The important thing is start with a smaller bit and LIGHT pressure. Let the bit do the cutting (with the engine runnng). I used two smaller bits, 3/16" and 5/16" before finally using the "R" bit.
If you can't find an "R" bit, I can get you one, but you'll need to send me $13 to cover the bit & shipping. PM me if you want one. You should be able to find up there. Check at auto parts stores or industrial hardware suppliers.
I had to go to several stores to get a cobalt drill. After drilling the manifold, I really don't think it was necessary as "solly" said it is soft metal. I had the engine running and oiled the drill bit during the drilling. Everything went fine. Also, as SixPak says start with a pilot hole. Good luck!
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