I have Magnum 91, 4 stroke that I want to put a Tru Turn, or the like, on. The shaft has two prop nuts, the outermost conical and jams the first nut that has a recess for the "cone" of the jam nut. This leaves virtually no thread to mount an adapter. What to do?
Second, how do you figure out how long a a screw is needed?

Thanks all!

True turn makes adaptors that fit different engines. Ask you lhs to get the proper true turn part.

Yep, get a spinner adapter nut. Generally this will replaced the stock jam nut.

And most the time you have to cut the spinner screw to fit. Once in a great while I have to replace the screw with a longer one but not very often.

use the adapter nut as a jam nut...You want 2 nuts on a 4 stroke