I have been trying to remove the 18mm nut on one end of the drop arm. It has got all the way to the end but now the thread spins with it. I am pretty sure that there is an allen key slot, I am assuming allen key anyway, on the top of the thread. Does anyone know what size it is?

put a bottle jack under joint and jack up put enough pressure to press joint into taper and it will hold it while you undo nut,same for tightening