well a 12mm 1.75 or 1/2" SAE nut fits it. It goes on so far and locks up. So I was told it is a whitworth 1/2" 12. nope. Then a BSF 1/2" 16, nope. I think it is just a faulty part. The BSF goes on and will get the entire nut started then locks up. This is a Morgan part. I will tried the 10 but it won't go on the stem. For what they charge for this, they should've sent the nut with the stem
