Ok, this might be a dumb question, but I'm pleading the "the only stupid question is the one that doesn't get asked". So here it goes:

If you buy a crank that is already internally balanced, can you justify having the crank re-balanced with the flywheel (or flexplate) and the harmonic balancer? Or is that simply a waste of time and money?

Thanks

 

Most Cranks are Internally balanced. However when GM made the SBC 400 and the BBC 454 they made them Exsternally balanced. The Harmonic balancer and the Flywheel had weights on them. So no one part was balanced but when they were bolted togeather they were. When a production engine is produced they calculate the weight of the Rods and Pistons and balance everything. Then as long nothing changes weight wise of the componets then every future engine should be balanced. Now lets build a custom engine of your own, probably a 383. You are intalling a 400 Crank in a 350 block. You now have the choice of doing it GM's way of Ex balance or doing it interally. No body knows whos Rods or Pistons you are useing or what they weigh. At this point nothing is balanced. The exception to this is if You buy a Kit that has all the above parts sold togeather as an balanced assembly.

 

Good question.

It depends on how fast you're going to spin a motor. On a stock motor, no, you don't need to balance the flywheel (other than it's as-built stock) and pressure plate.

OTH, on performance motors - where hp is king, and the motor lives in its powerband - here's the formula one ounce of static unbalance when spun to 4000 rpm creates 23 lbs of unbalance. Every ounce you're spinning at unbalance costs you hp and longevity

Force due to imbalance (lbs.)=1.77 * oz. * in. *(rpm/1000)^2

note, how the farther the imbalance is out from the center of the mass the greater the force

 

Iirc you can go neutral balance on the fly wheel and balancer and theoretically not have to. But in my opinion you should still have the balance checked.

 

I just went through this and my engine guy was VERY specific that he HAD to have both the flywheel and balancer attached before he would attempt to balance the crank. He also resurfaced the crank prior to balancing, though that probably didn't have any impact on the complete balance.

This is because the flywheel is SOOOO heavy and large and a slight difference on that single component can wreck a balance because of the huge forces it generates.

He also mentined during discussion that he DOES NOT worry about flex plates nearly as much with balancing because they are so light and generally self-balanced. This is the same rationale for the clutch assembly (well, the pressure plate, at least) not being spun with the rest of the rotating assembly.

A "Balance Job" is a VERY personalized service with YOUR parts TOGETHER as a SYSTEM. I believe the "waste of time and money" would be to perform a balance without all of YOUR parts that will be part of the system present and accounted for.

 

The tolerances for weight and balance used for factory (or aftermarket) is cost vs function.
Randomly pick up a crank, rods, pistons, etc and put them together and they will function within acceptable performance.

When you have yours balanced, they match that crank to those pistons(rods, pins etc) to a much closer tolerance. And match the pistons, rods, pins to each other, etc.etc.etc.

The end result is a much smoother (consistent) loading on the motor. On the upper end, better performance, less loss of power etc (as stated above) but even on a fairly stock motor it will be running smoother, which can't be a bad thing.

For the cost (compared to the price of anything else involved in building the motor, balancing is very reasonable), for me, it goes out for balancing.

 

to underline what I didn't say well above....

to balance or not to balance is a function of cost. The less you put into a motor the less it's necessary. E.g. a re-ring job with new bearings and a quickie valve lap rebuild - you'd actually spend more to balance the assembly than you would in everything else.
The middle point is when you change parts - replacement rod, or piston... balance, but without the flywheel and pressure plate
OTH - when you've got 20k into a motor, balancing everything is of paramount importance

And seriously, 9 out of 10 times, if I have a shop tell me that I must spend extra money for things that aren't necessary - I tend to run away from them because in 10 out of 10 times not only will they do the optional, but they'll do the frivolous without asking and demand you pay for it (just one case: guy wanted to balance, wasn't necessary, but he insisted - then did a blueprint as well.... on a DD truck motor)

 

100% get it balanced regardless of internal or external balance.

The difference between a balanced and unbalanced engine is one that acts like a truck engine and one that revs freely.

The balanced engine will last longer and put less strain on parts.

I was going to buy a 383 Camaro recently to put in my vette to replace the 355 I bought, he took me out and this thing was a dog, way slower to rev than mine, I asked him if he had it balanced when he had it built?.. and he said it's 'externally balanced', I said no, did you get the machine shop to balance the rotating assembly??... And he said no.

The guy who balanced mine balances aero engines and he weighed the pistons and rods and made them all the same, he balanced the crank with the ancillaries and the results are amazing. The engine revs up super fast and sounds like a race car, it's a very happy engine.

I'd never not get an engine balanced, it's free HP in that the engine can get into it's powerband quicker

..And as others have said, you need to give the machinist everything he needs to balance, I personally give him the short block and he gives it back assembled and I do the rest

 

The term "internally balanced" is how it is done with that particular rotating set. You can't buy a balanced crank per say. You have to weight the big ends of the rods you intend to use.......... make all the pistons and rods equal. Figure out the bob weight. Drill and or add heavy metal.

You are balancing a matched set either internally or as in external, drilling or adding metal to the damper and flywheel/flexplate

 

If you are using a rotating assembly that was already balanced, you can get away without balancing. If you are assembling any new parts, you have to balance. I would always balance with the flywheel and damper, although you could make an argument for leaving these out if its an internally balanced engine.

 

I am always the odd guy out. I weight match the pistons, rod big end, rod small end and then assembled combination of the above before anything gets balanced then always make sure the balancer, flywheel and pressure plate are included when everything goes to the balancer. Flywheel and pressure plate should be indexed and marked. I think this is one of the most important procedures and a little more time taken here adds up to a lot smoother running, durable and free reving engine especially at high RPM. Broken cranks and high RPM vibration I believe are many times caused by not enough attention paid to this easy step.

 

That's the way I understand it. Just because you buy an internal balanced crank doesn't mean it's balanced to your particular rod/piston combo.
A crank may be built for either internal or external balance, but the rotating assembly with crank, rods and pistons are balanced as a unit.

 

Mr. 63 Mako, why does your description make you the odd guy out?

Sounds like a textbook procedure to me.

That is exactly what was done to my current build...

 

[QUOTE=63mako;1583241988]I am always the odd guy out. I weight match the pistons, rod big end, rod small end and then assembled combination of the above before anything gets balanced then always make sure the balancer, flywheel and pressure plate are included when everything goes to the balancer. Flywheel and pressure plate should be indexed and marked....QUOTE]

Nope, doesn't sound odd this time.
The guy who does my balancing is a good friend and that's pretty much exactly what he does. Weight match everything based on the distance from centerline, and then index and balance as much as they supplied.

His biggest complaint is that more often then not he gets the orders from the engine builder and frequently they don't even give the flywheel (unless it's an external bal one) and rarely does he ever get the clutch etc. since they don't have those parts.

Then they assemble the engine, give it back to ??? and they put the engine to the trans and put it together.
Then the owner complains about a "shake" at whatever speed.
Mooser

 

THink what Mako meant was its called "doing it right"

You see "balanced" assys being sold out there.
Taking those pieces in and having them done by a pro is how I translate his post, I trust no production machine shop.

There is a joy of having a motor that sits perfectly still and is seamlessly smooth through the rev range no matter how ratty it sounds.

 

yep, got mine balanced with the flywheel and clutch, he put paint marks on the stuff to show me where to bolt it back on

 

Balancing is cheap insurance for anything beyond a plain vanilla, stock rebuild IMO
Parts individually may be within spec but once you put it together, you may be way off due to tolerance stacking. Why risk it?
ALWAYS provide the damper and flywheel (pressure plate is a good idea too) when balancing.

 

My point is, You weigh all 8 pistons. You match them all to the lightest one as close as possible. Then weigh all 8 rods, 2 scales @ jig, Big end and small end. Weight match both ends to the lightest individually. Then complete assembly should be equal weight. These are small, time consuming, labor intensive things that all add up and are not done at most shops yet when done with attention to detail will really pay off in the end. Most shops will assemble the pistons and rods out of the box, figure bobweight off the info with the rods and balance accordingly. Most decent components are "weight matched" from the factory. The thing is how close. You can balance +/- 2g or +/- .2g. There is a big difference @ 7000 RPM. The closer the better. Figure 4 Grams for oil. Bearings, Pins Locks, rings Ect. All get weighed accurately, all weigh the same and bobweight calculation is exact as is the balancing process.
A properly decked block, cced head chambers ect. to end up with as close to uniform as possible cylinder compression is the same thought process. A smoother running, quicker reving, more durable engine at high RPM.

 

I replaced my factory forged rods with new Eagle forged rods to take the "unknown" out of the mix, get the larger cap screws and clearance, and get the full-floating pins. I would have been most of the way into new rods getting new studs put into my old LT-1 rods.

While I would have done well to plan on a balance all along (I was TRYING to do a "budget build"), replacement of the rods tossed me into a 100% imperative of needing a balance job.

In the end, I paid $425 for my balance. It took the guy the better part of a day and this represents the better part of 10% of my total build.

Worth every penny. If for nothing else, the peace of mind. But I'm sure the engine will run like butter.

 

2 grams? that's 1/28th of an ounce..... so let's do some math? shall we?

Force due to imbalance (lbs.)=1.77 * oz. * in. *(rpm/1000)^2

FDI = 1.77*(2/28)*3*(8000/1000)^2
= 24.27428571428571

24 lbs out of balance at 8000 rpm? that is going to blow up a motor?

really? change the viscosity of oil and you've got a greater problem than that.... have oil dripping off the cam and onto the crank and you've got a greater problem than that....

I get being accurate, but 24 lbs at 8000 rpm seems a bit over-the-top to spend several hundred dollars. If you own a machine shop, that's just awesome, to the rest of us?? Let's not even mention how much strength is built into the motor.... without looking it up (again) can't a steel crank handle somewhere around 80,000 lbs of force before it breaks? And you're asserting that 24 lbs will make a difference.... sure, you can do the math about how mosquitos in florida slow down a racecar but still - in the end, it's just squished bugs.

I apologize for the rant, but I absolutely loathe people who post their stuff on the 'net without context - it's said, and repeated then becomes gospel truth.... and no one stops to do the math....

as for peace of mind, I spend more time worrying about whether or not I can pay the bills than I ever do about whether my Corvette's going to blow up because I'm flinging around another 24 lbs on my crank.... trust me, at 8k rpm, oil will not be the problem