Also remember - you must use proper tapping technique or you will screw up both your tool and the threads you are trying to create. Use some cutting oil (3 in 1 works ok) & as you are tapping make sure to back out 1/4 turn for every 1/2 to 1 turn you go in. This breaks off the thread shavings as you go so they don't clog the tap.
As for your big hole - you might try tapping & pluging it with the the next size up of thread & then retapping somewhere else for your EGT - try using the thick bosses on the bottoms of the manifold - they are just the right size.
edit - whoops the guy above beat me to it!