Regarding the Tamiya M-05 ball-diff:

Is it 'normal' to be able to adjust the diffs tension by rotating the front wheels in opposite directions? I can change the diff action from fully tight to fully loose by doing this, and I'm not sure if I've built it wrong or fitted it the wrong way around (looking at front of car, screw on left side, nut on right side).

I first ran the diff with the original screw/nut combo and it seemed to work OK for a few races, but I wanted it a bit tighter, and inevitably cracked the nuts plastic housing trying to tighten it. I then bought the TRF screw & nut, tightened the diff down, and it went completely loose 5 laps into the next race.

To be honest, right now I'm thinking about just going back to the TA03 balldiff, as you can make it as tight as you like, and with a bit of threadlock, it stays like that for good.