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I want to somehow get an M5 insert into a 7.5mm diameter hole in solid steel. (The hole has already been drilled with 7.5mm Dewalt cobalt-tipped drill bit)

Is there such a thing as a threaded insert that is suitably constructed to be able to go into solid steel? Some have external threads, some have that rough/raised diamond pattern on the exterior instead. I'm not sure what might work. How about something I could hammer in, if the insert's diameter is finely tuned to be *just* above 7.5mm?

Any advice is much appreciated. Thanks

For a one off hole a helicoil kit would be expensive S.fix eg # 887FR

Search fleabay for "nutsert" and/or "rivnut" - you can get a setting tool & a quantity of inserts for around a tenner. Do the job then resell it.

Drill the hole out to M8 or M10, fit a bolt and drill/tap that for the M5 thread.

The nearest Tapping size for what you already have drilled is M9 which is 7.70mm. Not easy to find a 9mm tap or even an M9 bolt.

As already suggested. Go one size up and then drill the core. The Tapping drill for M10 is 8.4mm. Insert some M10 threaded rod and (if necessary) centre punch the work piece/rod interface to lock the rod in place. You haven't stated as to whether this is a 'through' drilling or a 'blind' drilling.

The Tapping drill size for M5 is 4.20mm

Drills of these sizes (and the Taps) are available from RS Components but don't come cheap. It's now some years ago that I bought some 8.4mm drills but they came as a pack of 5 for about £25. And I have yet to wear out the first one. - I keep the one in use 'inverted' in the plastic container packaging.

The other (and perhaps cheaper) alternative is to get someone (Garage/Engineering firm) to 'puddle weld' the existing hole and then start again.