Buildup -- Step 1: Differentials

The Mad Force kit includes differentials that are already 100% pre-assembled. However, they're not assembled well. The Inferno diffs it uses are meant to use four spider gears, but Kyosho got extremely cheap here and decided to use only two apiece! We're going to remedy that, fast.

Pick out one of the axle tube/link assemblies; it doesn't matter which you start with. The first thing you want to do is remove the two self-tapping screws and seven machine screws (all phillips button heads) connecting the diff case/axle tube halves. This will allow you to split the halves apart. The input yoke & shaft stub will fall out with its dual ball bearings and pinion gear (fig. 1). The pinion will be lubricated with white grease, so be prepared. Remove the diff assembly from the axle tubes and note the orientation (which side the ring gear is on). If you put this back the wrong way, you'll end up with one or the other of your axles spinning the wrong way under power -- not a good thing.

Fig 1	Fig 2	Fig 3

Next, go ahead and disassemble the diff unit itself by removing the four screws and separating the ring gear from the diff cup. You can see in fig. 2 how only two spider gears are included. You can add a second set by purchasing Kyosho part #'s BS107 and IF102 (fig. 3).

The factory puts a dab of automotive-style joint grease on the gears to lube them. This puts next to no resistance on the gears, allowing the diff to spin freely and "unload" (letting one wheel turn much faster than the other on a given axle) under normal driving conditions. To reduce this effect, introducing some limited-slip action to "tighten" the diff, you can use silicone diff oil as commonly used in 1/8th scale buggies (fig. 4).

Fig 4	Fig 5	Fig 6

Exactly what weights of oil you'll want to use will depend upon your driving style and the type of terrain you'll be tackling. I like to use throttle to help the rear of a vehicle break loose in on-power turns for more steering, so I decided to use heavier oil in the rear than the front. Specifically, for starters, I went with 3000wt up front, 7000wt for the back. This can always be changed at a later date. Remove spider gear shaft (with the two gears and shims), wipe off the existing grease from all of the gears, and pour enough of your chosen silicone oil into the diff cup to fill it about halfway. Then drop your spider shaft/gear/shim assembly back into place, taking care to keep the notch side up (fig. 5). Now make another shaft/gear/shim kabob using your new parts bags and drop this in with the shaft at a 90 degree angle, taking care to orient this shaft so that the notch faces down (fig. 6). The two notched shafts will gently interlock when pushed together (fig. 7).

Fig 7	Fig 8	Fig 9

Pour more diff oil into the cup (fig. 8) until the level is about 3-4mm from the top. The idea is to get as much oil into the cup as possible without having it overflow and make a mess when you re-attach the ring gear assembly (fig. 9).

Fig 10	Fig 11

Tighten the four screws through the ring gear and into the diff cup firmly, but take care not to strip the heads. Now re-insert the whole diff assembly into one of the housing halves, facing the same direction as it was before, set the input shaft/pinion/bearings assembly into place (fig. 10), and reattach the two halves with their 9 screws. I'd advise barely tightening all of the screws at first, and then finishing tightening them in a star pattern to ensure evenness, as you would when installing a wheel on a real car. Don't overtighten these screws -- they're threaded into plastic and can strip the holes.

Now repeat the whole process above for your other axle. When you're done, what you're left with (fig. 11) looks exactly like what you started with. It just happens to be significantly stronger, and may go a long way towards improving your truck's initial performance.