Best way to balance the rotating assembly

[bigern45]

What is a best way? And then a way that will suffice or be better than just throwing rods in a car? JHM website says some have installed rods without any issues, some old hot rodders in town have suggested just assemble each rod and piston, weigh each one and make the rest weigh as much as the lightest one? Obviously good results with that in older big block fords and Chevys, but what are there's and cons of doing that with our 2.7??

And in the end, wouldn't this be specifically what a machine shop would do anyhow??

[A62TURBO]

Don't really understand but balancing a rotating assembly should be done by a machine shop with proper knowledge.

[julex]

It depends what you're doing. If you just replace rods and replacement rods are within few grams of original ones and are matched between each other weight wise, then you can get away with not balancing. Prime example of this is replacing stock rods with IE rods which weight almost the same to the dot.

If the weight is different, you plan using some other pistons, it is a stroker oryou want to rev unusually high - have the shop balance the assembly. Not only does the weight of each rod/piston pair have to match, but the counter weight on crank must be properly adjusted as well in such a case.

[richmonds4]

I took my crank and factory pistons with new rings to a local machine shop that builds performance engines
He then balanced the crank with bob weights and drilled out spots on the crank to bring it back in balance .
I used pauter rods which are lighter then stock .

It cost around 250.00 worth it in my opinion ! Do it rite the first time .
Engines running great ! 1k so far .

[Monty23]

My built bottom end has 2k+ miles on it so far with no issues and all I did was swap rods with no balancing....

[slow ride]

Balancing the pistons and rods to match the lowest weight units is what a machine shop will first do. Then you will build bob weights to simulate the pistons/rods and drill or fill (heavy mallory metal) the crankshaft counter weights to zero out everything. If you are not doing the full balance you might as well make all the parts weight the same as they NEVER are exactly the same. I've always run across a 1-5 gram difference in rods and pistons. It's not really that much though and factory parts would not be exactly the same either.

[bigern45]

Very insightful responses, I appreciate it guys.

Much food for thought, as even if each piston and rod are close, the crankshaft itself needs to be rechecked, with the other options on...

[haggard69er]

Balancing the pistons and rods to match the lowest weight units is what a machine shop will first do. Then you will build bob weights to simulate the pistons/rods and drill or fill (heavy mallory metal) the crankshaft counter weights to zero out everything. If you are not doing the full balance you might as well make all the parts weight the same as they NEVER are exactly the same. I've always run across a 1-5 gram difference in rods and pistons. It's not really that much though and factory parts would not be exactly the same either.

best way imo.

[bigern45]

Yeah, thought I could get away with just weighing/correcting each piston rod assembly, didn't take into consideration how the more/less weight than stock would effect the cranks balance as well.

[julex]

Yeah, thought I could get away with just weighing/correcting each piston rod assembly, didn't take into consideration how the more/less weight than stock would effect the cranks balance as well.

As I said:

"Not only does the weight of each rod/piston pair have to match, but the counter weight on crank must be properly adjusted as well in such a case. "

[haggard69er]

here ya go.

[ddillenger]

If you took apart a factory engine that had never been run and gave it a GOOD balance you'd be surprised how much better it would be. Take it to a reputable shop.

[julex]

so what you're saying is that balancing the assembly is not that important after all since factory balance is fubar anyway and yet main assembly outlives the life of engine (which dies from balancing unrelated issues) and the car itself?

[sweets4style]

Your first example of weight matching piston rod combos and in turn having a balanced assembly is incorrect. While in inline applications (not V designs) it is true that balancing the crank seperately and then weight matching the piston rod combos does indeed balance the rotating assembly. That is how internally balanced inline motors are done. V designs are done by simulating weights of piston rod combos in teh form of bob weights. This is because the forced are no longer in line with the crankshaft center line as in an inline motor. You have forces in the intersecting angle direction during the entire stroke process. You do not have to balance each rod/piston combo. You can choose to do so. But in many cases this will end up adding weight in the form of a heavy tungsten ball to teh crank to balance it. Dont get me wrong you can certainly do it but there is no need. External items like pulleys and flywheels should be balanced externally of this rotating assembly. You do not balance the flywheel and pulley with the rotating assembly. This is not an externally balanced motor. There isnt a good way vs bad way of doing it. These are the only ways to balance the assembly. Whether you believe 2nd-4th order vibrations will indeed damage your motor and cause issues enough to require balancing is your own respective decision. But short answer, it will lol

[ddillenger]

so what you're saying is that balancing the assembly is not that important after all since factory balance is fubar anyway and yet main assembly outlives the life of engine (which dies from balancing unrelated issues) and the car itself?

No, I'm saying balancing has benefits even on pre-balanced parts. If there is an improvement over a factory calibration, imagine the difference once you start mixing and matching parts.