Not sure which part you are disagreeing one ????
No.... the 318 and 273 were set up early on with the same bobweight. The 273's used a heavier thick wall pin to make up for the lighter pistons and reach the same reciprocating weight portion. (Where they MAY deviate is when the 318's switched to the heavier rod in the mid 70's... still digging on that question....)
But this is why I have presented the info that I have.... I have been digging up and measuring weight info for quite some time on all of this. Actually computing bobweight when the data is known is trivial.... the formulas are easy to find.
- 273 2 BBL ACTUAL WEIGHED AND CONFIRMED: Bobweight = 2168 grams (4 BBL pistons weight more but the pins are lighter so the piston + pin weight is the same, so bobweight is the same.)
- 5.2 Magnum from reseached part weights: Bobweight 2161 grams
If you have better numbers or actual part weights, I'd really and truly LOVE to have that info. But if you don't have better info, then I am not sure on what basis you are disagreeing.....????
The 340's were set up for a different bobweight. Not any issue there.
One other aspect of balancing that most folks don't realize: It is rare to actually do a 100% rebalance on all 4 journals. Usually what happens is that weight is changed on the 2 large end counterweights. This does nothing to directly change the weighting for the 2 central crankpins (for cylinder 3&4 and 5&6). What is being counted on for those 2 is that, IF the rods and pistons are reasonably well matched, then the 2 pairs of pistons counterbalance each other for the most part. It is more complex than that, but the bottom line, is that 'balancing' only really goes part way to an optimum balance. Since the small counterweights by those 2 center crankpins usually go untouched, and the other small counterweights by the end crankpins often don't get touched, then it really is not all that well optimized. Field re-balancing not as scientific as you might think.
So, that is why for lo-po stuff, you can swap out STD bore pistons with overbore ones, never touch the balance, and never feel it. The bobweight changes but not all that much as to be felt in lo-po use.