HPI Savage XS SS Build – Part 4 – Rear Differential

The Build – Part 4
The build of the rear differential is exactly the same as the front. I would have liked to build both at the same time but because the rear diff parts were 2-bags deep in another location, I opted to just build it per the instructions.

Build Notes:
Since we’ll be working with the differential, you’ll obviously be dealing with differential oil. Make sure your area is nice and clean and have plenty of rags handy for clean-up.

As I’m sure you’ve learned from the front diff, a high-quality small Phillips screwdriver is essential in screwing the screws in. Find one.

The first thing we’ll want to do is take the parts shown and move them to the side. These are for the next step (Rear Gearbox). Note the input shaft and gear; the longer shaft and taller gear are the ones you’ll set aside.

Take the diff outdrive and slide it into the gear as shown.

Place the black o-ring and shim over the outdrive shaft and slide the pin through the hole. In this picture the o-ring is already installed. Make sure it seats all the way down into the notch.

Place the green gasket onto the gear face as shown. Make sure you line the holes up. If the gasket does not stay, a dab of black grease will keep it in place. Align the notch in the back of the sun gear with the pin on the shaft and press into place. Again, if it does not stay, a dab of grease will help.

Slide the 2nd diff outdrive into the diff housing.

As we did with the diff gear, slide the o-ring and shim into place.

Make sure the o-ring seats all the way down into the notch.

Take a thin tool (one of the ones HPI supplies will work) and slide it into the hole in the diff housing. You will want to rotate the diff outdrive until the hole lines up with the tool…

…then slide the pin through the hole in the housing and into the hole in the outdrive. It is possible to do insert it by dropping the pin in from the top but this was was much easier. Align the notch in the back of the sun gear with the pin and drop into place.

Insert the set screw into the diff housing and tighten it down until it is flush. Rotate the outdrive to verify that you have not tightened it too much…you do not want the set screw to hit the sun gear.

Gather the parts shown and build two satellite gear assemblies. Note the location of the shims and the direction of the gears.

Slide one of the satellite assemblies into place as shown. Make sure the flat spot on the shaft is pointing up.

Slide the second satellite gear assembly into place as shown, making sure the flat spot on the shaft is pointing down. You want the flats spots to be resting against each other.

Fill the differential about 95% up with diff fluid. I used 5000wt fluid in the rear. Rotate the diff outdrive to help circulate the fluid through the entire housing. Re-fill if you need to.

Now take your diff gear assembly and turn it over onto the diff housing assembly. You want to make sure the screw holes are aligned; neither the gear nor housing are keyed to make this easier. Just take your time here.

Using the dainty Phillips screws, secure the two assemblies together. This is where your quality Phillips screwdriver is almost a necessity.

Slide one shim and one bearing over the outdrive. Make sure it drops all the way to the face of the gear.

Add two shims and a bearing to the housing side of the diff. Make sure they seat all the way against the housing. Mine didn’t so I had to do a little sanding of the housing ‘nub’.

Part 5 – Assembling the Rear Gearbox.
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