After much research I found my manifold holes had been enlarged so I had to make a stepped stud. In my case a 7/16” UNC for the manifold and the standard ⅜” UNC/BSW (see below) for the brass nut.
Standard studs are ⅜” UNC both ends, and I used a 2” stud and purchased ⅜” BSW brass exhaust nuts as I could not obtain UNC.
Also used ⅜” spring washer, not sure how long the high tensile spring would last though.
Note:
TPI COUNT
The reason that almost all BSW and UNC threads can be interchanged is that they have the same thread per inch count (TPI). This term means the number of threads present within a radial measurement along the fastener of one inch.
THREAD ANGLE
The angle of the thread is slightly different, with BSW having a 55 degree angle and UNC having a 60 degree angle. The thread angle is the term given to the name for the angle between two thread flanks.
However whilst there is a 5 degree difference, it is not noticeable to the eye – and both threads will still screw into each other.
EXCEPTION TO THE RULE!
Most rules have an exception and this one is the same!
The above is true for all thread diameters except ½” BSW and ½” UNC.
This is because in this ½” thread size only, BSW has a thread count of 12 TPI whilst UNC has a thread count of 13 TPI. Frustratingly close but enough to make them non-matching!
