I didn't think my post would get this many responses, thanks for those, and I had no idea that there are actually tools available that were designed specifically for this purpose. The Cal-Van stem spreader looks like a snap ring pliers. Does this tool actually work?

To clarify, I am speaking of a stem stem, not aluminum. I plan on installing a set of 22mm steel drop barss into a Raleigh stem typically found on the 3 speed Sports models.