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Home » RC Build Blog - HPI Savage XS SS Build » HPI Savage XS SS Build – Part 4 – Rear Differential

HPI Savage XS SS Build – Part 4 – Rear Differential

HPI Savage XS SS Build
HPI Savage XS SS Build

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.

HPI Savage XS SS Build - Part 57
HPI Savage XS SS Build – Part 57
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.

HPI Savage XS SS Build - Part 58
HPI Savage XS SS Build – Part 58
Take the diff outdrive and slide it into the gear as shown.

HPI Savage XS SS Build - Part 59
HPI Savage XS SS Build – Part 59
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.

HPI Savage XS SS Build - Part 60
HPI Savage XS SS Build – Part 60
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.

HPI Savage XS SS Build - Part 61
HPI Savage XS SS Build – Part 61
Slide the 2nd diff outdrive into the diff housing.

HPI Savage XS SS Build - Part 62
HPI Savage XS SS Build – Part 62
As we did with the diff gear, slide the o-ring and shim into place.

HPI Savage XS SS Build - Part 63
HPI Savage XS SS Build – Part 63
Make sure the o-ring seats all the way down into the notch.

HPI Savage XS SS Build - Part 64
HPI Savage XS SS Build – Part 64
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…

HPI Savage XS SS Build - Part 65
HPI Savage XS SS Build – Part 65
…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.

HPI Savage XS SS Build - Part 66
HPI Savage XS SS Build – Part 66
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.

HPI Savage XS SS Build - Part 67
HPI Savage XS SS Build – Part 67
Gather the parts shown and build two satellite gear assemblies. Note the location of the shims and the direction of the gears.

HPI Savage XS SS Build - Part 68
HPI Savage XS SS Build – Part 68
Slide one of the satellite assemblies into place as shown. Make sure the flat spot on the shaft is pointing up.

HPI Savage XS SS Build - Part 69
HPI Savage XS SS Build – Part 69
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.

HPI Savage XS SS Build - Part 70
HPI Savage XS SS Build – Part 70
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.

HPI Savage XS SS Build - Part 71
HPI Savage XS SS Build – Part 71
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.

HPI Savage XS SS Build - Part 72
HPI Savage XS SS Build – Part 72
Using the dainty Phillips screws, secure the two assemblies together. This is where your quality Phillips screwdriver is almost a necessity.

HPI Savage XS SS Build - Part 73
HPI Savage XS SS Build – Part 73
Slide one shim and one bearing over the outdrive. Make sure it drops all the way to the face of the gear.

HPI Savage XS SS Build - Part 74
HPI Savage XS SS Build – Part 74
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.
Back to the HPI Savage XS SS Build List

About Tony Phalen

Tony Phalen - As an avid RC enthusiast, Tony has been building, bashing and racing RC Cars for over 25 years. He has raced everything from 1:18th scale trucks to 1:5 scale motorcycles and everything in-between. He's also worked on both sides of the industry fence; working at and with many major manufacturers (as well as being a sponsored driver) to working for a high-profile industry magazine. During this time he has learned many tricks, tips and techniques and has transferred that knowledge to CompetitionX - the most informative RC website on the internet!

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