AC idler pulley removal

I'm halfway through changing the timing belt cover, it's all simple and easy except how do you get the A/C idler pulley off the old cover and onto the new? There's a threaded hole in the new one, but no obvious (obvious to me anyway) way to lock the wheel in the old cover and unthread it.

Crap - I posted that by accident - something about typing with two greasy fingers. Meant to go hit the thing a few times to see if there was a dust cap or something before posting. So I did, and it is. Problem solved. Next I'm going to see if I can ease out that crank shaft dust seal and re-use it until I'm back in the land of Land Rover parts. Doesn't seem like a smart thing to do, but it does seem necessary if I'm to have a Land Rover to drive for a the next few weeks. Anybody done this?