I used the proper drill, drilled out the passages, tapped for a 1/4-20 thread and used a set screw with sealant to plug the holes. If I recall the holes are something a bit more than 1/8" so a No.10 thread set screw wouldn't work. I felt it was a much more positive isolation than depending on sealant around head bolt holes on head gasket. But it is a bit more work.