170 harmonic balancer bolt size

What is the proper size and length of the harmonic balancer bolt on a 170? I got a 3/4 coarse thread 1.5 inches and it doesn't feel like it's the right size. It's a 66 170. I want to have a crank bolt on it so I can turn the crank if I need to adjust timing and I just feel more secure that there's a bolt there even if it doesn't make a difference.