I put a threaded prop hub off a US 41 on my ryobi 31cc. Its a short shaft. Seems to fit well.This is my first conversion. Is this ok to do?

Since tkg is not here to answer, I will pass along a tip I learned from him:

Paint the shaft taper with a permanent marker, then install the prob hub (without the woodruff key if it came with one) and turn the hub around the shaft a few times. When you remove the prop hub, if the permanent marker paint has been worn off very evenly, then you are probably okay. However, if it wore unevenly, or only the front of rear portion of the taper is worn off, don't trust it. Even if you are a little bit off, the prop will wobble around and cause mayhem.

Volfy: A good tip! Tapers can be frustrating but that is a great method to check them.

As a non-machinist, I generally trust that the taper is fairly true to the face. For a nice fit (taper shaft to hub) I use some fine valve grinding compound on the taper area and rotate. the alum high spots wear off increasing the surface contact.

Also used this method on a flywheel with molded key which got broke off. Gave a nice tight fit that did not rotate once it was installed and operated.