[37 Coupe]

If someone has a known 1937 Trico wiper motor could they look at it and tell me what I need to know. The original standard black arm from my coupe and a couple 37 Bob Drake ones I have ,they have a serrated hole that attaches to the motor shaft. My question is the motor shaft what is it supposed to look like ? Mine is brass ,threaded on the end for the nut and has a taper not a matching serrated diameter .Is this correct? I don't have the original wiper motor from my car,it was replaced. The Bob Drake arm no matter how tight I draw it down with the nut comes loose after a few minutes. Got the motor from Ficken years ago as he had issues fixing my original. Scarey day yesterday in a downpour. Thanks