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I'm having some electrical upgraded and I need to drill a 2-1/2" hole through my 8" foundation. The foundation is poured concrete and has small rounded stones inside. This house is almost 100 years old so I'm expecting the concrete and stone to be harder to get through than a newer style straight concrete foundation.

I'll be borrowing a Bosch (Bosch hammer model) 11236VS with an SDS Plus bit that has both hammer and rotary function.

What type of drill bit is ideal for this job?

Is a 2-1/2" x 9-1/2" Diamond Dry Core Bit ideal? Or, would a carbide tipped hole saw do the job? The diamond bit is more expensive, but I'd be willing to spend more if it means get the job done easier.

I won't be hiring a core driller for this job, I'm a DIY and am doing this on my own, so any feedback on doing it myself vs. suggesting hiring someone appreciated.

Any insight around the bit, method of drilling, and what to expect are appreciated.

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    I suspect that you already know the answer to your own question... you will probably smoke a carbide bit long before you get through 8" of 100 year old concrete and stone.
    – Jimmy Fix-it
    Commented Oct 25, 2023 at 15:29
  • @JimmyFix-it I figured this, but in some Amazon reviews I see some people say they have done it successfully so that's why I figured there might be a chance. Better to go with the diamond bit then? I suspect no rebar within this foundation, but lots of different stone certainly.
    – CodeVar
    Commented Oct 25, 2023 at 15:32
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    That's what you get for trusting Amazon reviews... ;) Also, modern concrete has small stones in it, too. It's called "aggregate" and it's part of what turns cement into concrete.
    – FreeMan
    Commented Oct 25, 2023 at 15:43
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    I've been making 4" holes in masonry (and concrete, that doesn't have rebar in it...) with the same carbide core bit from China for at least a decade (uh, I could be on number two; but a shaft snapped; wasn't tooth loss). - Might just be my ancient hammer drill, but if you don't keep the slack out of the hammer action, it will spin you in a circle. Sometimes I put a pipe on the handle to make it longer. I've never bothered to do it wet and don't usually even use the vacuum attachment. willing to spend more if it means get the job done easier... then rent the kind you bolt to the wall. ;)
    – Mazura
    Commented Oct 26, 2023 at 4:23
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    Call me "Amazon review". I did the exact thing. 2.5" diameter, 8" deep, cheap carbide-tip hole saw. Took me several batteries and left me with bruises, but the bit didn't give out.
    – hobbs
    Commented Oct 26, 2023 at 20:21

1 Answer 1

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Core drill, preferably wet. Look into rental for the tool.

If doing it with a dry bit, stop often to vacuum the dust out and allow the bit to cool. If you overheat diamond it either turns to charcoal or comes unbrazed from the backing. A wet core drill will flush out the swarf and cool the bit as it goes.

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  • Thanks @Ecnerwal. I called locally and nobody had an actual core drill. Instead just hole saws with hammer/rotary style drills. My electrician said he normally just rents this setup and drills them without too much headache so I may attempt with this smaller type setup. Worse case I let the electrician deal with it.
    – CodeVar
    Commented Oct 25, 2023 at 15:45
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    The most cutting with the least hammering is best, whatever equipment you end up with. Also consider that 'old' can mean hard and brittle, or starting to deteriorate. There have been so many drills I have -not- rented because the teeth were beyond dull, missing, or worse. Good ones, though, will have no problem with stones or anything else that belongs in concrete — except for steel, of course.
    – Steve I
    Commented Oct 26, 2023 at 14:50
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    You can probably get by with a standard drill if you wet drill. You may need a bit extender though to bore 8" deep.
    – Huesmann
    Commented Oct 26, 2023 at 23:16

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