L6 engine balancing?

Traditionally, L6 engines of any type have excellent (but not "perfect") balance, with minimal vibration.
They all have counterweights, yet many comments suggest that "they don't need to be balanced", when of course they already are.
My question: assuming the engine will see severe service, any reasonable method to keep the lower end together is worth examining.
The question: has anyone had any L6 balanced? What was done, and why?

[please: don't cut-n-paste from Wikipedia, we all know what it says]

Thanks!

Some Slant guys balance, some don't. I have had my 232 over 7000 a couple times, normally shift at 6500. It was not rebalanced after we changed rods and pistons.

Don't all in line six motors fire 120 degrees apart from the previous piston and cylinder, two pistons at top dead center at the same time correct? Is there any 180 degree in line six motors?
That being said why wouldn't you want to balance the rods, pistons and crankshaft like any other motor?
I know most of the factory specs on balancing production motors is no where near as good as any decent Hi Po balance job would be, + or - one gram on all of the rotating parts
If it is apart and you want to twist it tail to make it howl, balance it

720° ÷ 6 = 240°, firing order is the usual 1 5 3 6 2 4, one pair of rods & pistons at opposite ends of the center main are at the same position (but different parts of the combustion cycle). 1 & 6, 2 & 5, 3 & 4.

Balance? But... how do you do that?

Not real sure of what you’re asking, but the cranks do have “cast in” drill bosses for balancing. If I remember correctly, the cranks are under counterweighted, and the drill bosses are on the rod end of the crank.

And the counterweight mass is determined by...?

I'm not talking about the math, aren't all in line six cylinder rod crankshaft throw 120 degrees apart, 3 throws divided into 360=120 2X120=240 240 divided into 720 =3 6 divided into 720=120, correct?
All the four cylinder crankshafts I've seen are all 180 degree cranks

I know when I did the stroker 4.0 I6 for my Jeep I balanced all the rods and pistons but the shop spun the crank with no bob weight on it.

Yes, the pins are at 2 at each 1/3 of a circle, but rotates twice for a complete combustion cycle.
The L4s have to be, 720° ÷ 4 = 180° .

w/r/t "the shop spun the crank with no bob weight on it"
That's where I get nervous.
If the reciprocating weight is zero-ed out by the other piston doing the same thing that only means that no compensation is needed for the piston, rings, pin, and rod small end.
But... isn't the rod big end still rotating weight, which the counterweight must oppose?
If not, the crank has been "balance factored" (including only some of the weight) like a V8... by accident.

Eeek!

I don't recall if he had to add or subract wieght from the crank, I was standing right there when he did it but that was like 14 years ago. It runs very smooth though I can give it that.

Note: the "perfect" L6 balance is not possible due to the engine (any L6 TIKO) design, some rocking couple always exists.
It's not a quality control problem, you just can't have the counterweight exactly opposite the crank throw - the compensating weight is always offset to either side.

The issue as I understand it is a harmonic that comes into play just over 6K. The engine would hit a wall power-wise. This was a struggle to "beat" in a race-only build I was party to. The engine was a Chevy stovebolt. Fixes included aluminum rods, severe lightening of the crank (really cut down counterweights), and high quality balancing of the assembly. That allowed the engine to pull cleanly to 7K. It's been many years but with methanol and single turbo it was running in the low 9s in a Studebaker.

high quality balancing of the assembly

... and what was done?