[vector1]

Okay, I know what you are talking about, had to dig through two books to find it. I'll type fast....

The high performance 289 crankshaft is of higher nodularity than std, and comes with add-on counterweight for optimum high-rpm use.

additionally

the hp 289 unitis of higher nodularity than others, and it also comes with a .150 thick add-on counterweight at the front to improve high-rpm balance. If you are using this crank, with the extra counterweight, be sure to use the proper timing chain crank sproket; and if you want to install a multi-index crank sprocket, mill .150 from the back surface to retain proper alignment. Ford used two different types of timing gears, a thick set and a thin set. pre 65 engines use the thick gears and timing chain, and crank sprocket has a spacer that fits behind it. all 65 and newer engines use the thinner chain and gears, and crank sprocket has a shoulder which spaces it properly from the crank. Entire sets can be interchanged from one engine to another, but do not mix componenets from a thick or thin set.

I also think the counterweight was for the thicker rod bolts or heavier rods, but just a wag. Your machinist is going to balance it in the end, the counterweight deal might have been a bandaid.