Maybe it's a bit fuzzy after 40 years, but I could be wrong. I think I probably meant "within one gram", which would be +/-.5 gram. I haven't read the articles I linked to in a while, so I don't know what a "precision" balance would be these days. You definitely hit a point of diminishing return with this stuff, and it doesn't make any difference beyond a certain point. To do it to .001 gram, for instance, would be meaningless. I know my machine shop had State-of-the-Art Stewart-Warner gear, and was highly respected by the "pro" guys. He cost a bit more, but did superb work.Some good reads in there. A few that I've seen before but that Hot Rod page in particular was a new one. Pretty much reinforces my thought that the crank and lower rotating parts need to see a machinist. No way in hell could I afford a balancing machine. I could probably build a gravity balancer out of some old flexplates and bearings with a steel frame, but that seems a little excessive for the moment when a machinist will typically do the job much more accurately with a digital balancer for $50 or better. Maybe an interesting endeavor for the future, though 🤷♂️
An entire gram still seems like a large deviation for the effort, imho. There are manufacturers that sell sets within that threshold. It shouldn't take much longer to get down into at least the tenth range.
I was looking at one of my old pistons today and did notice how much material there is around the wrist pin holes that could likely be removed safely. I've also read that the bottom of the skirts is a good area to take from if you make sure to smooth it out well while grinding.
Besides, my Dad knew his Dad from the Navy, and my Dad had sold him his Bridgeport mill, Logan lathe, and Hisey-Wolf surface grinder.
As far as balancing the pistons, cast pistons are generally closer together than forged ones, but I don't remember the percentage difference. I've seen people drill holes in the pin boss area to remove metal, but "My Machinist" didn't like that because it left sharp edges that could lead to stress cracks, fatal in cast pistons, and definitely Not Good even with forged ones. That's why he milled the bosses equally to lighten them.
I saw him do a crank once, and it was very interesting to see how they detected the amount of unbalance, and it's position on the crank, so you'd know how much to drill, and where. For the amount to drill, he had a chart from S-W indicating drill bit sizes and depth of vs metal amount removed for different metals. "The Book" also had tables and charts for the bobweights used on the crank throws. Really cool stuff for geek like me to learn about.
I'm sure things have changed int the last 40 years, but the basics are still the same. As Mr. Scott would say "...Cap'n, I Kenny Violate The Laws of Physics!"