Couple things I've heard from old time engine rebuilders years back.
1. Its OK to balance numbers 1 and 2 pistons and rods the same, then to balance numbers 3 and 4 the same, but its not necessary that all four be the same.
2. Rather than take metal off the late style "lightweight" pistons to achieve balance, its OK to add weight to the three lightest pistons by pressing in part of a head bolt shank into the wrist pin hole.
What do you think?
Personally? Remove metal to even them out. All 4 as close as you can get them.
Yeah, What Charlie said. Head bolt shanks pushed into a wrist pins sounds like a good way to score up some cylinders. And I've never thought it was smart to put extra foreign material into anywhere it didn't normally belong in an engine. Just get yourself a good scale and remove material to get all 4 as close to each other as possible.
If it were mine, I would spring for aluminum pistons. If I were to run the later cast iron pistons I wouldn't worry about balancing them or the aluminum ones either for that matter.
I have not personally studied any aluminum pistons for our cars. If they are designed like modern pistons, they should have a "balancing pad". That would be an area with a little extra metal, designed so that it will not affect the strength or function of the piston when material is removed for the purpose of balancing.
FWIW
Roar
You want your pistons all the same.
Rods that weigh the same, is not balance, and really does not do any good, until both ends all weigh the same, and then are the same total weight.
Where the mating comes in at, what you were talking about Richard, is when the rods were all balanced, and you had two rods of one weight, and two rods of another weight, may be a gram apart, them you put together as a sets on 1 & 2, and 3 & 4. But you want all the pistons all the same. We zero everything!