It's either 8M1.0 or 8M1.25, I don't remember which. 8mm is very close to 5/16", enough that props I reamed using either 8mm or 5/16" reamers will work interchangeably. If the thread pitch looks to be very close to 5/16-24, then for sure it's 8M1.0 since 1 inch = 25.4mm.

5/16-24 = 24 threads per inch
8M1.0 =~ 25 threads per inch
8M1.25 = ~ 20 threads per inch

Ralph, I found that the G-23 doesn't like to be propped down too much. I got not much higher higher RPM as you did with an MA 16x8, and was kinda disappointed. Swapped in a 16x6 APC, and the little Zenoah came alive. I'm even think about changing to a 15x8 and really let her rip. Plus the GP Spacewalker it's on flies kinda slow with the 16x6 anyway, so pitching up a little should help speed it up. I would love to get a G-26 to play with, but right now I've got enough of these little Echos already.