Perhaps a word about the reasoning behind it all will help.

On small motors the key is installed parallel with the taper. That causes significant protrusion from the crank surface at the lower end, because it locates the cam as well as the flywheel. It also gives deeper depth into the flywheel keyway.

On 1962-up Big Twin series, the shaft and flywheel hub diameter were increased. But they couldn’t increase the O.D. of the hub because it still had to fit between the coil laminations. So, if they had increased the I.D but not the O.D, that means the hub wall gets thinner at the lower end. Furthermore, if the keyway in the hub were parallel with the taper, it would come dangerously close to cutting the hub wall in half at the lower end. So, the solution was to use a short key and keep it close to the upper end of the taper, and cut it parallel to the centerline of the shaft Thus, the keyway in the hub barely makes it to the lower end and does not weaken the hub wall.

Summing up, small motors key parallel with the taper, 1962-up over 28hp motors parallel with the shaft centerline.

Hope that’s right.