RE: Propellers for Norvel engines
.049, .061 - M3 x 20
.074 - M4 x 25
All are partially threaded. If you use a fully threaded prop screw, the spinners will fit loosely. I usually buy partially threaded allen head screws in 25 mm and 30 mm lengths and then cut to length. The partially threaded shank is a better match for the id of the spinner.
5x3 to 6x3; 6x3 max for the .049 and .061 Big Mig; the AME needs to spin up more, so probably a 5.5x3 or less. A lot of folks have had good performance with the 7x3 on the .074
andrew