I had a chart for torque specs for different sized screws, but I can't find it anymore.

For head bolts I use 10in/lb for 4-40/2.5mm and 12in/lb for 6-32/3mm. That will cover 99% of the nitro motors out there.

I found a post on MECOA's forums, all in in/lbs:
2 56 - 2.2
4 40 - 4.7
5 40 - 6.9
6 32 - 8.7
6 40 -10.9

Those numbers sound a little low to me, but it also depends on how many threads are engaged. A backplate screw would be fine at 4.7 in/lbs and would probably strip at 10. A head bolt would be way undertorqued at 4.7.