All bolts are measured by the thread diameter, not the hex head size. Thread sizes are usually stated like this: 5/16-18 x 1. This bolt would have a 5/16" thread diameter, it has 18 threads per inch (this is called thread pitch), and the threaded part is 1" long.
You should be able to get a bolt sizing tool (it is a flat piece of plastic with several different sized holes in it) from Lowes or any decent hardware store. While you are at it, you will probably need a thread pitch gauge. It looks like a small set of feeler gauges, except the gauges look like small saw blades.
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These are generally accepted sizes for general machinery and most carry over to automotive sizes. However, the thread size is what's important. As stated above, a thread gauge is your best bet.
Also, when buying the bolt they will be spec'ed like 3/8"-16x2.
3/8" = thread diameter
16 = thread pitch, or the number of threads per inch.
2 = grip length, or the distance from the end of the thread to the bottom of the bolt head.
If you are dealing a critical component such as an A-arm, caliper, ect. where fastener failure could mean your life then please educate yourself about fastener grading systems. Grade 8 is generally accepted for critical components, but even here there is a lot of room for error.
You don't want a stainless steel head bolt.
Regular bolts will not work for head bolts.
The stretch and spring tension of the head bolt is critical to it holding the head.
Regular cylinder head bolts are steel and the shank is thinner than the threaded area to allow the bolts to stretch and hold tension on the cylinder head. They really funcition like super stiff hold down springs.
Unless you're buying genuine ARP or SPS stainless alloy bolts, don't even THINK about using "stainless" bolts for any kind of a stressed application (suspension, steering, brakes, engine, etc.); 90% of the "pretty" stainless bolts on the market (other than ARP, SPS, or Brooks) are SAE Grade 2, which is about the same as the lag bolts farmers use on gate hinges. After 37 years of dealing with highly-stressed premium automotive fasteners every day, I see this "pretty" stuff at car shows all the time and it gives me the willies.
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