Hi,
I think I have finally got a good setup of the ball diffs. This is how I have them now:
Normal outrunners, Bfast copper shim, Bfast ring, Caster Ceramic balls, Steel gear. The screw has been changed to a screw and t-nut from Ass (for the B4, M2.5 instead of M2) and the pressure plate has ceramic balls from Bfast. Grease from Bfast. Nut goes in the longer outrunner together with the sping and it's shims, you might need a few more shims so the nut ends up far enough out.
Really smooth diff, and it almost feels like the nut is broken when you turn the wheels opposit each other, you can have it so loose without slipping.
Note that I had to enlarge the hole in the outrunners slightly to accept the M2.5 screw. I also had to enlarge two of the shims for the same reason, can't remember if it was for the preassure plate or spring. Bfast shims for the preasure plate worked fine though without enlarging. To enlarge the hole you need a diamond tool or something like that since the outrunners are hardened, I found a narrow tipped tool from an engraving set I had.
Normal run in of the diffs and now I have put two packs through them without the need to tighten them.
Really fun to drive around on my grass/dirt track with slicks now (normal tires worn down).