Quote Originally Posted by Neale View Post
Could you clarify what you are trying to fit to what? Somehow from what you've said you I don't think that you will find an off-the-shelf fitting but if we know what's needed it will be easier to advise. For example, is your "normal everyday 3/8" thread" UNC, UNF, BSF, BSW? Not to mention the finer threads in the ME series... Or maybe nearest metric size? It's all a bit of a rat's nest

BTW, I don't know of any thread where measuring the inside of the thread (as per your picture) will give you a recognisable dimension. Usually threads are measured over the outside - in effect, the diameter of the rod that an external thread was cut on. You can use internal diameter to find actual thread size but you will need to know what you are looking for in thread tables (possibly core size, give or take a bit of clearance).
Ok Neale thanks for the reply, I'm trying to get to a point where I can screw in a 3/8" bolt to the bush after I've ground the head off and fit it into a chuck capable of taking up to 13mm. if that makes sense ?