Mini live center from bolt - video

[Frank S]

As has been said, bolts are a great source of known quality metals for making things.
Imperial or SAE grades commonly used here in the USA generally start out with no markings on the head I call them mud and fat, not all that different from plain hot roll steel. and go all the way up to grade 9, usually only found in black anodized socket head cap screws and even those may or may not always be equal. There are some extremely special hardened bolts found mostly on certain parts of Earthmoving equipment. Caterpillar track bolts are among some of the hardest bolts you will ever encounter, because the case hardening on them is much deeper. Construction grade bolts such as A307 and A325 in general fall into the gr5 category but are specifically designed to withstand forces not as commonly found in automotive grade bolts the A325 bolts generally have larger head sizes such as a 3/4" Dia bolt instead of using the wrench size of 1 1/8" will be 1 1/4" inch for larger contact area. Metric bolts grades are usually 5.5, 8.8, 10.3 and 12.9 in general will coincide with the SAE grades, in hardness and tensile strengths of 3,5,8, &9 but not exactly. This pertains to steel bolts only other metals such as aluminum brass Stainless, and Titanium may or may not have similar markings on them. usually not. The exceptions being for Stainless and titanium.