11-05-2020, 11:21 AM
I find that an 8mm stud extractor is near enough to grip the stud properly, and is widely available where imperial sizes are no longer so. To get a centre for drilling out a broken stud you can drop the head back on the remaining studs and use a 5/16" or possibly a fractionally larger (metric?) bit which is a good clearance fit for the hole in the head. Only use it enough to create a centre in the top of the broken stud, then remove the head to check that it is nicely centred. If not you may need to do some careful work with your centre punch. If the centre is right it can be useful to pop the head back as a guide for keeping your drill vertical, at least until you have gone right through the brken stump. With care you can work slowly up to tapping size, and if you are lucky you can wind the old thread out with a scriber. If unfortunately you finish up slightly off centre things can be recovered using a mousetail file. In my experience techniques required vary a bit with how stubborn the stud or how lucky or unlucky you are.
Robert Leigh
Robert Leigh