Cotten wrote: But by back-calculating a Factory Pan, I found it to be about 57%, so I suspect the "factory service school" was making it up as they went.
And "getting the weight of the connecting rod small ends close to matching each other" sure sounds good.
The problem is that the top ends can never weigh the same, unless you use two male rods!
Hey Cotten, I don't really disagree, but the 57% factor you found does beg the question, Did the engineers specify 50% and poor quality control resulted in 57%, or did poor record keeping over the years result in the claimed 50% figure? The reason that I gave all of those figures was to illustrate exactly the point that you make; that balance factors are somewhat arbitrary.
As to the issue of getting the reciprocating weight close between front and rear cylinders, of course you are right that there is nothing you can do with stock rods. Still, S&S felt it was worth retooling their rods to get closer, so I have to think that it is a legitimate issue. Of course, I believe their ulterior motive was to allow them to use several different piston weights without changing the actual balance of the flywheels.
How about this. Since the reciprocating weight includes not only the wrist pin end of the rod, but also the piston assembly, what if a guy were to use a lighter wrist pin in the rear piston? I just went back in my records and found 5 XA rod sets that had differences in front and rear reciprocating weights that ranged from 5 to 21 grams (rods only: not including pistons). A stock wrist pin weighs about 93 grams (probably some variation there too). Axtell used to sell taper wall pins that weighed 83 grams and they still carry tool steel straight wall pins that weigh 67 grams. Looks to me that it may be possible to get those total reciprocating weights pretty close in some situations. Would it be worth it? I don't know, but it would be interesting to find out.