Problem: long shaft needed, but short shaft installed. Object: swap the metal gear and bushing from the short shaft to the long shaft.
It took me a while to realize, Heng long gearboxes basically have two length of output shafts (the part your sprocket bolts onto). The output shafts are either 38 mm long or 48 mm, and if you have a gearbox with the long shaft where the short one is needed or vice versa, the gearboxes do not interchange. However, the shafts do interchange, and not much work is required to do this either! And there is no need to spend any more money on replacement output shafts (Heng long store, on Ali Express has them, they are about $20-25 a pair with shipping).
OK suppose you bought a tank that came with plastic gears and you buy a gearbox set with metal gears to upgrade. When the gearboxes arrive, you realize you have ordered ones with the wrong length output shafts for your tank. So your options now are, Send them back, buy the right shafts wait for them to arrive and swap them? Well why do that when maybe the fastest and easiest way, just swap your wrong shafts for the right ones, that you already have from your old gearbox that you don't want anymore anyway, all without spending a dime?!
Facing this problem I realized the last option was best, since the shafts can be swapped, and without much work either. All you need is a vise, (or even a make shift piece of wood with holes in it), a hammer (rubber or leather might be better - although you can just pad the shaft with a piece of wood while you pound on it) and a punch.
Disassembling the Gearbox
So, here's what you do. (If you are swapping the metal gear for the plastic one, that's described after this part) I loosen, but not remove, all 4 of the screws first on the gearbox, (just in case there is one tough one that doesn't come out easily). If you have a screw that won't turn, first try a different screwdriver, the heads are different sometimes. If it still won't turn, there's a trick: clamp the shaft of the screwdriver in vise grips, closest to the end (nearest the screw itself). Press down hard on the end of screwdriver, now carefully turn the screwdriver applying pressure on the end of the vise grips. If one of the spacer bushings (I don't know what else to call them) on the gearbox is turning while you try to unscrew it, just attack the screw on the other side instead, and hold the spacer bushing with vise grips (I pad the spacer bushing first with some tape or piece of cloth).
Now all your 4 screws are loose, just turn the gearbox on its side, remove all the screws. Lift off the metal plate part of the gearbox, try and disturb as few of the gears as you have to, and remove the output shaft. (Best to do one gearbox at a time - in case you need a reassembly model - more then once I've mixed up the gears somehow, and had to look at another gear box to figure out where they all go).
Swapping the output shaft gear
OK let's assume the whole reason you are doing this is to upgrade from plastic gears to metal. So, you want to swap the metal gear, from the un-needed shaft, onto the shaft you want to use. This is not too hard, since the gear is a light press fit. The splines on the output shaft are what hold the gear in place, and not much force is required to pound off the gear.
So just take your plastic gear shaft, spray a little bit of any kind of lubricant like WD40, oil or anything really, to help when you ease the gear off. Now place the output shaft in the vise but do not clamp it at all. Just draw up the jaws so the shaft hangs free, and the jaws support the bottom of the gear. Now take a hammer, tap on the shaft until its flush with the gear, then use the punch and hammer, and tap out your shaft. Remove the bearing.
Repeat this process with the metal gear shaft. Before you tap the gear off, note which way the gear faces, and also, which way the brass bushing faces as well.
In the photo above, the long shaft has had its plastic gear removed and swapped with the metal gear and bushing, pressed out from the short shaft and pressed ON to the long one. Long shaft is the one on the right.
If you like, since everything is apart, now is a good time to put a drop of your favorite grease or oil inside the cupped bushing of the output shaft and anything else you want to grease.
Remove the output shaft and replace it with the one you want to use. Reassemble, and tighten all the screws (even the ones you didn't even touch before, since they are often loose, even on new gearboxes ).
I prefer not to use loctite on these screws. Th screws are soft and easy to strip out, and if you ever have to take the screws out again loctite might make it that much harder. If I had a problem with the screws loosening I'd just use lock washes, loctite only as a last resort for screws that loosen repeatedly.
Ok now assuming you have the gear you want on the shaft you want, all that's left is a function check to double check that the gears will turn and not bind.
Spin the gears by hand, or just hook up power and run them if you prefer, while watching each gear's movement. Sometimes something will be out of place after reassembly and bind things, if that has happened you want to know that now when it's easy to fix not when it's back in the tank of course.
...have thought about doing something like that...but I usually just pop for another gearbox...mostly because... I know I can use the improper one in another tank
