Connecting Rod balancing question

Hap

Member
Been MIA for way too long...
Back to the process of building a new SBC bullet and wanted to confirm something about balancing connecting rods.

My memory from prior engine builds is that the big end weight plus small end weight almost never adds up to the total rod weight, and this number is usually lower than the total rod weight.
Is this correct?

Am now questioning this as have seen YT videos where they only take weight off the big end to get the total rod weights all even.

Appreciate any feedback / comments.

Cheers.
 

Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member





http://www.enginebuildermag.com/2014/03/weighing-balancing-work/

http://www.hotrod.com/articles/ccrp-0803-engine-balancing/

https://www.amazon.com/Proform-6646...rd_wg=YpFUS&psc=1&refRID=AASHNARD6A99RCQQ18EH

pro-66844_w.jpg

http://toplinedigitalscales.com/300...le-weight-modes-ACDC--FREE-SHIPPING-p523.html

http://garage.grumpysperformance.com/index.php?threads/belt-sanders.13148/#post-68577

http://garage.grumpysperformance.com/index.php?threads/what-con-rods-would-you-buy.942/

http://www.hotrod.com/articles/ccrp-0803-engine-balancing/

http://www.angelfire.com/ca4/CorvAIRCRAFT/RodBalance.html


https://jet.com/product/American-We...650-G-X-01-G/78d673f7668d470d9a93b875aa96928c
American Weigh Scale Ac-650 Digital Pocket Gram Scale, Black, 650 G X 0.1 G _ Jet.com.html

http://garage.grumpysperformance.com/index.php?threads/connecting-rod-strength-h-vs-i-beam.1168/

http://garage.grumpysperformance.co...kshaft-and-connecting-rod-compatability.9320/

http://garage.grumpysperformance.com/index.php?threads/measuring-crank-bearing-journals.5478/

http://garage.grumpysperformance.com/index.php?threads/engine-balancing.3900/#post-57943

http://garage.grumpysperformance.com/index.php?threads/measuring-rod-and-pin-heights.3760/

http://garage.grumpysperformance.com/index.php?threads/cast-or-forged-rotating-assembly.253/

http://garage.grumpysperformance.com/index.php?threads/rod-bolts-rpm-vs-stress.341/#post-68856
 
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Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member

generally the process starts by weighing all the connecting rods
the basic process is that your trying to produce a matched set where all 8 connecting rods wight and measure as close to identically as you can get them.
you won,t generally be doing much to the better aftermarket rods because the manufactures take a good deal of effort to package and sell them in matched sets.
the lightest rod is generally selected as the target weight and accurately weighted, on both ends and in total , the the others in the set are matched in weight buy use of a belt sander very carefully used to remove very small portions of the connecting rods counter weight balance pads while you constantly test weigh the rods until the weights match.
and yes you'll constantly and carefully inspecting them for flaws and obviously it will help to have some precision measuring tools like inside mics.

ENGINE BALANCING

http://garage.grumpysperformance.com/index.php?threads/engine-balancing.3900/
balk10.jpg

http://www.angelfire.com/ca4/CorvAIRCRAFT/RodBalance.html

http://www.angelfire.com/ca4/CorvAIRCRAFT/RodBalance2.html

I posted a message to the CorvAIRCRAFT e-mail list, and I shortly had my answer.



balk1.jpg



Don McGehee sent me a private e-mail with this attached drawing of a jig which was drawn from memory of one he saw in a hot rod engine shop.

balk2.jpg


From his drawing, I made this jig.

I turned 2 phenolic disks which slide freely over a shaft, which lowers friction and allows me to change the disks from one shaft to the other, for balancing each rod end.

Although the centers of the shafts are the correct distance apart, the chain all but eliminates any unwanted side loads.

I've been able to remove and replace the connecting rod on this jig, and if it's not exactly the same weight when I make the change, it's only 1g difference.





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http://garage.grumpysperformance.com/index.php?threads/connecting-rod-balancing-question.13403/
One good thing I've found out from this exercise is that the stock connecting rods are very close anyhow. From my heaviest to my lightest, there's only a 7g difference.
 
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Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
“Connecting Rod Balancing”

By Ted Eaton

An engines connecting rods exhibits traits of both rotating and reciprocating mass and hence, must be match weighed end for end to insure these two masses are kept independent of each other. As a point of clarification, the reciprocating end is the small end of the rod or the portion of the rod that is representative of up and down motion in the cylinder while the rotating end of the rod is the bearing end which rotates with the movement of the crankshaft. Your balancing shop will have a rod weighing fixture that’s designed for separating these two masses and then being able to have all the rod small and big ends match in weights throughout the particular set of rods being balanced.

Simply finding the lightest rod in a set for total overall weight and then reducing the weight of all the other rods without any regard to which part of the rod the weight is being removed from to match the lightest does not make for a balanced set. This is because the weight being removed is most likely being taken from the wrong spot on the rod and thus actually making the rods even more out of balance than before attempting to weight match them. This method fails to take into account whether the mass being removed is reciprocating or rotating mass which is a major consideration in a dynamically balanced engine.

Connecting rod balancing requires a fixture that allows each end to be weighed independently. There are several different fixture designs available on the market but all utilize the same concept; each end of the connecting rod is isolated from the other for weighing purposes.

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Similar in concept to the match weighing of the pistons, the ends of the rods must be weighed with the lightest small and big end of each rod within a set being found and isolated. Very rarely will the same connecting rod from a factory installed set have both the lightest small end and lightest big end on it. After finding the lightest ends, it is then just a matter of taking the remaining heavier rods and making the ends match the previously found lighter end weights.

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Your balancing shop can employ one of several different methods in which to reduce the connecting rod end weights. Typical tools for this operation can vary from using a grinder, belt sander, or a milling machine. The design of the connecting rod in itself can dictate what machining or weight removal operation will be used. Most stock style connecting rods have a balancing pad on each end which is a convenient spot from which to remove material for balancing purposes. Many of the newer aftermarket rods and especially the H-Beam style do not have these balance pads on the ends and do require some forethought before attempting to remove any material from them. For many of these newer designed rods, material from the big end is removed at the rod bolt edge instead of the very bottom. The small ends for rods without balance pads are usually best done on a belt sander using a nice rounding motion in which to remove material evenly from around the pin end. Regardless of the method used for weight removal, it’s important that the metal not be unduly overheated. This may require repeated quenching if excessive grinding must be performed in which to remove the required amount of material. Excessive overheating of the big end can cause out-of-roundness to the big end bore which can prove disastrous to bearing clearances besides affecting the structural integrity of the metal itself on either end.

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After all the connecting rods have been weight matched, the reciprocating and rotating end After all the connecting rods have been weight matched, the reciprocating and rotating weights are then recorded on a balance card or work sheet for the upcoming bobweight calculation. All that remains at this point is to clean the connecting rods of any debris or grinding/sanding residues caused by this particular balancing step and rebox them until engine assembly takes place.

The next article in this series will cover the nuances involved within the bobweight calculation in preparation for spin balancing the crankshaft. Until then, Happy Motoring.



 

Hap

Member
Thanks to all that replied!
May have stated my question wrong, as I know the process of balancing rods, have the proper holding jigs, good scale, etc. and have balanced many sets (mostly aluminum) of rods in the past.
I balance the rods and pistons, calculate the bobweight and provide my numbers to the machine shop that balances the crank.

My question is: for those of you that balance your own rods, do you find that the big end plus small end weight never equals the total rod weight?
For example, the total rod weight is 590gr; the balanced big end weight is 400gr, and small end is 166gr.
400 + 144 = 566gr, which is lighter than the total rod weight of 590gr.

Am just curious about this stuff now that I'm older, as never gave it much thought before.
However none of the engines I balanced before blew up due to vibrations.

Grumpy is correct in that the current aftermarket production steel rods are sold weight matched, and are close to within less than 1 gram.
My rod jig looks similar to the one below.

pro-66844_w.jpg
 

Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
think about that question a second
My question is: for those of you that balance your own rods, do you find that the big end plus small end weight never equals the total rod weight?

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the scale indicated weight on either end of the connecting rod as the scale will indicate it, when combined with the the scale indicated weight on other end of the connecting rod as the scale will indicate it can NEVER equal the total weight of the connecting rod weight as a single unit for the simple reason that when a single end is weighed the other end is partly supported by the holding jig, that non-weighed end is exerting some weight on the jig itself

 

Hap

Member
Thanks Grumpy your explanation makes perfect sense to me...and as I said never gave this much thought until seeing that video talking about how they balanced con rods.
If I recall correctly, they first match all the big end weights, then weigh the total weight of each rod. Any difference is then taken off the small end; as the small end weight should equal the total rod weight minus the big end weight.
Suppose in theory this should work.

I'll just keep at it the same way I've been doing for the past 30+ years.

Thanks all for the replies.

think about that question a second

balk7.jpg

the scale indicated weight on either end of the connecting rod as the scale will indicate it, when combined with the the scale indicated weight on other end of the connecting rod as the scale will indicate it can NEVER equal the total weight of the connecting rod weight as a single unit for the simple reason that when a single end is weighed the other end is partly supported by the holding jig, that non-weighed end is exerting some weight on the jig itself
 

NOT A TA

reliable source of info
think about that question a second

balk7.jpg

the scale indicated weight on either end of the connecting rod as the scale will indicate it, when combined with the the scale indicated weight on other end of the connecting rod as the scale will indicate it can NEVER equal the total weight of the connecting rod weight as a single unit for the simple reason that when a single end is weighed the other end is partly supported by the holding jig, that non-weighed end is exerting some weight on the jig itself

The rod cap in the pic above is to the right of the pivot point on the stationary end counteracting some of the weight of the small end of the rod.
 

Strictly Attitude

solid fixture here in the forum
When I balanced mine they did add up to the weight I used a small level to make sure I was absolutely strait. I also was able to use an expensive calibrated scale from job. There was a 2 gram difference between the cheap one vs. The expensive one. But the cheap one let me get most of it done at home prior to doing mine during my lunch break. I found the cheap scale hard to keep results consistant.
 

Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
yes quality tools are rarely dirt cheap,
you DO tend to pay more for better and more accurate precision tools
paint, marker etc. tends to wash off, you really should lightly die stamp the main caps

https://www.harborfreight.com/36-pi...mping-set-63675.html?_br_psugg_q=number+stamp

rods that use bolts with nuts like pictured below will be weakened if excessively clearance ground
rod-grinding.jpe

stroker profile rods offer more clearance to cam lobes, and yes the stroker clearanced profile rods are available in both (h) and (I ) beam designs
scatrdx.jpg

http://garage.grumpysperformance.com/index.php?threads/precision-measuring-tools.1390/#post-68861
 
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Hap

Member
"you DO tend to pay more for better and more accurate precision tools"

Right again!
When balancing rods, the main factor is making sure your jig can replicate the weighing points and maintain a consistent rod center distance.
If the holding jigs bind or the weighing points change even a few thousands, obviously the numbers will come out different.
 

Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
Video: How An Engine’s Rotating Assembly Is Dynamically Balanced


By GREG ACOSTA OCTOBER 03, 2018

An engine’s rotating assembly is nowhere near “lightweight.” Sure, you can get a “lightweight” crank and rods, but that’s an extremely relative term. The fact is that you have a large mass spinning at significant speeds, and that means that even the slightest imbalance in the assembly will be magnified many times over.

On the minor end of the spectrum, an imbalanced (or improperly balanced) rotating assembly can lead to annoying vibrations felt inside the vehicle. On the opposite end of the spectrum, a significant enough imbalance can lead to high-speed unplanned disassembly of the engine, while it’s still in the engine bay.

Knowing how critical it is to have your rotating assembly balanced, most people undertaking a performance build will accept the extra cost associated with the task and have it done. However, do you know what’s actually happening when the assembly is “balanced”? YouTuber Eric The Car Guy put together a comprehensive video which not only shows the process in detail, but explains what is going on as they balance his Forged 302 Windsor rotating assembly.


When weighing the rods, the big end and the small end are weighed separately as the ends move in different manners. Once all of the rods have been weighed, step one on the overall balancing process is to get the group of rods to within 1 gram of each other.

Step one of the process is to weigh both the “big end” and “small end” of each individual connecting rod. Each end is moving in a different manner – the big end rotates, and the small end reciprocates – so the weights of the individual ends is important. As Eric points out in the video, with today’s manufacturing processes, getting eight connecting rods to be only a few grams apart from one another, out of the box, is far more common these days, and shooting for the +/- 1 gram variance is an easier task.

Once the rod weight measurements are completed, and the group of rods weight-matched, the whole assembly for one cylinder – connecting rod, wrist pin, piston, and piston rings – is weighed and a little math applied, to create the target weight for the bobweight. The bobweight is a weighted clamp which will simulate the two rod and piston assemblies which will normally ride on each crankshaft journal (in a V8 application). Once the target weight is achieved, the bobweight is attached to the crankshaft before it is spun up to speed on the balancing machine.


The bobweight is carefully calibrated to simulate the weight that will be on each crank journal (two complete rod and piston assemblies, in this case). It will then be bolted onto the crank journal for spin balancing.

In an externally balanced application, the balancer and the flywheel or flexplate needs to be mounted on the crankshaft, as they play an integral part of the balancing equation. Once those are mounted on the crankshaft and the bobweights are affixed to the four crank journals, they can actually spin the assembly up.

One the machine goes through its technical magic, it tells the machinist how much weight needs to be added or removed, and where that weight is needed, in three dimensions. With that information, the machinist can then drill holes to remove weight, or drill holes and press or weld in heavy metal slugs (which are heavier than the removed material) to add weight.

Once the assembly is balanced within 1 gram or better, it is called good, and ready to be installed in the block. While this video is a bit on the lengthy side, it offers a great look inside of a very common process you may or may not be intimately familiar with.


This is the initial readout from the balancing machine. It is indicating that 194 grams needs to be removed from the front of the crank at 14 degrees of crank rotation, and 223 grams need to be removed from the rear of the crank, at a radial location of 41 degrees.

 
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