I have been disassembling some stock 440 rods from pistons and reconditioning/balancing them and was wondering how much does balancing big/small ends versus overall weight matter. The reading on the internet is all over the map in my opinion. The sets I have worked on always have a rod or 2 that are out of wack compared to the rest. Stock rods coming out of motors probably in factory condition.

The latest set had 6 rods around 852gm with big ends at about 604gms. But #1 was 875, and #8 was 865. But the lightest rod at 851 had a big end at 612. I was able to bring #1 in at about 855 and 604, and #8 about 854 and 604 but clearly the density distribution of the lightest rod is way off. Swap it out or is that close enough considering total weights. This is not for a high performance engine, but just curious how this is viewed and not the first set of rods I have seen where one or two rods way off from the factory, or the big end versus total weight mismatch. Thanks,

i don't believe balancing is an exact science and it may depend on how "exact" the user wants to be. for me i've gathered up 2 or 3 sets of rods and picked 8 "good ones" out of the bunch. i think the factory service manual states plus or minus 4grams for rod weight but doesn't specify which end so i just took that as a total weight. i think the service manual does give specific small end/big end weights for hemi's. if you have several sets just try and match them up.

Big end (rotating) weight is 100% compensated in the CW.
Small end (reciprocating weight) is 50% compensated in the CW (for V8)
50 grams light on the small end = -25 grams in the CW
50 grams heavy on the big end = +50 grams in the CW
Net: +25 grams heavy in the CW

You could under one big end and hang it with the heavy one. Bob weights are set to one journal (2 assemblies). But I think you've done a good job to get where you are at with only 8 rods to work with. I don't think over all weight is that important. Nowhere on a typical bob weight work sheet is overall, just top/bottom.

Understand the process. Some methods have you weight big end and subtract it from total to get small end weight. Others recommend weighing both ends. So in the end total weight is in play. Just not used in the bob weight calculation. If I took off 8 grams to get big end to match, I would be well under the small end at that point as total weight should be down too.

I guess the point of big end compensated for at 100% matters, plus the small end is only a small portion of the reciprocating weight considering piston and pin. So better to have a difference on the rod small end, versus the big end which is the major contributor to rotating weight.

Appreciate the inputs.

if the pins are real long you could grind some off the ends, none floating type just a thought

would the pin weight be then added to the rod small end weight ?

Was under the impression you were to find the lightest small end and match the other 7 to that weight. Then do the same for the big end.
Why wouldn’t the overall weight be the same then?

Not any different than when you check weight distribution on a car. Measure the front then the rear. The total Is the total.

From the standpoint of geometry and physics, only the piston pin, rod bushing, and some surrounding material is reciprocating weight (and above the pin centerline it's moving in the opposite direction).
Only the crankpin, big end shells and part of the cap and beam are rotating weight (and below the crankpin centerline it's moving in the opposite direction).
The rod beam between these 2 centers is both reciprocating and rotating weight, with the percentage of each determined by its portion of the rod's length (not the weight).
More of my comments on my site here: http://victorylibrary.com/mopar/crank-bal.htm

You are correct, MY BRAIN misfired backwords LOL

"V" engines cannot be perfectly balanced. Even engines theoretically in perfect primary balance (L6, V12) have rocking couple.
The 50% frequently used in V8 is not mathematically calculated, but "what works".

You end up only using .5 of the recipt. less critical in my mind.Get pairs of bottoms the same(as a total), get small ends as close as possible. Remember we are dealing with 8 stock rods here kind of stuck with what you got. So you make the best of it.

Every machine shop that I have had balance my stuff uses the same procedures.
No exception.

Equalize the pistons, pins, small ends and big ends to the lightest one. Add each including the bearing shells and rings using a standard for oil volumes to the formula.

You can jerk yourself around all day about how this process was developed and find faults with it but it works.

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.

Like I said before - it’s no different than measuring a car across the scales. Measure the front and measure the rear and add them together and you get total car weight. In order to correctly weigh a rod the beam HAS to be perfectly horizontal and you need the proper fixture to suspend the rod at the small or big end. Using the wrong tools or using the correct tools incorrectly will get you the wrong numbers. Density, cap size, shape is irrelevant.

I tell you what I’ve decided even in my stock builds. By the time you have a competent performance machine shop, press the old pistons off, install ARP rod bolts, resize the rods, go to the extra work and the extra expense of matching/balancing the rods, checking the lengths and then pressing them back on the new pistons, you can buy a nice set of new rods. Even Molnar’s are in my builds budgets.

This person is dealing with 8 stock rods with one rod 20+grams off the others and variations end to end. Buy a set of h-beams, a gram here or there, no problem. Put yourself in their shoes. What do you do? Grind 10-20 gr off a cap/little end ? Do some of you even know what 10-20 grams of grinding would look like.These are the skills/knowledge we are losing. I see most people anymore just spend lots of money, let someone else do it, and watch youtube videos. This would make a great shop class test Just one more comment, you should grind perpendicular to the rods bores center-line.

(op)What are the 8 big/little end measurements now?

Man, it's crazy shops are charging you $700 to rebuild a set of stock rods. It's nothing like that around here.

Not sure if mentioned but my stock Hemi rods are all rounded on the small end, I don’t see how they could ‘make them the same weight’.
Same with the h beam rods I have

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