[Canuck_Targa]

The fit of the thread is not determined by how the threads are machined (either rolled or cut) they are defined in classes. Class 1 having the most clearance and class 5 being an interference fit, a stud would have a class 2 or 3 on the end that you would be attaching or removing the manifolds and a class 5 on the end that threads into the cylinder head. You could have all these classes in either formed or cut threads, it is merely 2 different methods of machining threads. In the case of the picture of the studs, I think it is just two different manufacturers way of producing them (one formed and one cut).

You are correct James, if the holes were through holes into cooling jackets, you would need to use thread sealer so they didn't leak, but I haven't seen many exhaust studs that go into the cooling jackets (but that certainly doesn't mean they it doesn't happen).