G'day vegaome.
The stud is 5/16 unf on the manifold side and 5/16 unc on the engine side ,Going into the head.
I dont know what the hole will look like when you have finished removing the easyout so let me suggest a fix that I use with success every time first heat the easy out ,try to get it red hot with out to much heat on the head then when it is still hot use a Pin Punch and give it a tap in then try to work it out the opposite way ,once you remove the broken easyout and providing the hole you have drilled is near on centre of the broken stud ,using small drills and patience drill right through and keep increasing the size of the drill say one sixteenth at a time till you can see the old thread,when you are at this stage get an old Screwdriver a small one and grind the end into a small chisel,find the END of the thread and using the chisel gently ease out the old piece of stud thread Again with Patience the thread will come out like a spring if you are lucky ,once removed retap the hole using tapping compound just to get the best result you can .
If on the other hand when after removing the broken easyout the hole is to big you can tap it out to 3/8 "any thread but get a new stud first.
The other way is to drill and tap the hole out to either 1/4 npt or bsp Both are tapered threads ,install a Plug of the correct thread using loctite thread locker ,leave for 24 hours ,then either hacksaw or carefully grind the plug end till it is flush with the Exhaust surface on the head, mark your new hole using the new gasket as a guide and CAREFULLY drill a new hole ,try not go to deep and you will be back to new again.
just a word from my experience the word easy out tends to suggest that if one was to drill a hole and tap the EASYOUT in there all that is required is to undo the broken part ,definitly not so in fact the design of an easy out if you look at it when used it will EXPAND the thread and if anything make it even less likely to come out, My opinion on the spiral easyout if you have a set Give them to someone you dont like and buy the PROTO broken stud remover,it is tapered ,square in section with a sharpened leading edge ,it will come with user instructions.
So again Patience is the best tool to arm yourself with.
happy days.
Hutch.
