After changing the cam cover gaskets which gave me some problems getting the things to seal properly, I had a bit of time yesterday to check for any leaks and also clean up some of the old oil around the heat shields, engine block and surrounding area.
As I was cleaning the area under the dip stick mounting bracket I noticed that the forward most exhaust stud with its nut still secured just seemed to drop off! Broken right off.

Impossible to have broken whilst I was wiping away some old oil stains but this action would have been enough to dislodge it if it was already cracked and ready to 'go'..
Only way this stud could have got to this point is with over tightening. The stud isn't bent in any way and there's no corrosion around the break point. I doubt very much me raising the engine recently to replace the compressor would have caused this; may have hastened the stud falling off but not causing it to snap this way. It was overtightened, it must have probably just been 'hanging on' ever since and ready to fall off at some stage.

Question is when would it have been overtightened, who would have done it and why?
I've owned the car since 24,000 miles (39,000 km). It came with a full Jaguar service history and had one for several years thereafter. At no point did it have any work done to the exhaust manifold.
I've never experienced any exhaust fumes or manifold noise and it's exactly the same since the stud fell off.

I would think it impossible to extract the broken piece of the stud still lodged in the head unless the engine was raised out high enough to clear the strut tower/inner wing in its way. I doubt whether these mobile stud extractor experts would have enough room to work in the engine bay. Unless they can perform miracles with mirrors and right angle drills!

It's very strange.



Stud in place before I started wiping the old oil stains from around that area.

Stud had just fallen away! Photo shows dipstick and dipstick heat shield moved out of the way.

Wider angle view.

Close up view. Exhaust manifolds are nicely shielded from heat and oil drips.

Close up on what appears to be an overtightened stud at some stage of the cars life.

 

I agree.
As far as I know the hole is deeper than the length of the stud, so the remains of the stud will probably not be stuck fast in the head.

Perhaps not an option with the engine in the car but a left-handed (reverse) drill bit in a centralising sleeve using a flexi drill extension and an anti-clockwise electric drill may well do the job.

A couple of years ago now I remove a recessed broken head stud from an aluminium engine block successfully using this method, but without the need for a flexi drill extension.

Mellow

 

With virtually no access in situ to extract the broken remainder, is there any harm in leaving it as is? No exhaust leak, right?

If that idea drives you nuts and you've just got to remove the broken piece, be EXCEEDINGLY careful with a screw extractor. Those little devils can only apply a small amount of torque before snapping. They are only designed to extract broken fasteners that will easily spin free. If you have a fastener that is seized (corrosion) or is physically jammed in the bottom of a blind hole (overtorqued), the screw extractor will snap well before the fastener will unthread. This will turn a bad situation into absolute misery.

Unless I know a broken fastener will unthread easily, my screw extractors stay safely in my tool chest, where they can't do any harm. I'd rather drill out the entire broken fastener with left hand bits and then retap. Deal with a broken extractor once and you'll swear never again. But back to your situation, I'd leave it alone if at all possible.

 

Thanks m-e-l-l-o-w, I'm reasonably well set up but would probably pay for one of the local mobile specialists to do that job, he's good but very busy. I had him Helicoil all the inlet manifold threads on the alloy heads of my Triumph Stag many years ago when I had far less confidence.
I know one of the mechanics at Porsche (Australia HQ) down the road and he often gets them out of trouble apparently.
Good to know it can be done with a centralising sleeve.
May drive me a bit nuts but I'm taking heed of Karl's advice and leave it alone. Very good advice about those limited screw extractors. I've bought a couple over the years but have never used them. The cam cover bolt I overtightened and snapped I ended up gripping the small bit still poking out with my power drill in reverse.
No exhaust leak or noise, just a big surprise it just fell off!