Well, I was happily stripping a 3SGTE engine and, after cleaning out the head stud caps I wasn't able to insert the splined bit in one stud and found that the cap has been stripped by some clumsy previous rebuild, see pic.  Welcome to 30year old engines#@$%, too many previous owners.
 
Any suggestions?
 
I know, tons of oil varnish.  This is a spare engine I've had in the garage for a few years and only now opening it up to see if worth rebuild.  Long story of buying it from Bris wrecker, looked good in photos but disappointing when it arrived.  So I'd left it in the corner of the garage until I had more time to look further.
 
 

If that's the last one you may be advised to re torque the others to hopefully reduce the load on that one.
Then find a very slightly larger spline or even a hex bit and with just one solid hit with a good say 4 lb hammer.
Having driven the bit deep you may be able to then tack weld it to the bolt.
This is not the time to fiddle fart about. You will only get the one shot at it.
It's made from very hard material so almost impossible to drill the head off it.
Someone else may have a better solution ??

Thanks Falcon, I'd only just cracked the tension on a couple of head bolts before I found this one where I couldn't insert the splined bit.  So not much to gain by retorqueing?  I think that a 10mm hex shaft will be a very close push/hammer fit.  I'll see if there are any other ideas as I dread welding splatter in this area.  I do have a 10mm hex shaft socket which I can sacrifice.  I'll also look at plugging the nearby oilway into the block and making a shroud around the weld area.  I think I can get access for a tack weld on two sides, top & bottom in the above pic.

Falcon, brilliant, your 4lb hammer worked!  I scratched away with a dental pick to clean out the last of the grunge and there was about 1/3 of the original splines at the bottom.  It looked like whoever torqued the head previously didn't have it fully inserted and it smeared the splines at the top.   The top of the cap was still too tight to insert the splined socket so I wondered if it might even get some bite there if I could get it fully inserted.  I taped the splines on the 1/2" socket to the head bolt socket depth, carefully lined up the 1/2" spline socket so that it was aimed at the splines at the bottom of the cap, got my 4lb hammer, and gave it a couple of whacks until it seated at the taped depth.  Got my long 1/2" socket bar and the head bolt came free!  I can't thank you enough.  Onto the next problem:-)