BSF Bolts failing when tightening

[David.H]

I keep a copy of this on the lid of my toolbox as a guide. Note that is is for dry assemblies recommending a 10% reduction for oiled ones. There is a metric one available  which I must admit to using more often!

http://electronicfilters.tpub.com/TM-10-...3P_104.htm

The plating on bolts & sets etc also affects the tightening torque. Most basic testing would have been done on unplated fasteners as that gives a known reference point. Cadmium plating is "slippier" than the zinc (with clear or yellow passivation) that generally replaced it (less toxic, but also less corrosion resistant) and so test results should say what state the fastener was in.
The point about M6 & 1/4 BSF is well made....1/4 UNF with 28 tpi have been known or "persuaded" to almost work...