what prop should I run on ST 40X

That would depend on the airplane and the purpose to which you put it. Even then there is personal preference.

So. . . What plane, what purpose?

Ken Erickson

Quickie 500 pylon racer

Not really a good airframe for an X-40, but if you really want to try it, I would go with something in the 8 1/4 to 8 1/2" diameter with the pitch around 7" to 7 1/2". Do not use any sport plastic prop, since you need to reach at least 21,500 on the ground and better would be 22,500. In full race trim, these engines really like to unload in the air, but that takes a clean airframe and a prop that is thin yet strong enough not to flutter. On a F1 they would run between 27 and 28 K in the air in level flight, and all the props were made from maple.

See if you can find an old Rev-up pylon prop, they show up on auction sites on occasion. Just cut down the diameter, balance, and test run. Thinning down the blades will bring up the rpm.

Also expect the plastic film covering to melt where the fuselage meets the wing due to vibration.