While I had the rear offside brake stripped to sort out the stick handbrake, I decided to drill out the broken disc retaining screw.
I bought 8 stainless screws off e-bay before christmas, and they have been sitting in the garage since.
After removing the disc (see here for further details MG TF Rear Disc & Pad Change
or here fro the front discs Changing AP Discs and Pads
) it revealed the broken screw.
A nice heavy blow with a centre punch will provide the start for the drill and may even free the threads up a little.
The next step was to drill down the centre of the screw
to allow access for the screww extracor. For those of you who have never used one of these, it is a bit like a big self tapping screw with the opposite thread to that which you are trying to remove. You tighten it into the hole, screwing it the opposite way to tightening the screw, and the theory is that it bites in and undoes the remains of the screw. That is the theory, but it very rarely works, unless the screw is fairly loose in the first place.
So, that didn't work. Don't apply too much force as the extractors are hardened and brittle. If they snap in the hole and cannot be removed you are stuffed as you cannot drill these.
The next step was to use a bgger (5mm) drill)
and tap a 6mm thread which is the normal thread of these bolts. This could work, or could end up a mess as you are tapping a 6mm thread into an existing 6mm thread and could just end up stripping out the existing thread. It is worth a go, as the next step would be drilling a larger hole and tapping and fitting a larger, non standard, screw.
Well, the operation was a success. I now have 2 nice new stainless screws in the rear offside disc.
Now that just leaves the other 3 discs.