MAke sure the threads are clean all the way down the hole. It's a cast iron manifold so it's pretty tough and forgiving. I use a pocket screwdriver to loosen the gunk in the bolt holes then blow them out with compressed air than follow up (carefully) with a thread chaser. I don't use a tap. You might have enough good threads for a stud and some Locktite. (might) Clean the threads on the bolt if you are reusing that.