Hobson Zingo Fixed Cup Wrench 36mm
It fits the flats and tightens traditional fixed cups on cup-and-cone bottom brackets. A $140 shop tool is better, but this hand tool will do the job, too.
Here’s how: First turn the cup counter-clockwise (on most non-Italian frames) until it’s as tight as you can get it without the wrench slipping off.
Then have a friend hold the wrench onto the fixed cup and keep it from sliding off by pushing the heel of a hand on it, against the bottom bracket while the other hand stabilizes the wrench handle.
Then you get the side of a metal hammer and whack the handle of the wrench with the side of a hefty hammer to screw it on there really snugly.