Ive searched here, the net, and all the engine building mags I got around the house, and Im still slightly confused as to balancing the rotating asselmbly. Ive had people tell me that unless its gonna be a road race motor, i.e. engine sees mid to upper RPMs alot more than low end like street driving. I bought my crank, rods and pistons seperate. Rods are balanced balanced as a set, as are the pistons, and the crank has an 1870 bobweight. I can get an electric scale that can weigh the bob weight of the pistons, rods and all, but how exactly do I do it? Like the formula that goes along with it?

And lets say I take it to the machine shop to get it balanced. And I take them my flywheel and balancer. What kinds of mods do they do to the balancer/flywheel to get everything balanced? I want to learn all this to sace me some money in the future.

 

When I had my engine ballanced. The machinist did not want my ballancer or flex plate. I used a Streetdamper and he said that it would lie to him and give a false reading if he used it.

I would just spend the $150.00 or so and have it done. Then you will know its right.

 

Theres some kinda tool i can get from Comp, that supposedly can get more accurate readings than most machine shops. I dont see how, but thats what Ive read.

And to quote David Vizard,

"If the budget exists, balancing the rotating assembly is recommended. But be aware that your motor will be just as reliable and run just as well without balancing"

That being said, if it will be as reliable, and run just as well, why balance it?

 

It will be "reliable" in that it'll run.

Run as well? I guess so.

Make as much power? No.

Get as much life out of the bearings. Probably not.

Think of your tires. When you get new tires installed they need to be balanced with the wheels they're on otherwise they'll vibrate through the whole car.

The crank is the same way. The crank throws are counter weights. In order to "counter" the weight of the reciprocating mass as well as possible, they have to equal the weight they're counter acting with.

The smoother the rotation of the rotating assembly, the more efficient the engine will be (more MPG and more HP) and it would make sense to me that bearing life would increase, similar to tires that are balanced to the wheels they're mounted to have a longer wear life.

 

Agreed 100%

 

ya, that sounds reasonable. Thanx.

But, I m kinda curious, has anyone here balanced their own rot. assem.? I know a lot of people are gonna tell me to take it the machine shop, but Ive just heard that I can get more accurate readings if I do it my self, i.e. the mathematical way, rather than having a shop do it. Again, I havent seen any proof to prove that its best to do it either way, just curious.

EDIT:

Lets say I balance the crank, rods, pistons, locks, pins, bearings, all that good stuff, all myself with a scale and the formula that goes with it. Can I just bolt on the flywheel and balancer and be done with it? I mean when it gets balanced at the shop, is anything done to the balancer or flywheel itself, or just the crank and internal components?

 

my understanding on balancing rotating assemblies...


most internally balanced engines need to have parts of the counterweights drilled so that they have a certain grams of weight on them each to make up for the extra weight added from different rods and/or pistons, or a new crank.

This can be difficult since you have to drill certain amount of grams of metal off of a crankshaft counterweight, but there's no telling how many grams you've taken out until you give it a spin again.


Also in internally balanced engines the flywheel/flexplate and balancer are neutrally balanced, this is why your machinist would not need these when balancing your rotating assmebly.

In externally balanced engines (the 400 SBC for example) there's no plausible way (without mallory metal... but we'll leave that for another discussion) to change the crankshaft counterweights, so they use the flywheel and balancer to balance the assembly.


I hope that all of this information is correct, this is all that I've gathered while researching on all that I'll need to have done to my block and what I can and cant/shouldnt do myself

 

Except for externally balanced engines such as 400 SBC and 454+ BBC, harmonic balancers and flexplates are neutrally balanced and are not needed for balancing the rotating assembly. The harmonic balancer is actually a poor name. Technically it's a vibration dampener since it has nothing to do with balance. The externally balanced engines just have extra weight or cutouts to compensate for the weight needed on the dampener.

I have a 4340 steel crank. I use aluminum rods and have lightweight forged race pistons. To balance the crank for these lightened parts, they have to add a heavy mallory weight to the crank to offset the reduce weight.

Taking all the parts to the machine shop for balancing is needed so they know exactly how much weight is needed on the crank. The dampener and flexplate are never adjusted on an externally balanced engine. Rods are weighed on both ends and the weight is adjusted to the lightest rod. Even a matched set of pistons is checked and adjusted to the lightest one. A proper balance will also compensate for the piston rings, wrist pin and rod bearings since they all add weight.

Whenever you use non stock parts, you should get everything balanced. If the engine never gets above 4000 rpm like in a long term, daily driver, then slapping everything together will probably last for many years but if you're using non stock parts then you're probably going to be well above that rpm range. I run my engine into the 7000 rpm range so balancing is a must.

Things to take to the machine shop

Crankshaft
Con rods with rod bolts/nuts
Pistons and wrist pin
Wrist pin locks if they're floating pins
Rod bearings (one set)
Piston rings (one set)
Dampener and flexplate/flywheel if it's an externally balanced engine.

The example above about balancing tires is good. You can feel a thumping of an unbalanced tire. Eventually it will flat spot the tire. The engine can do the same thing to the bearings but you would never feel it. It's possible you may feel an engine vibration if the balance is really out.

 

one of the only things i had done was balancing,....i passed on most of the other "recommended" procedures. But during assembly i blueprinted the entire motor, made sure my clearences were all okay and double/triple checked all of my measuerments, such as ringgap, rod and main bearing clearences, valve adjustments, cam degree, spring height....etc. jsut make sure it's all w/in spec. haven't started mine yet, but i'd venture to say it's no longer rocket science....just follow the rules.

 

Check this out, very good explanation.

http://www.strokerheaven.com/engine_balancing.html