So on a 300-6 water pump there is an approximately 1.25" diameter boss cast on the back of the main outlet. On some pumps, this boss is left alone while on some it is drilled, threaded, and plugged.

I've noticed a difference that on pumps meant for older 300's (app. prior to 1985) this boss is always there and plugged while the newer ones are left untouched.

What is this boss/hole there for?

 

I've seen this also,and don't know for sure what was intended. These engines were,have been, and are used in everything, including larger trucks. My guess-- cooling for an air compressor for air brakes. Someone here will know, or try the large truck forum.OBCB