Drilling a cast iron soil pipe

denso13 · Friday at 6:20 PM

Fine on a downpipe which will also have water going through it I'd say. Just fit a cheap neutraliser if there are any concerns.

dilalio · Saturday at 6:54 AM

endecotp said:
I’ve done this dry; what exactly would you use for lubrication?

Drilling cast iron is self-lubricating.

endecotp · Saturday at 6:36 PM

So.....

Buoyed by my success with the scrap bit of pipe (post #13), this afternoon I have attempted to drill into the real pipe.

The material seems to be massively harder. After maybe 30 minutes of drilling (with breaks), it has barely got below the tip of the drill. And the drill seems to have worn down significantly in the process. It hasn't got to the point where the hole saw is in contact with the metal.

I've actually changed the pilot drill for one from another hole saw, previously used only on wood I think so still quite sharp, with no improvement.

I think these pilot drills are just regular HSS. So I could get a 1/4" drill better suited to cast iron and use that to finish the hole. But then I still have the actual hole-sawing to do, and I imagine that the saw I have will be similarly useless.

Please share your advice. Especially if you've ever actually done this!

Thanks.

trojanhawrs · Saturday at 7:44 PM

30 minutes rofl. Cast drills easy with a sharp drill and you should be through it in seconds if it's cutting. You'll have work hardened that spot now and it will likely be very difficult to drill. Try a brand new drill roughly the size of the pilot in another spot - slow speed setting and good pressure. Burst it a couple seconds at a time, make sure you can see swarf coming out - if its not discard and get a new drill immediately. Once youve proven you can get through give the hole saw a bash - if it struggles get a tungsten carbide one, they're not that dear

Stuart981 · Saturday at 8:10 PM

A consideration - an external condensate drain needs to be of a large enough diameter to avoid freezing in winter and when connecting such a drain to an external stink pipe be careful not create a choke point.

endecotp · Saturday at 8:20 PM

trojanhawrs said:
30 minutes rofl. Cast drills easy with a sharp drill and you should be through it in seconds if it's cutting. You'll have work hardened that spot now and it will likely be very difficult to drill. Try a brand new drill roughly the size of the pilot in another spot - slow speed setting and good pressure. Burst it a couple seconds at a time, make sure you can see swarf coming out - if its not discard and get a new drill immediately. Once youve proven you can get through give the hole saw a bash - if it struggles get a tungsten carbide one, they're not that dear

Thanks for that! “Work hardening” sounds like a plausible and interesting possibility.

Regarding drills - some include cast iron in their “suitable for” and some don’t. I now wonder if I should take any notice of that. Tomorrow, I could get a pack of 5 “suitable for metal” Erbauer drills and/or one Bosch “suitable for steel and cast iron” drill.

dilalio · Saturday at 10:45 PM

Cobalt pilot

endecotp · Saturday at 11:45 PM

dilalio said:
Cobalt pilot

Thanks.

At one point, I got the impression that cobalt drills might be less suitable for cast iron. Now why was that….

Ah yes:

Bi-Metal Hole Saws for Wood, Metal & Plastic | UK Drills

Bi metal hole saws for clean cuts in wood, plastics and metal using the correct arbor and speed. Fast UK dispatch from a specialist supplier.

ukdrills.com

Note that their “bi-metal” hole saw says “suitable for: … cast iron”. Now tap on the “cobalt” tab and it says “For wood, plastics, steel and stainless steel”.

But then look at drills, rather than hole saws:

Cobalt Drill Bits for Stainless Steel & Hard Metals | UK Drills

Shop M35 cobalt drill bits for stainless steel, harder metals and heat-resistant drilling. 5% cobalt alloy drill bits with fast UK dispatch.

ukdrills.com

and it’s the “cobalt 5%” drills that say “cast iron”.

Razor900 · 2026-07-12T08:31:11+0100

dilalio said:
And you can't discharge condensate into a cast iron stack either!

Are you sure about that???

endecotp · 2026-07-12T15:13:59+0100

Latest update:

trojanhawrs said:
Try a brand new drill roughly the size of the pilot in another spot - slow speed setting and good pressure. Burst it a couple seconds at a time, make sure you can see swarf coming out - if its not discard and get a new drill immediately.

Success, with a new Bosch HSS drill I was through in about a minute, using bursts as described. But:

trojanhawrs said:
Once youve proven you can get through give the hole saw a bash

Brand new hole saw, same model as before. It did the first couple of mm OK but then stopped making any progress.

I do wonder if it gave up when I first squirted it with water - might that cooling contribute to hardening? I wasn’t using any water when I did the experiments outside.

trojanhawrs said:
- if it struggles get a tungsten carbide one, they're not that dear

I guess that’s the next step then!

dilalio · 2026-07-12T15:30:33+0100

Razor900 said:
Are you sure about that???

Well, I was!

Razor900 · 2026-07-12T16:45:43+0100

dilalio said:
Well, I was!

Its fine to put it into cast as long as its in the wet bit ie it can't be the top connection to the stack.

As long as you have a bath, basin or wc above it there's no problem

trojanhawrs · 2026-07-12T17:06:08+0100

endecotp said:
Latest update:

Success, with a new Bosch HSS drill I was through in about a minute, using bursts as described. But:

Brand new hole saw, same one as before. It did the first couple of mm OK but then stopped making any progress.

I do wonder if it gave up when I first squirted it with water - might that cooling contribute to hardening? I wasn’t using any water when I did the experiments outside.

I guess that’s the next step then!

Possibly, might mean you're sort of quenching the metal. I drill a lot of stainless at work and never use water or cutting fluid, you just need to keep an eye on it because when it stops cutting its heating and you'll never get through it. Those carbide hole saws are the tits though, drilled through tons of stainless and 10mm plate with them

endecotp · 2026-07-12T17:27:39+0100

I note that e.g. this TCT hole saw doesn't mention cast iron:

Starrett SM22 22mm / 7/8" TCT Stainless Steel Hole Saw

For cutting high alloy and stainless steels, non ferrous light metals and reinforced fabrics. . Tough tungsten carbide tipped teeth have a positive cutting angle which provides fast and efficient removal of chips and swarf. . Ideal for production cutting. . Tungsten carbide tipped teeth provide...

www.lawson-his.co.uk

Oh, what to do?

The test pipe in the garden was so easy. Drilling the pilot hole this morning with a fresh drill was so easy. Now there's actually a hole in the pipe, so it would be preferable to finish that one. Possibly using a TCT hole saw, if that might be better at removing the now-hardened metal? Or maybe I should just start in a new spot and drill without any water like I did on the test piece and be through in a couple of minutes.

Related, unhappy, question: what's the best way to fill a hole you've abandoned in a cast iron soil stack? I'm thinking a blob of epoxy, or something of that sort.

trojanhawrs · 2026-07-12T17:37:16+0100

Believe me it'll fly through that, hardened or no.

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Drilling a cast iron soil pipe

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