If you hold both rear tires or hubs you should just barely beable to move the spur. do a rebuild with new diff balls and bearings, be sure to use the best diff lube on the market from www.BEEFYPRODUCTS.com tighten down to the sweet spot and you should be good to go....
...also, depending on what type of axle it is...check to make sure the threaded stud isn't pulling out of the axle causing the nut to not tighten properly. (Had that problem on some of the OLD BME "White Fiberglass" type axles)
Make sure you clean everything before you build the diff, clean the rings and balls with motor spray, don't want the oil that is on them for shipping. Also, there is a good side and a not so good side to every ring, ensure you have the best side toward the spur.
Also something to check is the spur gear you are using.. I have a few PRS ones where the thickness of the gear at the diff balls is 1/8" so what happens is when I build a diff using the gear the rings grind into the gearas they are riding on the balls...
Also use #600 paper versus 2000.. the more you polish the ring the more preload you have to put on the diff to get it to lock... I can build just as smooth of a diff using #600 and using less preload whick keeps my right outer bearing alive longer...
Kenwood is exactly right! you don't want to polish the diff rings, you want to use 600-800 grit to take them down level, the more you sand your diff rings you will notice how much better your diff gets,it's because everytime you sand them they get a little bit more smooth,you can see the high spots if you put some bluing on them as your sanding them,or I just use a sharpie.
Had a problem also were the diff was slipping to much
and me and Kenwood found out the diff tighten nut was
stripped out were it would not tighten any more no matter
how much you cranked on it. Something to look out for.
Yup kev is correct.. I started using the aluminum associated diff lock nuts for the diff.. I washaving issues with the threads on the plastic nuts.. The hole on the plastic ones seems to be too big from the get go and the threads never cut deep enough..
One area that I really enjoy is building good diffs. I don't mean to sound boastful, but so far, at my local track, nobody has matched my diffs. I've tried all of the different methods that people talk about including sanding with 600 grit. I no longer agree with 600 grit. My latest and best diff that I built had new diff rings that were polished. The diff was race ready tight, yet you could take one tire and spin it and the tires would easy spin for 1-2 seconds (4-5 revolutions)! That was smoooth. And it was all done with polished diff rings. So I am now a believer of finer grit sandpaper.