HPI Tutorial – How to Maintain your Savage X Differentials
The Savage X, like most other 1/8th scale nitro buggies and trucks,
uses gear differentials front and rear. The differentials allow the
outside wheels to rotate faster than the inside wheels in turns,
improving the cornering performance of the truck. The differentials
in the Savage X have a hard life; they transmit all the torque from
the centre driveshafts, through 90 degrees to the driveshafts that
turn the wheels of the truck. They are durable but it may be
necessary to perform periodic maintenance on them. Inside the
differentials there are a number of gears. These gears may wear and
cause the differential to slip under power. This may result in a
loss of drive through the differential and thus a loss of drive to
the wheels. One of the new features of the Savage X is the ‘Easy
Access Differentials’ which will make diff maintenance very easy.
In this tutorial we show you step by step the procedure to follow
to maintain the differentials in your Savage X to keep them in top
condition. These are the parts that you may need when maintaining
the differential:- You will also need some Heavy Duty Grease,
HPI part number Z164 to re-grease your differential when
reassembling your truck after cleaning. Also you will need some
lighter grease, HPI part number Z160 to re-grease the front and
rear gearbox gears. In this tutorial we show you how to remove the
rear differential from a Savage X. Removal of the front
differential is the same as removal of the rear differential.
Step 1 – Removing the differential
First remove the 2 screws from the underside of the truck. Then remove the 4 screws that attach the
upper part of the gearbox housing. For the removal of the rear diff you also
need to remove the 2 screws that hold the rear turnbuckle links to
the gearbox casing, so you can completely remove the casing.
Now you can remove the upper part of the
rear gearbox housing and the alloy brace from the truck. With the gearbox housing and the alloy brace
removed you can now remove the differential from the truck.
Now with the differential removed from the
truck you can start the task of dismantling the differential.
Step 2 – Dismantling the differential
The first step when dismantling the differential is to remove the 4
screws that hold the 43-tooth bevel gear to the differential
housing. With these 4 screws removed you can lift off
the 43-tooth bevel gear from the differential housing. Now you can remove the large bevel gear from
the differential and then the small bevel gears from inside the
differential. These small bevel gears should lift out with the
shafts. Also you can then remove the large bevel gear from the diff
housing. With the bevel gears removed you need to
remove the pins that pass through the output shafts of the
differential. The one on the side of the diff with the 43-tooth
gear should simply push out. On the other side of the differential
you need to remove the grub screw in the plastic housing so that
you can push the pin out through the hole in the diff housing.
With these pins removed it is now possible
to remove the differential output shafts. This will allow the
bearings and rubber o-rings to be removed from the differential
parts. Now you have disassembled your differential.
Step 3 – Cleaning and inspecting the Differential parts
With the differential in parts form it is easy to clean. Place the
parts from the inside of the differential in a suitable container
and clean them with HPI Nitro Cleaner or a similar cleaner.
Let the parts soak for a short while then
lift them out of the cleaner. They should be free of dirt and
grease. With the parts cleaned you are now ready to inspect the
gears for wear and damage. In the pictures below you can see the
difference between a new gear (on the left) and a worn out gear.
If you see that your gears are worn then the
will need replacing. Worn gears will cause the differential to slip
resulting in a loss of drive. You should also inspect the
differential housing for signs of wear. Wear in the differential
housing normally shows as an indentation where the cross-pins push
against the plastic housing. This wear will allow the cross-pins to
move and the bevel gears to become badly meshed. This may cause the
bevel gears to become damaged. You may also see wear in the diff case where
the large bevel gear sits. This wear will cause the mesh of the
gears in the diff to become loose and may cause the gears to strip
if used. The picture below show a differential housing that has
worn (right), next to a new differential housing (left). If your
differential housing is worn we recommend that you replace it as it
could cause the bevel gears to fail. With the differential parts cleaned, and
worn parts replaced it is now time to rebuild the differential.
Step 4 – Rebuild the Differential
If you have a replacement differential housing we recommend that
you replace the gasket on the 43-tooth bevel gear. The old gasket
should simply peel off. The new gasket is included in the 85044
spare parts set. With the old gasket removed peel off the backing
on the new one and stick it to the 43-tooth bevel gear. The next step is to attach the differential
output shaft to the 43-tooth bevel gear and the differential
housing. The differential output shafts push though the housings
and are held in place with a pin. This is easy on the side with the
43-tooth bevel gear as the pin simply slides into place. On the
side with the plastic housing you need to push the pin through the
hole in the housing and then screw a grub screw into the hole.
When building these parts don’t forget to
put the rubber o-rings in place. Now you need to apply grease to
the back of the large bevel gear that goes inside the diff. Grease
in this area will reduce the friction between the gear and the diff
case or 43 tooth bevel gear. This will reduce wear of the diff case
and the 43 tooth bevel gear. You should use Z164 Heavy Duty Grease
on these gears. With grease applied to the back of the gears
you can now slide the larger diff bevel gears onto the pins that
pass through the diff output shafts. You need to put some more of
the Z164 Heavy Duty Grease onto the front of these gears. Now you need to assemble the small bevel
gears and the cross shafts. Place 2 small gears onto one shaft and
drop it into the diff housing, ensuring that the flat on the shaft
is facing upwards. Now assemble the other 2 small gears onto the
other shaft and drop them into the differential housing, ensuring
that the flat on the shaft is facing downwards. With these small bevel gears assembled in
the diff housing place plenty of Z164 heavy-duty grease on them.
With grease applied to the bevel gears you
need to place the side of the differential with the 43-tooth gear
over the differential housing and screw the two together. Now your
differential is rebuild and ready to be fitted to your Savage X.
Step 5 – Refitting the Differential
Refitting the differential to the Savage X is a simple task, due to
the easy access differentials. Before replacing the differential
into your Savage X you need to add some grease to the 43T bevel
gear. We recommend HPI grease part number Z160 for this. The differential should now drop into place.
Remember to insert the driveshafts into the differential outputs
when inserting the diff. When the differential is in place you need
to replace the top piece of the gearbox, remembering to replace the
alloy brace. With these parts in place you need to
replace the screws that hold the gearbox in place. Now you have
refitted the differential and your truck is ready to run again.