I have a Saito 56 on an H9 Cub and I needed a ton of nose weight. I used a Harry Higgly brass prop nut, with a HH brass lock nut... don't know if I'm using the right name.

Basically, mount the propeller, and install the HH lock nut. It's a thin brass nut, but large diameter. Uses a 17mm wrench IIRC. I tighten that lightly, then install the large heavy prop nut. I tighten that up against the lock nut. Works great. I've never had a problem, and I don't use the Saito prop nut or washer at all.

I use a 13x5 MA K prop on mine, and get about 10,400 max rpm, and then richen it up to 96-9800. I'm a little surprised that you guys aren't getting more RPM with those 12x5's.

I recently tried a wooden 12x6, and I think I like it, though I'll have to test more. Definitely winds up faster than the 13x5 MAS.