I should have said " equivalent SAE Grade 12". Grade 8 bolts are 150,000psi minimum tensile strength. Socket head cap screws and bolts are 190,000. Unbrako's Engineering Guide gives their metric SCHS as equivalent Class 12.9, 1220mPa or 177000psi. Whilst I make a good part of my living selling metric taps and dies, personally I do not use metric fasteners if I can avoid them. I much prefer UNF/UNC/UNS. Too many pitches in use in many metric diameters, sometimes 5 or more, a lack of uniform standards (ISO Coarse is the only standard. Others are not ISO- per Sidders Guide to World Screw Threads, Machinery Press New York 2005). The warning in the Unbrako Engineering Guide is one other reason.
"A WARNING TO METRIC FASTENER USERS- Metric socket cap screws are NOT sold in a single strength level like U.S. inch socket screws. The use of metric fasteners in the worldwide market has led to the creation of many standards. These standards specify the fastener requirements; dimensions, material, strength levels, inspection, etc. Different standards are the responsibility of various organisations and are not always identical."
My experience with metric taps and dies has found the following standards or published thread guides:- ISO (Coarse), French Metric, French Automotive, Swiss Industrial Standard, Swiss Horological Standard, DIN, Swedish Inst. of Marine Engineers, JIS, and there used to be a few from the Eastern European and former Soviet Union countries. The only saving grace of the entire crowd of metric systems in my view is that they all use the same 60 degree thread form.
For those not familiar with North American custom built heavy trucks, all chassis bolts are Grades 5 or 8 and most are fitted to reamed holes. This is a standard to strive to achieve with your 4WD modifications.
