** Identification Markings on Bolt Heads: ASTM and SAE Standards
http://shopswarf.orcon.net.nz/boltid.htm
** Aircraft Hardware: Overview on bolts, nuts, washers etc., etc.,
http://www.zenithair.com/kit-data/ra/hardware.html
Quotes from the above article regarding aircraft quality :
Do not assume the engineer role by using hardware types or sizes that are contrary to your plans or assembly manual.
Commercial grade hardware found in hardware or automotive stores is legal to use on an experimental airplane but should not be considered for even a moment. Why? Let's look at bolts as an example. Common steel bolts purchased from a hardware store are made of low carbon steel that has a low tensile strength usually in the neighborhood of 50,000 to 60,000 psi. They also bend easily and have little corrosion protection. In contrast, aircraft bolts are made from corrosion resistant steel and are heat treated to a strength in excess of 125,000 psi. The same comparison applies to most hardware items. So, use only aircraft quality hardware on your airplane. Save the other hardware for your tractor.