Building the differential is pretty straightforward. A very good diff action can be achieved with just a little care.
First, sand the diff rings on 600 grade wet and dry on a flat surface. I use an old hub to hold the diff ring while sanding it so I don't sand my fingers!
The diff rings are pressed from steel so are not completely flat. Sand both sides of the diff ring slightly and you will find that one side has the outside sanded but not the inside. This is the side of the ring that you want facing the spur gear. Sand this side to both roughen and flatten the ring. I sand the ring until it is rough almost to the inside as can be seen in the pictures below.
The 12x comes with a 95 tooth spur gear. I normally use a 10.5 for both stock and sports modified racing and this requires a smaller spur in order to get the correct gear ratios. I am currently using the Hot Bodies 88 tooth delrin spur which I find to be very accurate. I also use Edit ceramic diff balls. They stay round and the diff requires less maintenance, though they can slip a little more than steel diff balls.
Below are all the parts ready to build the differential.
I add a 0.01 inch shim to the axle first. This stops the outer edge of the bearing from catching on the axle. Then I smear a small amount of the supplied diff grease where the ring is going to go, purely to hold the ring while assembling the diff.
I then put the ring on making sure the sanded side is facing the spur. I then put the bearing onto the axle. I put the smallest dab of the supplied diff grease on each diff ball on each side, barely enough to see it, and then put the spur onto the axle. The spur gear has a lip on the inner hole which means it will fit on the bearing easily one way only, so put it on this way round rather than forcing it on the other way.
Then put the bearings into the outer hub, add a smear of diff grease where the ring goes and put the ring on, again making sure the roughened side is facing the spur. Then put this assembly onto the axle. Add the thrust cone and spring washer, making sure the inside of the spring washer is facing the outer edge of the axle. Then screw on the plastic nut enough to prevent any movement of the parts but no more. The differential should be adjusted properly later on when the car is built. This is the only part of the build where an imperial tool is required, with the nut being 11/32 inch.
Enough rear axle shims have been supplied to space the axle out for zero offset rear wheels. Most of the wheels in use currently have a 2 or 3mm offeset, so I would recommend buying another two packets of the shims as seen below.
It is not possible to say exactly how many shims should be on each side of the axle at this point. It can only be adjusted properly with the wheels on the car, so as a starting point put 8 shims on the spur side and two on the clamping hub side.
Put the axle on the car, slide the clamping hub onto the axle with the wider side of the hub to the inside, and then tighten it so there is a tiny amount of movement of the axle from side to side. If you can feel the axle move from side to side but not see it, it will be about right.