It's not a gib head key and the key isn't tapered. The slot in the pulley is cut at an angle to the axis of the pulley so it is deeper at one end than the other. We bought the mower in a well used condition and the seller included some spare deck bearings. I now think that at some point the...
I know this is an old thread but I've got the same problem. It seems the pulley shaft is parallel but the keyway in the pulley is tapered. As the pulley is otherwise symmetrical it's easy to put them on the wrong way round. The pulleys on my deck are fitted with the wide end of the taper...