Hy 46Sparky, I think that hole/port is a 1/8" pipe hole for mounting a hydraulic brake light switch. 1/8" pipe thread is 27 threads per inch, 3/8" u.n.f. bolt thread is 24 threads per inch, pipe thread is tapered and the more you tighten it the better the seal. I can't see them using a regular bolt thread anywhere that intersects with the hydraulic system unless there is a flat machined surface for a copper washer to seal on, hope that helps.
