Looks like an easy solution to get your fan clutch to come on earlier or later, so your engine will run cooler or warmer. Seem that gm designed there bbc to run hotter in 68 to reduce emissions. So if you have bought a fan cluth that was built after 68, it was designed to activate at a higher engine temp.
http://midamericachevelles.com/Fan_Clutch_Adjust.pdf
Looks like an easy solution to get your fan clutch to come on earlier or later, so your engine will run cooler or warmer. Seem that gm designed there bbc to run hotter in 68 to reduce emissions. So if you have bought a fan cluth that was built after 68, it was designed to activate at a higher engine temp.