I don't repair cars for a living anymore, but I have used these tips-

A great suggestion passed along is to use valve lapping paste which is gritty and fills the voids. This works as long as the head is not too enlarged.

Depending on the size of the head, using a chisel to help start the bolt twisting can also help sometimes.

On the old '77 280SE I had to drill out an exhaust manifold stud ... I was very concerned as messing it up meant a new head. I used an E-Z-Out. I was told to get a high-quality drill bits, use a center punch to start as mentioned above. I nailed the broken stud in the center (whew!) and the E-Z-Out worked as claimed.

haasman

__________________
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)