Limey,
Here are a few thoughts. Because the manifold is made in the USA the thread configuration would be NPT. Looking into a threaded hole trying to determine pitch can be a though one.
My book of knowledge tells me that, although they are close, BSPT threads cannot be threaded into NPT. The thread count is one off, and the pitch angle is 5 degree different.
I think you have two choices..find a supplier of NPT fittings in your area, or call your cousin in the states and ask for help.
I think a BSPT pipe tap of the correct size might be run into the existing hole to re thread it. Because the NPT thread is so close to the BSPT size, you probably would end up with a semi-bastard configuration..but it would probably work fine. To be sure, instead of pipe dope, coat the thread with a two part epoxy and let it dry.
It's always better to start with a thread configuration that you have access to..swapping back and forth from this size to that size always ends up in disaster, not to mention a funky looking plumbing job. Stay with BSPT!
Go to your hardware store and pick up some threaded adapters and see which one is close. Don't use BSPP as this isn't a tapered thread.
Stuart
