drill bit selection

Rapid Robert said

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

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Cast iron drills easily with High speed steel bits (HHS) Cobalt will hold an edge and last longer. Your choice but you may be limited because of the length you need. Drill slow with medium pressure and keep pulling the bit out of the hole to clear the chips. You are just enlarging a previously drilled hole I am guessing.