Places like Edmund Scientific, and at one point, Surplus Shed, sell optical spanner wrenches. It's a bar with a fixed spanner and the other side slides to the proper diameter. This is the best method as you won't have to worry about them collapsing and scratching an element.

Also, some lenses are all cemented together. I have had great luck with third-party lenses, they just fall apart. But, a decent lens from Nikon or similar have a tendency to have the retaining rings cemented in. Making it very hard to disassemble. But, older and simpler stuff like an enlarger lens will be easier.