I used to work in the mines, and this is the way we fit and removed all bearings/bosses/bushes. Most gas supply shops sell liquid nitrogen, and many will lend you a decanter. It's relatively cheap, and not classed as a dangerous good. It has some fun uses besides shrinking things, and is a lot less dangerous than you might think. Do wear gloves though.
The surprising part is that it works both on external and internal bearings/bushes. A lot of people have trouble understanding that, but bearings will shrink in all directions. We used to pour it into a galvanised dustbin and leave the bearings in there a couple minutes. You don't have much time to get them in once they are out of the liquid nitrogen.
Also great for freezing water pipes so you can do work on the mains side (generally when replacing a broken mains tap).
Shaun
