have an axle to do it with and it already has a keyway
Just saw that, and whoa down, skippy. If you plan to line up keyways like that, just drill smooth all the way through your extension piece of shaft, and use it like a stack of washers. Otherwise your chances of the threads lining up for the finish torque are about zero. You'd twist the keystock in two. The statement I made about a rough slice for the keystock should have been more specific. That slice is only for the keystock to slip through on its way to the motor shaft, a convenience thing. None left sitting in the rough slice, all of it in the machined keyway of the motor shaft.
Picture 2 pieces of shaft butted together with a piece of keystock bridging the 2 keyways. Lock 1 in a vise, grab the other with a pipe wrench, and twist. Keystock snaps in 2, boogers up both keyways, etc.
