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My experience with cobalt bits is they are better for drilling very hard substances (stainless, hardened steel, etc.). For mild steel, use lots of coolant and a high RPM - probably about double what one would normally use with high-speed steel bits.


That said, I would think that bolt would be hardened steel but perhaps someone can shed more light here?


My metallurgical vocabulary is limited. I was thinking about a metal file as opposed to a grade 5/8 bolt. I'm sure there is a difference in the characteristics.

89 3L V6 5spd SWB, Aisin Manual Hubs, 2 (suspension) seater Raider in Wheeler, OR