Streetiron85

I realize that a balanced rotating assy is desirable, but I'm also aware that many engines are rebuilt without being balanced. Nor do they seem to come custom balanced from the factory.
I'm doing a stock type rebuild on a motor that will probably redline @ 5200, with cast flat pistons, factory rods, factory crank. But the crank is not the one that was originally in service with this block/ rotating assy, it's a new factory crank.
This motor is a back up motor, but it WILL be used while I'm doing a build on my "serious" motor, and I feel like "factory balancing" whatever that is, would be adequate for this purpose.
So here's the question:
If I put this new crank into a block with the parts mentioned above will the balancing be as good as it would be in a factory motor?

If I can I'd rather put the $150 into one of my other motors.

Thanks

MrDude_1

i would balance it anyway. its cheap insurance...

*shrug*



edit:

and the factory does balance them... they just have higher tolerances.

RB83L69

If the crank is new, it isn't balanced to anything.

The way the factory does it (or at least used to do it) is to take rods as the come off the line, and measure them for big end weight, small end weight, total weight, length, etc.; and store them in a n-dimensional array of bins. Then when one bin gets 8 rods in it, they balance a crank to it and build a motor.

New cranks have never been balanced to any set of rods & pistons. That's what balancing is, after all; matching all the parts to each other, and then matching the crank counterweights to the weight of the other parts.

If you use a used crank and the SET of rods that came WITH THAT CRANK, and I don't mean 8 random rods from who knows where, you can usually get away without having it re-balanced even with new pistons, as long as the pistons are close to the weight of the originals. Any time you mismatch the rods and crank, it needs balancing.

Get your stuff balanced. You'll regret it if you don't.

Streetiron85

Thanks for explaining that to me.

jpk91rs

IMO I would use the money in your serious motor. I don't believe balancing is necessary on a stock type motor. Just my opinion though, not the gospel.

88IROC350TPI

If its a stock rebuild motor I wouldn't waste my money. Sure it may be a good idea to do it but in reality you're never going to know the difference. Maybe your motor will feel a little smoother and maybe your motor will last slightly longer ...maybe, maybe not....

Streetiron85

Thanks for all the replies guys.
I've decided I'm gonna try to sell the above mentioned motor, if I do I'll be money ahead, and I'll have fewer things to trip over in the shop.
It's a +.060 327 partial shortblock - See my ad in the classifieds if interested.

Happy New Year

Need4Speed1387

So what does someone mean when they say that their engine needs to be balanced? Do you take it to a shop and use weights or something? So if i used the original crank and rods from a 350 block that I'm building up, i wouldn't have to get them balanced when i start rebuilding it? This engine should put out about 375hp @ the crank, and probably never see the high side of 5500RPM. Should i get it balanced or should i spend my money elsewhere? And what's the average price of a engine balance?

Streetiron85

When a shop balances an engine, With the rods and pistons disassembled they weigh the big end of the rod, the small end, the overall weight of the rod, and the weight of the piston. Or the 'reciporicating mass'. After that, they grind metal from each of the parts til the weight of everything is matched within a gram or so.
After that they attach weights to the crank according to what the reciporicating mass is and spin it on a machine and remove or add weight to the crank til it spins in balance.
That's all I know.
Some shops rebuild engines without balancing cranks. But as RB posted above, they're balanced to a certain extent at the factory. Yet I've seen engines built from a pile of mismatched parts, and they ran, but not as smoothly as one that's been balanced, I'm sure.
My guess is that in the majority of cases, a motor built out of mismatched parts would be only 'moderately unbalanced' due to the inherent in-balance character of the V-8. Although, it seems like the potential would be there for the assembly to be 'extremely unbalanced'.
I'm not sure either, what an extreme example of an unbalanced rotating assembly would be like. I'm sure that somebody out there does know tho. I'm pretty sure I wouldn't want to be the owner of a motor that's 'extremely unbalanced', it would probably vibrate like a Harley.
The shop I usually work with charges $150 for balancing.

88IROC350TPI

I threw my motor together in a hurry and didn't get the rotating assembly balanced. Next year I'll probably tear it down for checking and get it balanced but as of now my motor feels VERY smooth and I spin it to 6600RPM. Maybe due in part to the fact that I have a top-quality harmonic dampener.

Streetiron85

88IROC350TPI-
What rotating assy did you use on that engine?
Was it just some random stock factory rods or some aftermarket rods that came in a set?
Cause I'm thinking that most likely if the rods are pretty closely matched in weight, then the engine would end up running pretty smoothly. Usually a set of high quality pistons will be pretty closely matched to begin with, I think. A set of Speed pro hypers that I bought needed to have only about 1 g. removed from 2 of them to make a weight matched set.
I can't claim to know much about the science of balancing, but in a V8 for each set of rods/ pistons, there's there's another set offsetting it on the opposite crank throw. And it seems to me that if the individual parts were close to being matched in weight, and they were attached to a crank that came from a smooth running engine, it would still be OK.
In my case I'm thinking about using a new factory crank that has never been run in anything and never been 'factory balanced' and I'm attaching a set of factory rods to it that are out of an aftermarket rebuilt short block. And I know that a lot of those rebuild shops just have a box o' rods that they use. So it seems to me like there could be a a good possibility that it might not be well balanced.
I've decided to play it safe and go ahead and balance mine. I'm guessing that chances are, the parts are not matched as well as they are in a motor like yours.
That's if I don't sell it.

Thanks for the reply

ronterry

How would you like to have a ceiling fan over the dinner table that's not balanced. Yeap they even balance those. If not they make lots of noise, vibrate wildly, and only last a fifth of there service life. Those little bearings get trashed.

Forgive me, but this story needs to be told!!!
I had a friend that had a ceiling fan over his bed, and one night the damn thing exploded while he was sleeping , and than - no kidding - it f'in started a fire in his bed. and that was the only light he had, cause it tripped the circuit.
Again - I'm sorry for the story, but watching him vacuum up pieces of fan blade was funny to me.

BTW: You can buy pre balanced asemblies from mail order. It's a time saver.

Ron

88IROC350TPI

My motor is pretty much parts collected over the years thrown together: TRW forged pistons, C&A forged rods and an Eagle cast steel crank.... I'm going to push it to the extreme this spring with new heads and silly amounts of nitrous ...we'll see if it holds Even if I melt a piston I'll blame it on not being balanced

Viprklr

iTrader: (5)

I didn't get mine balanced as suggested by somebody I know. Turns out mine was one of those "extremely unbalanced" assemblies. I guess the PAW kits don't come balanced. Anyway I put everything together and it ran fine. There was a vibration right from the beginning but I figured it was from the poly mounts I threw in. As I finally rolled 5K miles I decided to rip it back out. This car was practically a rolling vibrator at this point. Upon teardown I found that one of my main bearings had worn through to the copper.
The machine shop where I took everything to get balanced ended up saying that they almost had to replace a rod, one end of the crank was 31 grams out and the other was 42. The pistons were quite off as well. They ended up taking weight off every piston/rod assembly and adding weight to both the balancer and the flexplate.
I can't wait to get this thing back in the car. Maybe it'll run as smooth as a 16 cyl jag.
I'll be balancing any motor from now on. $300 additional in parts because I didn't do it the first time.

Streetiron85

Sorry to hear that you're one of the unfortunate ones who tried your luck and lost.
Interesting to hear your story tho.
Ya gotta know that something spinning around at 800 - 6000 rpm can potentially vibrate. I've been lucky so far myself, but now I'm putting together engines using parts that weren't originally together, and I think I'll quit rolling the dice.

Insomniac92z28

I can't help it!
Chick magnet

Need4Speed1387

If i'm using the same crank and rods that came out together, i don't see why they can't go right back in together and not be balanced all of a sudden. Can i just go and get the pistons all matched to the same weight and be fine?

MrDude_1

no.


unless all of your new pistons, pins and clips match the exact weight of the old ones.

remember its spinning... and this isnt a 180* engine, so theres some counterweights on the crank..

Streetiron85

We'll assume that your engine was running smoothly before.
I can't give you the answer to your question with any level of certainty. But if you can get an accurate enough scale and you've got some ingenuity, you can weigh the reciporicating parts yourself. And do a lot of the work if you're careful.
Separate the rods and pistons or have a machine shop do it, and weigh the pieces.
Let us know what you find out. I'm interested if no one else is.
My guess is that if you find pistons that weigh the same as your old ones, the engine will be smooth. But maybe your pistons are all mismatched in weight to begin with ...Then what?

Viprklr

iTrader: (5)

No need for a chick magnet. The wife liked it though.

Unless you can get a scale that measures in grams I wouldn't waste my time.