BSF Bolts failing when tightening

[Bob Culver]

Completely dry threads are somewhat impractical., so oily is the more usual condition. This does not mean dripping SAE50 or 140 EP or moly or graphite or any other grease. A hard washer has huge influence An old  Newnes gives 7.5 ib ft for ¼ BSF 45 tsi, the basic HT grade presumably corres to R. Presumably your wrench accurate. Can check using a lever and spring balance, scales etc

It is unusual for the thread to fail before the core, for any strength.

M6 can be mistaken for ¼ BSF.....