Quote Originally Posted by Frank S View Post
The other day my neighbor brought me a bolt and asked, do you have this size metric bolt......... As it turned out it was in fact a 5/8-12 UN bolt. I figure it was proprietary spec. by the manufacture in an attempt to make you buy replacements only from them
The new math, where 12 can be odd!

Regarding proprietary threads; a few come about through practice of 'design for manufacturing'. A lot of measuring tools have this, they sought a certain degree of thread engagement [or movement per revolution] not offered in standards like NC or NF. Included here, the continued use of old drawings and equipment. The replacement part was seen as a side benefit; being that historically the era's products outlast anything offered later.