First we need to know the size of the tap ? I would use a spark eroder but that’s because I have one. What metal is it in ? What engine. A steam chest with a missing stud or bolt in a working engine is not a good idea.

I broke a 1/4″ tap in aluminium after drilling undersize in error and after setting it up securely in a mill, used a solid carbide drill which got it removed easily. You have to be careful to have positive control of the feed, a drill press will be likely to break the drill.

I have drilled out taps with a single flute carbide cutter.

For this to work you need a rigid machine, preferably with a quill and a quill stop.

The problem:

M4 IIRC snapped in a part I had a lot of work in.

The cutter:

Made from a round piece of carbide on a single lip cutter grinder. Flat ended, with normal relief on the back

Chucked up ready:

Then peck drill 0.001” at a time, against the quill stop, and allow a bit of dwell.

The tap comes out as very fine chips:

Result 🙂

Once you are through then you can pick out the pieces and with luck the thread is good.

Dave

If it was working prior to your discovery, why not leave it as is. Let sleeping dogs lie as it were.

On 4 January 2026 at 15:49 bernard towers Said:

Yes but the broken bit will not be coated!!!

Think about the practicalities.  Dissolving top-down through the break alone will take forever because so little metal is exposed.

And as the tap is jambed deep inside the hole, one might die of old age before a coated tap comes loose.  Dunno about Bernard, but my days are numbered…

Dave

Sorry SOD but have done it and with elevated temp and agitation with a Hypo it goes quite quickly, last one was 2 days. All our days are numbered

You young whippersnapper, I’m at 27649 days