Question about balancing rotating assembly
There is probably no spec. The rods are all weighed and all lightened to the weight of the lightest one. All the reciprocating weight is added to the crant throws (bob weight), balanced on a machine and weight is added or material removed from the crank throws to balance the assembly. You can "weight match" the rods but it's not balancing the engine.
^^^^^^^^^^^THIS^^^^^^^^^
Balancing a crank is not just science. The very best there are that do it don’t agree on much anything. There is (just to name a few) things that make balancing a crank as much an art as a science, and so many things affect balance such as crank materiel, block materiel, the way the crank is made, the rods you are using, displacement (yes, it matters) RPM (that’s a biggie if you want to start a fist fight about crank balancing...BTDT) and other things.
There is the standard 50% balance job, which even the best minds have a hard time determining where that number came from. I suspect it came about empirically, but can’t say it for fact. There is over balance (51 or 52 or more % of the reciprocation weight IIRC) and there is under balance (49 or 48 or more % of the reciprocating weight IIRC) and THEN you move on to the argument about WHEN to use over under or 50% (be ready to fight if you have a definitive answer because people have their ways and they don’t always agree) or if you should ever use it.
The fact is for most street or street/strip stuff if you get the rod ends to under 2 grams difference and get the crank under 20 grams out on either end it will be 100 times better than the factory balance.
FWIW, on my street/strip stuff I was under a gram on the rod ends and under 5 grams on the crank at 50%. On steel rod race stuff I was under a half a gram on the rod ends and 2 grams on the crank at 50%.
If you were running 400 inches or less (a totally arbitrary number I pulled out of my hat based on general small block Detroit engines) or 500 inches or more (again, an arbitrary number I based of basic BB Detroit architecture) the I would look at the Bob weight and power output and maybe use the above race numbers and add a percent or two based on the Bob weight.
FWIW on my aluminum rod stuff I personally ran and shifted at 8500-8800 I was at 53% over balance. Every time I looked at the bearings after a season, the more over balance, the better they looked. And that’s basically how you determine how you balance a crank. It’s like making hot dogs.....
