Hi Tom,

The pin is called the 'captive cotter pin', and engages the PTO shaft as the nut is tightened, to lock the cutter clutch in position. To release it, you undo the nut until it is level with the top of the thread and then gently tap it down until it releases the clutch assembly.

It sounds like yours has seized, perhaps by corrosion, and by the sound of it the pin is now unserviceable.... frown

The fix is to now to soak the pin area with Penetrene; apply liberally and let it soak for some time so that it get right in, and then tap the pin again and get it to release. wink
Good luck mate, a good soak and it should move down.
cheers

Please do not PM me asking for support. Please post your questions in the appropriate forums, as the replies it may receive may help all members, not just the individual member.
Kindest Regards, Darryl grin