Hi Dave,

Depending on how long the manifold has been in service, and the condition of the other studs, it may be worth your while to pull the manifold. It may seem like more work, but you'll end up with new exhaust gaskets and copper nuts on the manifold to engine studs, 3 new manifold to header studs if you like, and an easier job of drilling and tapping a helicoil.

I often do this as a matter of course if the header bolts and nuts look like a dog's breakfast. It's an ungodly place to be working and I find it better to pull the manifold and header pipe out from the engine bay as a unit to where I can deal with it on equal terms. Just a thought.

regards, Peter