"Back then" a Chev line mechanic once wasted some two hours on this. Came into the store, he had already been to our fasteners shop "down the street". He had a bolt with a 10mm head "so he knew" it must be metric. The bolt shop was no help they could not identify it.........because it was metric, and was an odd thread
No, boys 'n girls, it was 1/4 USS thread.............with a 10mm head
I believe some of the magnums use these on the valve cover studs
