you should be able to get "exact enuf" by measuring a pair of intact studs per your pictures with a precision vernier caliper
- you have likely soaked the hell out of it with Kroil and put lots of pinpoint heat on it with an Oxy-Acety torch already ?? - sometimes the heat will help the Kroil penetrate
fwiw - i have successfully hand-drilled these things multiple times - first make sure the broken end is ground FLAT so you can center a self-starting bit on it - you start with a SMALL dia tough high speed steel RH drill bit & once you have a deep pilot hole you can progressively switch to larger left-hand bits - the things are tough, so expect to waste a couple bits
sometimes the thing will come loose with the left-hander when you have it drilled out to a shell (the stomski tool uses LH bits)
- or you can try an easy out once it is very thin (and soaked & heated)
worst case - if you miss exact center -
you can buy step studs to fix these 10mm to 8mm - just drill & tap that slightly offset hole to 10 - yup - i have also done that more than once
if you want lots of tips & good experienced advice on how to get studs out & what to do if they fail - go search Pelican for <exhaust studs>
start here -
broken exhaust studsQUOTE(saigon71 @ Jan 25 2015, 11:47 AM)

QUOTE(larryM @ Jan 25 2015, 03:01 PM)

look at the Stomski tool for ideas on how to design your jig project - it is not a direct application for your heads, but may help you figure out a setup
Stomski stud tool2 yrs ago, I b'ot one of these from Pelican to do my 3.2 - it is invaluable - i'll never ever need it again, but it was worth every penny at the time
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I saw that one this morning while researching...very nice. If I can get the exact measurements between exhaust studs, I think I can make a suitable jig out of flat bar steel that will keep the drill straight & true.