which bit?

[Rikko]

I've been using just a #7134 for all my cuts and it's held up like a champ. Just do a lazy circle when you make the first hole (like drawing a very bold period) and you're golden.
Just make absolutely sure that you keep the cutting area soaking wet. Some guys have had luck just spraying with a misting bottle, but I prefer hauling it outside and keeping the hose gently soaking the workspace.