Mark,
I had exactly the same problem as you
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fortuntely it was on a gearbox that I had bought under warranty and the gears we'ren't right either
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so refunded.
But I did try and tackle the knackered threads myself whilst in the car and I'm afraid I failed at it too.
You may have to settle with removing the box and drilling the hole out so its true to accomodate an M12 1.5 metric fine thread.
Use a that tap for a new thread.
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and to be honest, the way forward to is to bore out the holes for the engine mount, and the slave cylinder, whilst the box is on the floor, attach the slave then the engine mount.
Make sure everything is sorts on the floor before refitting the box as you simply can't see if the threads biting with a bolt or if its cross threading when doing it in a car.
I sympathise for you chap, unecessary problems.
though to be honest, its seems a fault of the last refit of the box.
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the bolts won't have gone in straight and caught between tightening the bolts into the alloy casing and catching against the engine mount.
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too much resistance early in the thread as its going in so will have pulled the threads out.
Theres simply too much load on that joint for chemical metal and the likes, once the box heats up and cools down several times, the substance will just break away due to the expansion and contraction rates of the surrounding alloy.
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bit like welding a solid steel flange to a cast iron manifold.
So recap
- box off if you want it done properly
