and bevel gear, but the sector gear itself was being very problematic. Adding the correct number of shim (using the thinnest I had available) to get it to clear the spur gear caused it to drag on the cutoff lever. Adding shims to get it to clear that piece caused the sector chip to drag against the inside of the gearbox shell (this without any shims having yet been added to the sector chip side). In the end, I wound up having to file down the sector chip a tiny bit so it cleared the shell, and also had to file down the brass tube that fits over the sector gear so I could apply a shim.
Like I said, no two gearboxes are exactly alike. You do enough of this and you will inevitably run into issues with certain brands and designs.
Some gearboxes are easy to shim, some are hard….
A gearbox like those used in a SAW or M60 is a breeze to shim because you have superb access to the gears from the underside. Version 3 gearboxes are also fairly easy because they allow access behind the trigger. Version 2’s are a little harder because they have slightly more closed off shell. There are other types and setups that are more difficult or easy depending on design
I could go on… but….
There is certainly a great deal more that could be said in regards to shimming, not to mention other fundamentals that I have not completely covered. It would obviously turn into an exhaustive and lengthy treatise, however 😛