Recently one of the 7mm studs broke which fasten the upper and lower crankcases together and it snapped well below the top surface of the aluminium casting.
In the old days we would have tried to drill a hole in the top surface of the stud down below the casting surface and try to remove it using a wrench and ‘easy-out’ tool. The result was usually a hole which ran off centre because the broken surface of the stud was not flat and if this was avoided the Easy-out snapped off, leaving a hardened tool jammed in situ.
The best approach would be to dismantle the whole crankcase, etc. and file two flats onto the now projecting stub and using a tight fit spanner or vice grips try to remove the stud by unwinding it out.
Not wanting to do the full dismantle job I discovered a new tool at Lee Bros at Parramatta of American origin which works like a charm using a 3/8 reversible variable speed drill, which is called a proGrabit and made by Alden in imperial sizes only. They are not cheap, but saved many hours and it removed the broken stud in 10 seconds. !/4 inch was close enough to 7mm to do the job.
Firstly I turned up a guide bush for an end mill which needed to be a light hammer fit and thus needed a filed angle under the head to allow removal with a punch. This ensured that the broken stud face was machined square, then another bush was fitted with a bore for a 1/8 in pilot drill to make a true guide for the Grabit pilot drill, which is left-handed.
Now the left-handed drill of the Grabit drilled a true hole of the correct size and the tool was then reversed and whipped out the broken stud.
If you Google 'Alden' or 'Grabit' you will find a wealth of helpful hints.
Fitting a New Stud
If there is not enough projection to use the common way of locking two nuts onto the thread of the stud a very effective driver can be made using an acorn or dome nut into which a small ball bearing is dropped and held by a little grease. These are particularly easy to remove after the stud is seated.
Frank Wetton, 2012