Equipping the SB Sport with bevel gear Diffs can be a good idea if you want to go brushless or convert your SB into a 4wd racing truck (did the first one, want to do the second one in one or two months)
First of all, you'll need an extreme durable drivetrain for the bigger torque of BL and rotational mass of the diff isn't such an issue since there's plenty of power. Ball diffs are not up for that task, especially when it comes to BL AND big tires, they won't do their job a long time.
A second "bonus" comes into play: by using gear diffs, your internal ratio goes up from 2,75:1 (44/16) to 3,18:1 (35/11) - that will compensate for bigger stadium Truck tires so you can just gear like a normal buggy
You basically have two options:
either go with the plastic gear diffs of the STR-4 Rally or throw in the diffs of Academy's RT-4 GP Gas Truck (guess what diffs I did choose
So here's the complete list of parts:
complete diff assembly
You can fill them with 10000cps and above silicone oil to get an LSD effect.
2 Pairs of joint cups that mount on the diff outputs and take the driveshafts. This is the steel version of the aluminium layshaft cups - so if you feel that they wear a bit too fast, just get RG-039
STT-07 und STT-08
These are the pinion gears and shaft for the front and rear gearbox. They are machined from steel. There's also a cheaper version with a black shaft and zinc pinion gear that carries number ST-17 & ST-18 or so...
RG-054 & RG-055
Some shims that might come in handy during the conversion. to shim the diff correctly, just use at least 2 10x13mm shims at the outdrive that is opposite to the crown gear.
BUT REMEMBER: While shimming metal diff gears, you'll have to be much more cautions because if you set the mesh too close, they'll destroy your bearings (whereas plastic gears would just break in a little more) Set it to loose and they wear. You have the right amount of mesh, when the gears turn smoothly and you hear a little "click-click" when holding the pinion gear and wiggling the crown gear.
You might also want to get some M2,5x14mm screws since the stock ones, that secure the crown to the diff case stand out a little with their FH and catch the gearbox. (the angle of the flathead is bigger making it hard to get a countersunk hole on the diff gear that cleanly accepts the screw - so just throw them away)
Once you're done, pack some teflon based grease into the gearbox and install the upper half. Be sure to check, if the gears are running smoothly when everything is secured with screws (important!)
If you feel the gears are binding, just add a 3rd shim opposite to the crown.
When driving, remember that these gears are made from steel and zinc; a comination, that is very wear resistant and has a hard surface, so break in will probably take a lot longer than your plastic gears.
Needles to say, that you possibly will never have to take apart a singe gear box any more, because you have the most durable drivetrain in relation to vehicle weight... I love, how academy shares parts between models.
Additionally, install an enhanced top deck that replaces the stock upper(! not the lower one!) deck
...and there you go.