After installing a new freewheel on my 1979 Motobecane 50V, I was disappointed to pedal and have the whole damn thing spin on the hub. When I removed it, I could see most of the threads were gone. I used some JB Weld and hoped the last few threads would be enough. They listed about a week before the freewheel broke loose again.
So, I thought making my own woodruff key might pin the thing. I got the freewheel snug and marked the hub and the freewheel . Then I removed the freewheel and used a file to notch both the freewheel and hub.
I cut the head off a nail I thought was roughly the size of both notches combined, and hammered the nail on its sides until it was square. After test fitting in each notch, I whipped up some more jb weld for the threads, then spun the freewheel on snug until the notches lined up.
I drove my woodruff key in tight, cut off most of the excess leaving a little exposed in case it ever needs to come off, and ground the edges smooth.
So far so good.