BSA Nut, manifold to head (1/4" British Standard Cycle)

This is the small nut that clamps the exhaust or inlet manifold flange down to the cylinder head, cut to 1/4 inch British Standard Cycle thread. It's a fastener you'll find in multiples around the head, holding the manifold stubs tight against the head so there's no leak path for exhaust gas or intake air at the joint.