I am in a machine shop of a retired racer. So we have all the equipment but it is 30-40 year old stuff not state of the art. I pressed the pins out, cleaned the rods, have a balance beam scale and the correct fixtures for weighing and hanging rods. Not the state of the art stuff, but older. So part of this is hobby learning and doing. My time no cost.

Regardless, the potential to induce an error by summing multiple measurements is greater than a single error on a single measurement (usually). Having measure rods before, my experience has been that measuring both ends never seems to add up to total weight. Even with best effort to be consistent in measurement, level, etc.... For these rods the sum was 4-6 grams different than a measured total.

So I always measure total weight of assembled rod first. Then measure big end. Then inspect why any major anomaly. In this case the heaviest rods had pads that where larger then the 6 rods that were equal in weight. Those two came in nicely with weight reduction of the big end. The problem was the lightest rod had the largest big end weight and its pad looked similar to the other 5 rods of near equal weight. So it had to be a density of the metal issue. When I measured the small end it was smaller then the rest too.

These rods all looked like same batch too. Same group letters and numbers. In this case DF-21 thru 23, and B on opposite side of LY. All the pistons where stock 1970.

I can't really reduce the big end of the lightest rod without driving total way lower. I then would have to reduce small end on all those other rods with out the meat to remove so it would be a spiral effort down.

I do have other rods I can swap in, but more of my curiosity was how critical is this really. How to approach this if you had to use it. Which I think has been answered. Moving rods to try to get pairs matched. Error on small end is less of a factor then big end. Assemble balance weights to account for it. We have that counter weight material too. We have a crank balance machine, unfortunately not fully operable, so final balancing is done by the local machine shop.

I will post where I am at later, left the info at the shop. Thanks for all the replies.

I think Transman is right. The total weight has to equal the sum of the individual weights. Differences in density or form that would shift the center of gravity along the axis of the rod would be reflected in different small and large end weights but it can't affect the total weight.

They were closer to being equal weight when made due to better controls and smaller volumes.
Every Hemi rod I have seen or used had contour machining on the small end so they were very close to start with.
They need very little work to get them equal.

I’m betting same applies to the aftermarket rod.

If you saw the rod machining line at Chrysler you wouldn’t have to wonder why the rods were all over the map.

That has all changed with PM rods and new forging processes.

Great subject and mis-understood by most lol.

You won't believe how many folks change pistons and don't get the crank re-balanced???

I always cut as much off the lightest rod and then bring the others down to match!
Big-end weight is the one to get the most off, it counts for twice as much on the crank CB.

Most Sixpack rods will come down to stock LY weight with clever grinding, polishing etc.

Some sets are a 'LOST cause' due to mis-matching or careless parts swapping from different engines.

I once bought a 340 with 3 different style of rods in there...

Best combo I have found is lightened Sixpack rods with KB Hyper pistons in a 440.
Goes straight on the stock steel crank without much 'jiggery', with balancing.

If you have ever worked on 70 and 71 Sixpack engines you will know how tricky it can get...

As any sensible engine builder will tell you.

''50 year old rods are doomed to fail, so go new''.

I did an engine for a Moparmate and the 'press-fit' sixpack rod went soft and the pin migrated.
Wrecked the bore and cost a rebuild, on my expense...

Factory cranks were balanced to a set 'Bob-weight'.

Then the rods were end to end balanced to an approx weight as long as they were fairly equal.

Some engines were close, others a country mile out!

Out of balance can manifest at different RPM's like a road wheel.
Sometimes you feel it and then you go a bit faster and it goes...

Somewhere around 3000 and 6000 rpm's.

You balance the crank at 1500 and the 3000 for better effect.

I've done hundreds of Mopar cranks and only ever found ONE that was right, a 340...
Big-blocks are 'suck and see' factory guesstimate...

[/quote]

It is not that simple because as mention the physic/geometry.

I measured the small ends today, and remeasured big ends. Then remeasured total, the problem is the added values do not exactly match total weight. One end is suspended and only one is on the scale. Plus there is a density of the rod issue and you do run into rods that have seem to have a higher density. I examined the pads and they are similiar in shape. Right now I have all the rods total weight correct, but there are differences in 2 rods where they have lighter small ends, but heavier big ends. I am also almost out of small area pads on these rods.

These are way better then as installed by Mopar where one was about 25 grams off, but frankly at least one rod is not really workable with the set. I do have a replacement I can use. It was just the weirdness that the lightest rod had the heaviest big end and a lighter small end. [/quote]

I think Transman is right. The total weight has to equal the sum of the individual weights. Differences in density or form that would shift the center of gravity along the axis of the rod would be reflected in different small and large end weights but it can't affect the total weight. [/quote]

I agree and understand the physic, but it is a very delicate measuring process. The ability to make those 3 measurements and get them to add up is not easy. Even cars require level garage, unsprung suspension, etc... Doing a big end has more of the rod over the scale, doing the small end less rod. Vertical and horizontal level of the hanging rod is critical to an accurate measurement.

I have weighed these rods 5 different times over 3 days and I can always get total weight to match. You get a few grams of difference when doing small or big ends. The whole key is repeatability. I trust the total weight to with in 1 gram, I would say the big end is within 5 grams. Not ideal for a race motor but probably pretty good for a street motor. Here is where I am at with measured total, big and small.
#1 852; 605; 242
#2 853; 605; 241
#3 852; 604; 240
#4 851; 605; 239
#5 852; 605; 238
#6 851; 604; 237
#7 851; 610; 234 Problem rod
#8 853; 604; 243

My latest measurement yesterday after adjusting hanging beam resulted in same total weights, but saw an increase of 5 grams on big end. Which would result in similar calculated small ends as weighed day before.

Hang it with #8 and forget about it ,use 606 for the bottom 240 for the top. just my

I assume you are working to a known 'bob-weight'?

You do end to end balancing along with beam 'parting-line' polishing to equal the whole rod.

Yes you can put a heavy rod with a light one and then mix and match pistons as well.

But anyone who have balanced a crank knows the slightest change can affect the overall balance.

This is why we buy aftermarkets, the whole balance issue and durability is removed.

In the production world we call this Gauge R & R.

Check that out. It will show you what you would have to do in production.

The variation can be understood and controlled.

But for what is being done here - I think you are close enough.

Aren’t all the rods forged? For me it’s hard to see difference in density, maybe just less metal around the small end? Measure inside the beam to see if a different thickness in that area?

Maybe homogenous is a better word. Regardless, the rod is different then the other 7. I inspected pads and they are similar, and as stated they are of a similar manufacturing lot. When I started #1 and 8 were heavy, #1 being almost 25gm heavy. Pads clearly looked like they were not machined like the others. But those rods came in symmetric to the others. # 7 always was lighter overall and heaviest big end. Seen this in other rod sets too. Consensus is it happens, and replace it if you can. I was just trying to balance them, not lighten them for racing.