San Diego Gear & Axle

TIP #1 Cleaning:
To properly clean a differential you must get all interior surfaces oil free and free of any accumulated metal. The metal will always build up where the axle tube end in the center casing and can become pretty solid on the bottom of the axle tubes. Use a long stick to push a rag with a solvent or brake parts cleaner on it through the axle tube to clean them. Use compressed air to dry and blow out any loose metal.
TIP #2 Pinion seal replacement:
The most common mistake we find when the pinion seal is replaced is that it is over tightened and the pinion bearing fail prematurely. The best way to ensure that you're not over tightening is to use inch pound torque meter to measure the rolling resistance when the pinion is turned. Record this number before you remove the pinion nut. When putting the pinion yoke back on after installing the seal you should put RTV sealant on the splines of the yoke to prevent oil from leaking through them. You should use a new pinion nut because they are crimp nuts and are only good for one use. Tighten the new nut down to where it just seats the yoke. Take an aluminum or brass drift and put it against the pinion. Now take a hammer and give the pinion a good whack. Now measure the rolling resistance with the torque meter. It should be with in 2 inch pounds of your original measurement. If it's too tight then back off the nut only 1 to 2 degrees and hit the pinion again with the drift, then remeasure. If it's to loose then tighten 1 to 2 degrees. This will help guarantee that your diff lives a long life.
TIP #3 Oil recommendations:
We have found that synthetic gear oil generally does not have the correct properties to allow a long life for the gear set in the differential. We recommend using quality petroleum based gear oil. We use Shaffer's oils in our work. Stay away from the cheap gear oil; it will only end up costing you a lot more down the road. In rear ends we use a 140 wt while in front ends we use a 90 wt. If you're differential is equipped with a L.S.D. (limited slip differential), posi, trac lock, etc. then you will need to add a bottle of friction modifier or you will experience a chattering effect going around turns and could damage the L.S.D.