THIS ^^^^^ I put a drop of oil on the tap, and slowly go back and forth to let is slowly cut the threads and avoid breaking the tap off inside the head. I always go deep enough so that the set screw is just submerged below the surface. That way, no grinding, and your manifold/header will fit flat and seal proper. I've done this way more than I wish to remember, and never had anything go wrong. You'll do fine