I know it is a long shot but what you need is a spark eroder.
Regards, Nick.

Thanks Boaty but can't slot them as they are deeply recessed. I don't think any glue will work either even the plastic steel I have.👍

You could always try super gluing a near as allen key into it and then try undoing it. if it doesn't some out you can remove the allen key with deactivator before you try anything else that would damage it too far. Or try using a Dremel tool and cut a slot for a screwdriver.

Try using a left handed drill I have a couple from Tracey Tools Devon. Very Useful

Much as i thought Doug Ta!👍

"the bolts have a plain shank so no danger of thread damage"
Well John, in THAT case you're laughin' 😁
Run an HSS drill up 'em (They don't like it up 'em you know! 😲As Cpl Jones would say😉) as mentioned above, the remains of the hex will keep the drill centered. Start with a small pilot hole perhaps.
Good luck, Doug 😎

Trouble is I can't access all my bits just now or I might try tapping in a spline (or two).On big stuff I would throw down a weld until I could grip it. Anyway a chance to use my new Mini Drills. Thanks again to all.👍 BTW Ste the bolts have a plain shank so no danger of thread damage.😜

Thanks chaps. All good ideas some of which will try. it has to be remembered I am chair bound so working "a la main"so to speak. Heat and oil /cooling has been tried. A very hot radiator to hand LOL. about 85 C. I think ball burrs could be the answer. They will cut through stainless.Eventually. Keeping flat mills central may be hard without a press of some sort. No current access to left handed drill unfortunately. I'll get there one way or three.😁👍

If there is nothing left to grip in the holes I would next try a left handed drill. The vibration, heat and left hand direction may unscrew the bolt.
Which ever direction drill you use the hole in the bolt head will keep it centred. Start with a smaller size and drill deep enough to be just below the head. Go up a size and drill again but don't get to close to the bolt thread diameter. Your ideal situation will be that as you get close the head will snap off. You could drill close then drive a screwdriver bit in and use that to snap the head off.

Steve

Good idea Colin 👍
Think you already tried the 'hammering' method didn't you?
I got the impression from you that there wasn't enough hex form left for that to work!
Cheers, Doug 😎
PS The fact that someone rounded out the hex heads suggests to me that the bolts might be seized in anyway 🤔 making success with improvised tools unlikely.

before resorting to milling/drilling I would get out my box of assorted screwdriver bits. in there I have an assortment of metric hex, imperial hex, spline, torgues, etc.
Find something that is too large and (gently) hammer it into the bolt head. You could even grind an oversize down to fit which would give a sharper edge to the hex which will help it grip.

Steve

Doug your right as usual, but prior to soaking in penetrating oil I would gently warm up in the oven and then apply the oil so that it gets drawn in as it cools.
Cheers Colin.

Mornin' John,
I assume this is on the cylinder head you mentioned?
If so I would hesitate to use any of the extractor methods in case it distorts the ally head.
I would be inclined to to use a flat ended milling bit to carefully mill the heads off, until you can gently prise the head off.
When the head is off you can probably grip the threads with pliers and get 'em out, soak with penetrating oil / WD40 overnight first perhaps!?
Using an HSS drill bit might also be OK, but with more risk of drilling into the head and/or spreading the shank of the bolt over the lip of the bore hole. Making it very difficult to lift the head off.
A milling bit in a decent drill press or mini milling machine (like my Proxxon) would be my choice.
Cheers, Doug 😎

Hi onetenor normally I would suggest stud/screw extractor/ but I would suggest that you drill it out using 2mm drill then if you have an tapered burr you could try and catch the screw with that, I managed it once, or you could drill it out with 2mm drill then use a smaller burr and carefully grind the screw out and if successful re-tape the hole for 4mm screw