If the crank bolt needs tightening it might be 15mm bolt and most standard sockets are too thick to fit inside the hole. There is a bike-specific wrench (of course - there is a freakin' bike-specific everything) but some regular ones can work. If it is a newer bottom bracket it could be allen or other new fangled setups. I frankly don't understand some of the new builds - I need to investigate as I have them on my new bike
