Buy an angle grinder If you are going to work on old bikes. no more "how do I get a siezed cotter pin out of the rusty crankset I'm replacing?" or "the seatpost clamp bolt is stripped, what do I do?" Just cut the S*it off and replace.

I just got mine yesterday (I've been meaning to buy one for a while, but the above mentioned cotter pin issue spurred me into action) It's Awesome! It turned out I also needed it to customize the new cotter pins to work with the "new" cranks and the french Spindle.

Also Rear derailleurs with cage pivot springs that are blown in a certain way so that the cage hangs loose but hold tension when rotated 360 counter clockwise can be used without issue by, well, rotating the cage 360 counterclockwise and then running your chain through it. it makes putting the wheel back on slightly more difficult but otherwise works flawlessly.