I know this might sound drastic but if these don't come out, what would stop me from either drilling them right out and tapping a bigger thread in there ? Or..drill them right out and bond some stainless studs in (JB Weld maybe).
Anyone ? think I might be starting to go mad thinking about this.

I now wish I could have laid my hands on one of these prior to tackling this job. It's a handheld induction heater, designed to loosen stuck bolts. Not cheap though.
Not sure it would do much on exhaust bolts though, I had mine glowing after welding a nut to it and it still snapped off.

http://www.sykes-pickavant.com/images/product/2590_2961_x.jpg

You may end up drilling and tapping and then using helicoils to maintain the right thread size.

The problem I would have would be to get the drill perfectly situated so that it was centered on the stud remains and straight. You'll need one of the jigs mentioned earlier in this thread. I went back and read the entire thread and saw that you'd tried my favorite tricks with heat. Sorry there was no joy.

I love marine epoxy (JB Weld) but I don't think I'd rely on it for the heads. Better to go the long way and do it right. Porsches are not a brand to do a half ass job on.

Drilling, use the best, brand new drill bits you can find and use lots of oil (spray WD40 will do.) And go slow! Wish I had access to a lift!

Brian
I had this made by a chap who machines bits for my motorbikes.

https://serving.photos.photobox.com/...47df5760cc.jpg
https://serving.photos.photobox.com/...246a3820f3.jpg
https://serving.photos.photobox.com/...88c3de847f.jpg

It screws onto the broken stud and allows a 3mm bit to start.
I've tried it out and the 3 left hand bit hardly made a mark in the stud.
So now I've ordered some hi quality 3/4/4.5mm bits for hardened/stainless steel and 2 different makes for comparison.
I've not given up just yet but i'm close.

Nice!

I had to find a way to get the dumb/soft lower left fastener off where the bell housing mates to the back of the engine. Soft bolt, who knows what size allen key, but one that I didn't have. So I had to drill in from the side to be able to get something in to break it loose.

I knew my drill bits were crapped out and wouldn't do the job, so I headed to the local hardware store and bought the ones they recommended--Irwin titanium bits. About $28 for a small set, 20 pounds in UK, I guess. I drilled the bolt head with a cordless Hitachi drill. Didn't exactly go through like a knife through soft butter, but with oil, and patience it did the trick.

In your neighborhood, I'd probably look for Bosch drill sets. I've used them and they are 1st rate.

Was that the T50 bolt , lower left hand side?
Just took that out yesterday, I had to order a new bit as mine had an 8mm hex shaft (to get a spanner on) which was too weak so I ordered one with a 10mm shaft meaning I could use a slightly beefier spanner (well 2 as I used the other for leverage.)
https://i.ebayimg.com/images/i/16186...1/s-l140/p.jpg

Took a week to arrive but did the job:)
Gearbox (Transmission) out last night , now need to make the tools for locking the cams so I can get the flywheel out and dig into the IMS bearing.

Im steering (blindly) towards this bearing..Vertex Eternal, I like the idea of cylinders versus the low contact surface of a ball.

http://www.design911.co.uk/uploads/i...0590102EPS.jpg

Actually to be honest I would rather be able to buy an exact replica of the OEM dual row bearing that is in my car.
Its lasted 17years and 80k miles...good enough for me.
Surely someone has a box of these in Germany somewhere that never got fitted.

So this is one of those jobs that i go to now and again...it's not much fun.
I tried different methods of welding to the broken studs but nothing worked. I put the welder away and I will be drilling them all out.
So far i've done 2 ..yes just 2. I work up in 0.5mm increments from 3mm to 6.8mm then use a tapered M8 tap re thread the hole. They seem good and the bolts that came out screw in really well.
The drill guides I had made up work really well for getting the first 3mm hole done.
Here you can see.

https://serving.photos.photobox.com/...b484910ba2.jpg

https://serving.photos.photobox.com/...719ae5ab88.jpg

I'm just going to take my time with this..no rush.