In a time before nuts and bolts were packaged into useless plastic bags in never correct quantities, gathering fasteners was as easy as heading down to your auto parts or hardware store and just asking for a dozen M8 45 millimeter exhaust studs with a 1.25 thread pitch and matching copper pinch nuts. While there are some auto parts joints and fastener suppliers that still operate this way, many more have gone down the dark path of plastic bags and blister packs. As knowing is half the battle when it comes to finding the right fastener, the nut and bolt measuring gauge or screw checker is an indispensable item to have in the garage or pocket. The nut and bolt gauge also comes in handy when aluminum treads unscrew right along with the with the stud or bolt. After swearing up a storm and peeling away the aluminum, knowing exactly which Heli-Coil® or screw thread insert to get is foolproof thanks to the nut and bolt gauge. The good news is that some of the places that will still give you a brown paper lunch bag full of nuts and bolts will usually supply a gratis nut and bolt gauge with purchase. Measure it twice. Fasten it once. Or at least until it breaks again.
Fastenal hands out (if you ask nicely) a pretty excellent nut and bolt gauge. It’ll do diameters and pitches for both SAE and metric hardware. It’s saved my bacon a number of times.