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I'm in the process of replacing my front discs and pads. Having soaked the Brake Carrier bolts with WD40 overnight, I proceeded to attempt removal the following morning. As I strained to get the leverage required, I could feel the red mist coming when suddenly there was a loud bang, my head shot up and smacked the underside of the wheelarch and the cars' alarm activated. After silencing the alarm and stemming the flow of blood from my cut ear, I discovered that far from freeing the bolt, I'd just rounded off the bolt head corners. Warily, I decided to attack the other bolt. Unfortunately, I think my first attempt may have damaged the socket. I could see rotational movement of the socket, but this did not translate in movement of the bolt.
I now have two stubborn bolts to which I am unable to apply sufficient leverage to loosen and am now having to consider more exotic means of removal
One option I am considering is to try drilling a hole through the centre of the bolt using successively larger drills until it approaches the minor diameter of the thread, weakening the bolt so that it unscrews conventionally or the head shears off. Unfortunately the threaded end of the bolt is obscured by the Brake Disc so this would have to be done from the head end of the bolt where access is limited.
Has anyone had experience of this problem before, and if so, any suggestions to overcome it
I don't really want to apply heat because of the possible damage it may do to bearings, seals and the like. The car is supported on axle stands at the front jacking points with my trolley jack providing a safety backup under the front crossmember.
Any advice would be appreciated.

...I proceeded to attempt removal the following morning. As I strained to get the leverage required, I could feel the red mist coming when suddenly there was a loud bang, my head shot up and smacked the underside of the wheelarch and the cars' alarm activated. After silencing the alarm and stemming the flow of blood from my cut ear, I discovered that far from freeing the bolt, I'd just rounded off the bolt head corners. Warily, I decided to attack the other bolt. Unfortunately...

Pure Gerard Hoffnung - 'Now the barrel was lighter than me...'

Sorry I can't be of any help (I imagine you've considered one of those reverse-screw stud removal bits) but I found your write-up hilarious. Good luck with the problem.

Sorry to hear about the problem.

I would also try an Eezi-out or similar reversing tool (tapered LH thread) before trying to drill out the whole bolt, as you'll stand much less chance of damaging the threads. If the Eezi-out doesn't work, you'll still be able to go down the bolt drilling route.

Good luck

Paul

I sympathise, as recently I had the same problem.

My front carriers require a 17mm spanner to do the job and the socket I used was the 'multipoint' variety. Like you, I rounded the bolt head and had to tidy it up with a file/grinder and buy a HEX socket with 1/2" drive. This I hammered on and it took 24" of leverage to shift the beast! (Much blood and tears, also..)
Once off, I cleaned up the bolt head to 15mm size. (Rears are 15mm anyway)

I hope you don't mind me reminding you that when facing the 'tail' of the bolt, clockwise rotation is required to remove them...!

Good luck.

Michael

Thank you all for your responses. Since I too used a multipoint socket, I will first splash out on a good hex. socket, dress the head and try again, as successfully achieved by Michael. Its unlikely I'll have the opportunity to do this for a week or so now, but will keep the board informed as this saga unfolds.
I also intend to buy some replacment bolts as I believe they have a thread locking patch which becomes less effective each time the bolt is replaced.

Good news and bad news!!!
I bought an impact wrench socket which applies the torque through the hexagon flats. With this I managed to undo the bolts using a jack under the end of the socket wrench with a piece of wood in between. A very satisfying crack was heard as each bolt freed up. (Thanks for the advice).
Unfortunately, on measuring the front discs, I have discovered they are 325mm diameter. - The front and rear discs I bought on ebay approx 3 months ago are 305mm. The description stated they were for the 4.2 litre XK8 2003 onward (not brembo). It would appear that some 4.2 XK8's have uprated discs, and mine is one of them. - So the saga continues.
What I'm not sure about is whether the rear discs are also uprated. I guess I'll just have to remove one of my rear wheels to measure the disc before I take things further.

Bad luck on the wrong parts.

If you want to sell the 305s could you let me know, as one of mine has been badly stone-scored . (mwest3139@gmail.com)

Michael

Have only just read this topic, the way I managed to crack loose the bolts
Is to spay with penetrating fluid then use a good quality ring spanner and mallet, make sure the spanner is the correct size. locate the spanner on the bolt head in a suitable position so you can clout it firmly, one good clout frees the bolt threds. this worked on my 1998 XK8. once loose carry on with socket wrench. I hope this information may help for future work of this nature.

Have only just read this topic, the way I managed to crack loose the bolts
Is to spay with penetrating fluid then use a good quality ring spanner and mallet, make sure the spanner is the correct size. locate the spanner on the bolt head in a suitable position so you can clout it firmly, one good clout frees the bolt threds. this worked on my 1998 XK8. once loose carry on with socket wrench. I hope this information may help for future work of this nature.

I am very pleased to be able to say that the ebay seller not only exchanged the 305mm discs for 325mm discs at no extra cost within 3 days of me contacting him, but also arranged for collection of the former from me at his cost. - Needless to say I was extremely impressed. (Michael, I can definitely recommend seller "Quality.car.parts" should you still be interested).
The Rear discs did not appear to be uprated and replacement proved uneventful.
Thanks to all who responded to my plea for advise.
One possible design weakness struck me that applies whenever removing the front wheels on the car. I expected to find the disc secured to the hub flange by a countersunk screw. This was not the case, which means the disc can be knocked apart from the flange allowing brake dust and/or rust deposits to migrate between the disc and flange potentially causing disc runout and brake judder once the wheel is bolted back. Often the wheel is seized onto the hub, efforts to remove the wheel can quite possibly separate the disc from its flange at the same time disturbing large quantities of scale, rust and dust. Smearing mating faces with copper grease makes it easier to remove wheels, but even so I think care needs to be taken not to disturb the disc as far as possible. Unfortunately its somewhat out of our control when tyres are changed.
Nick.