...the one time, on the one bike I had to do this, it turned out to be easier (because I have a bench grinder) to just take an odd 16 mm socket and thin the wall. i still have it in the tool box somewhere, and will probably never use it again, because of course when you reassemble the thing, you want to use a crank bolt with a smaller, 15mm head. Or one of those socket headed ones that precludes the need for a dust cap.