which of these drill bit types would be the best/hardest? titanium or black oxide. drilling block oil passages where if a bit breaks the block is junk should I use a cheapie if it would be less likely to catch & break or the best/hardest I can get? thank you for your time. RR
You can use either one of those or the cobalt recomended. Use a sharp new bit with cutting oil and don't go too fast. The cast iron will drill much easier than you are expecting.
If you do break one of those three types of bits, which are really just coatings on the bits, you will have one more chance to drill out the broken bit using a solid carbide bit. Solid carbide is excellent for drilling through broken drill bits and taps.