Newcastle hi tensile bolt Co is the bolt industry standard. Grade 8.8 is what your chasing. Usually sca has these with a zinc coating. (Stamped on the head) Zinc isn't great with temperature, but is ok. Ideally black will suffice, and goes better with antiseize. You can get grade 10.9 which is stronger again, or 12.9 socket head cap screws. First number is the ultimate strength, and the second number is the % of ultimate indicating the yield strength. E.g. a grade 12.9 screw has a ultimate strength of 1200mpa and a yield of 1200x0.9 = 1080mpa yield strength. Most bunnings stuff like there galvanised threaded rod is grade 4.6 which is like butter. Basically a mild steel equivalent. Don't get stainless as they weak as sh*t. Imperial bolts use a similar stamping on the head as well. 4, 8 and 10 I think.