Hex wrenches also go by the name allen key sets. A good set of hex wrenches is an all-important part of your bicycle toolkit. The most common sizes that you'll need are 4mm and 5mm, Which will fit a variety of common bike parts, including stems, headsets, seatpost collars, brakes, and lots more. It's also handy to have a 10mm size hex wrench in your toolbox when it comes to working on bottom brackets and some brands of bike pedals. You can get hex wrenches that are in an L-shaped construct as a packaged set. Or you can get hex wrenches that come altogether in a single tool.