No hammers please! If you need a hammer it means you missed something. BravoFox nailed it, you really need two 12mm Allens for this job. I have two 12mm Allen sockets which I can put on two breaker bars. One to apply a counterhold while I loosen the other one. One of the 12mm Allens is part of the axle while the other one is basically a nut that threads on to the other end of the axle. Keep note (or refer to the manual) which way the axle goes in and also note that the spacers are different widths so keep note (or again, refer to manual) as to which one goes on which side when putting it back together.