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Old 03-14-2009, 08:20 PM
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Should I get my engine balanced?

I just got my 350 bored .030" and will be installing a GM crank, scat rods, and speed pro hypers. Does this need to be balanced now? I plan on running it up to 6k rpm with 9.5:1 CR. Everything I'm reading says it should be balanced but I know tons of guys who have built similar engines and never had a problem. If the answer is yes, I should get it balanced, why?

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Old 03-14-2009, 09:04 PM
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Hi,
Yes you should have it balanced.
An out of balance crankshaft will cause uneven ware, leading to premature failure.
Rich
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Old 03-14-2009, 09:50 PM
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I agree with Richard, you want everything in sync and running smoothly. Your talking 6000 RPM? For sure you want everything balanced or Kaboom! Figure how fast your engine is turning. If it slightly unbalanced, that unbalance is magnified the faster the engine goes. If your putting all that money into the engine, why not be safe with your investment and pay the little extra cash to have it balanced.

Read this, it might help:
http://www.cwtindustries.com/whybalance.htm

Last edited by kleen56; 03-14-2009 at 10:00 PM.
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Old 03-14-2009, 11:02 PM
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Balancing engines is a touchy subject, the most I ever balanced an engine was to weigh the rods and pistons to makes sure they within a gram of each other.

99% of the time, they were close out of the box.

Was the crank out? Who knows but since the radius of the inbalance was close to the circle of rotation its likely it hardly mattered.

I once seen a balanced crankshaft for a circle track race engine, it was "zero" balanced so all that was required was to make each throw the same weight, it had one small drill mark on a balance throw. I had to question if having $300 worth of work done to the motor just to drill a small mark on the balance throw really made any difference.

I don't think it did.

One thing is for sure you would think that by reading all the magazines balancing the engine was required or it will blow up at high RPM, complete hogwash! If the engine was going to blow up because you didn't make that 2 gram drill mark on the crank throw then you had other issues.

I should point out that they have been building high rpm engines from the factory for many years and none of them are balanced. Accurately built? You betcha but none of them ever seen a balance machine.

To answer your question "Do you need to balance an engine?".

The answer is no, but you should check the weights of the rotating equipment to make sure you don't have some wacky thick piston in there from the reject bin.

I call it "checking the balance" and its a lot cheaper to buy a $30 gram accurate scale from a department store than to send in an engine for a 2 gram drill mark for $300.

Use your common sense.
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Old 03-15-2009, 12:04 AM
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I understand where 4Jaw is coming with his statement and he's correct, balancing is a controversial subject. Most new crankshafts that come out of a box are close in balancing and some may not even require balancing. I've also put a few engines together without balancing and they have lasted for several miles without any problems (in fact my engine in my truck isn't balanced). I'm not saying balancing is mandatory, but in this guys case, he wants to run the engine at 6,000 rpm. That is a high revving engine! Even if it is a 2 gram drill hole, the 2gram off balance might be increased quite a bit running an engine at 6,000 rpm. For a daily driver, or just occasional streetrod accelerator, I might not balance the engine and don't see a need for it. If I'm at 6000 rpm? You bet I want the engine to be able to sustain that rpm and stay together, thus I'd opt for balancing the engine, even if it's a 2 gram drill mark. What if it does matter? and the engine has issues because you didn't opt to balance it? It may cost you more in the long run. But that's me and my 2 cents.
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Old 03-15-2009, 12:33 AM
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Balancing?

The 327 (.040 over), in my Sedan Delivery, is'nt balanced. I started with this engine when I built it for my FED. Bought a junk motor, kept the heads, block, crank; everything else got tossed. Reconded a set of sm. journal rods, hung some TRW 2166 pistons on them, had the crank turned, threw a "mild" solid lifter cam in it; NEVER HAD IT BALANCED. Went 9.23 @ 146 MPH; that was 8500 RPM on the 1-2 shift with the Powerglide, and 8500 RPM in the traps. But, I got LUCKY!!! Now, I'm building a duplicate of this motor, only with a large journal crank, aftermarket rods, and the same TRW 2166 pistons, only in STD size. And, THIS MOTOR WILL BE BALANCED. Is balancing really needed? Not with stock lo-perf parts; anything else really should be balanced. The "factory balance" will get you 6000 RPM, and maybe fairly often. But, what would a precision balance get you? A LOT MORE PIECE OF MIND.
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Old 03-15-2009, 01:38 AM
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Quote:
Originally Posted by maxpower_454
I just got my 350 bored .030" and will be installing a GM crank, scat rods, and
speed pro hypers. Does this need to be balanced now? I plan on running it up
to 6k rpm with 9.5:1 CR. Everything I'm reading says it should be balanced but
I know tons of guys who have built similar engines and never had a problem.
If the answer is yes, I should get it balanced, why?
If the rods and pistons weigh the same as the production parts, the factory
crank balance will be OK. If not, it should be balanced.
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Old 03-15-2009, 02:54 AM
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I agree with 4 Jaw, balancing is highly over-rated. My self, and several friends, have run 350's, 383's, and 406 small blocks unbalanced to 7500 rpm (and beyond on the 350's) with no failures for years. These are street/strip and drag engines, components were selected to be either lighter than or to match factory weights and all the parts were weight matched to each other.
You just have to educate yourself enough on the part's specifications to know what to pick. The 350 Chevy is probably the easiest do this with because it is so common.
I've yet to see a engine fail that could be directly connected to not balancing.
Only time I would consider balancing is for continuous high rpms like circle track ( because of harmonics)or some kind of extreme stroker crank build.

Quote from "How to Build Horsepower" volume 3, by David Vizard: "All it will mean is that the engine shakes more than we like. Whatever it does is still likely to be less than a balanced 4-cylinder engine."
"Foregoing balancing does not mean that there will be more destructive internal engine loads present. What it does mean is that whatever forces are generated at one journal are not completely countered by forces generated at another. The result is an out-of-balance engine, but the engine is no more likely to fail than if it were perfectly balanced."

In your case, I'd weight-match the individual pieces if you have the ability, but it wouldn't be something I would worry over if you couldn't. 6000 rpm is nothing to a small block, I'd say aftermarket fasteners are far more important. Put it together and run the scat out of it.
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Old 03-15-2009, 05:00 AM
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I'm smiling at your self-imposed red line of 6K rpm.

You will soon see how fast that goes out the window- like the first time you're getting pulled away from in a street "confrontation"!

If you really plan on keeping that 6K promise to yourself, you had better equip it w/a rev limiter.

In a stock-type rebuild that reuses the same rods and crank, Iíd say generally speaking, no balance is necessary- provided the replacement pistons and pins are from a matched set (like most good sets are, now days) and rpm is to be kept reasonable.

A custom crank from a reputable manufacturer that has been balanced to a stated bob-weight can be used with a matched set of rods and matched set of pistons and will usually be fine, also. The difference in weight between the components and the bob-weight value that the crank was balanced to will change the percentage of balance, but will not impart the more harmful loads that are to be avoided. I would recommend a quality harmonic balancer in any event.

But this gets iffy when unknown parts manufacturers are brought into the picture. Was the Chinese woman paying attention that morning when your crank was balanced? Were the rods all pulled from the same +/- 2 gram bin, or were there not enough of them so some were pulled from the next bin? Was the guy at "Claimer Pistons, Inc." hung over? That sort of thing.

So, to balance your engine or not? Sure, if you can afford to, and you do the harmonic balancer and flywheel/flexplate at the same time, and if you want the best performing, longest lived build you can do. Or if it's to be a serious high performance or race build.

And donít forget the peace of mind that it brings. Satisfaction of knowing youíve done your best is worth something, after all!

Last edited by cobalt327; 03-15-2009 at 05:45 AM.
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Old 03-15-2009, 08:17 AM
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Now those are the kind of responses I was looking for. Thanks a lot everyone! I will probably continue to debate this with myself for a while
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Old 03-15-2009, 11:18 AM
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Rather than give an option as to whether or not you should have your engine balanced, I will give my latest experience.
This winter I did a complete rebuild of my 468 BBC. The rods and pistons were reused, but I had to go with a new crank and fluid damper due to the HP increase. All the research that I did led me to beleive that a "balance" was a critical step in the rebuild. The local machine shop did this at a very reasonable rate ( $175 ) but made absolutely no adjustments to the components. The pistons and rods were virtually identical in weight and the crank required no adjustments either. The only real job they performed was to slightly open the bore of the fluid damper because it was slightly undersized.
As far as my comfort level, I do not regret spending this extra money, but next time, I just may weight the rods and pistons before having the engine balanced and based on the results, decide if I want to make this additional inventment.
In a nut shell, "I invested a lot of time and money in this rebuild and I do sleep better at night knowing that it is all balanced", even though all of these particular parts were "dead on" from the manufacturer.
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Old 03-15-2009, 11:22 AM
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Hi,
I suppose you don't need to balance your wheels then either, according to the non balancers, it's only out a 1/4 oz why bother. I'm sure you have all driven a car with out of balance wheels, felt the vibrations in the steering wheel, what makes you think an out of balance engine is any different? You do as you please, I'll keep having mine balanced, that $300. is only a small portion of what the total cost of the engine is, and it makes me fee better.
Rich

Last edited by richard stewart 3rd; 03-15-2009 at 12:16 PM.
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Old 03-15-2009, 01:06 PM
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Quote:
Originally Posted by richard stewart 3rd
Hi,
I suppose you don't need to balance your wheels then either, according to the non balancers, it's only out a 1/4 oz why bother. I'm sure you have all driven a car with out of balance wheels, felt the vibrations in the steering wheel, what makes you think an out of balance engine is any different? You do as you please, I'll keep having mine balanced, that $300. is only a small portion of what the total cost of the engine is, and it makes me fee better.
Rich
X2
I had my 468 BBC balanced last year,it sure is nice & smooth now,that was money well spent !!!
Guy
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Old 03-15-2009, 01:15 PM
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Richard,
You took the words right out of my mouth. They marginal money it takes, I have piece of mind knowing my engine is runnig it's best. It's just something about opening a hood and seeing the engine not moving while it's running. Then again, not moving or shaking all ovet the place when you feather the throttle. This might sound silly, but everyone knows a balanced crank makes more power. I wouldn't build an engine without balancing the rotating assembly.

Also, I'm not sure where you guys get it done, but $300 is on the high side for me. I've gotten assembly balanced for as low as $120 (all parts). If you're building something you will cherish, keep for a while, get it balanced.

Just like an OD transmission lessens the stress of an engine on hwy driving by dropping the rpm's, balancing helps an engine by reducing the stress on moving parts. The guys that opt not to balance, generally do not have those few extra bucks in the budget to get it done. A balanced engine even sounds different.
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Old 03-15-2009, 01:55 PM
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Quote:
Originally Posted by maxpower_454
I just got my 350 bored .030" and will be installing a GM crank, scat rods, and speed pro hypers. Does this need to be balanced now? I plan on running it up to 6k rpm with 9.5:1 CR. Everything I'm reading says it should be balanced but I know tons of guys who have built similar engines and never had a problem. If the answer is yes, I should get it balanced, why?
Depends on your luck, note a lot of guys here claim they stuffed a bunch of parts together and turned 8500 RPM without balancing the engine. Of course to turn a SBC at 8500 RPM is a lot different than your model airplane engine. At 8500 RPM on an SBC you've got some big bucks on the line. If you're willing to risk that to save 200 dollars go ahead, it's your bank roll and you can do what you want.

For my shop, every engine goes to the balancer. It's just that the tightness of the job is a bit variable by how the engine will be used. For a street engine we hold 'em around a gram. For race engines they get tighter and if it's a high winding race engine we tend to overbalance the crank a couple percent.

The reason for balancing is not for passenger comfort by making a smooth turning motor, though that's an out come. The reason is to take load off the structure of the engine. An out of balance crank assembly will in whole or in part attempt to center the out of balance forces as it turns. If that center isn't the same as the physically established center of rotation for the crank, then the crank will try to run someplace other than where the bearings confine it. That difference is a force, often a big force to the tune of several hundred pounds at 6000 RPM from a very minor static weight deviation. That force of the crank trying to be someplace other than where its constrained has to be absorbed by the main caps, their fasteners, and the block on top of all the other running forces. A small imbalance shows up as wear on the bearings, one can really see this when disassembling an externally balanced engine, here you will see the journals with the biggest local imbalance eat into the bearings they will show their copper base on an edge and still have the flash plating unmarked on the opposite side. This is the result of a twist in the shaft distorting its shape, not a miss-machine of a part. For severe miss-balances turning high RPMs you get a structural failure of the crank or block. Trust me on an over balanced race engine, you feel the difference under 6000 RPM and don't operate there any more than necessary to get to and from race speeds. The overbalance is to adjust for the very extremely large power and reciprocal forces on the crank at high RPM, it's not necessary nor desirable on engines that turn less than 6000 RPM. Its for race engines that turn high revs frequently as a pro drag engine or continuously as a NASCAR engine, not for the street or the occasional racer.

Which brings us to the other balance issue which is rotation twisting along the length of the crank. A weight differential between two throws puts a twist in the crank. This twist changes as power and the reciprocating forces vary thru the 4 engine cycles. In other words the shaft winds and unwinds for each change in force as each cylinder goes thru its cycles. The dynamics of this motion can lead to an event called dynamic augment where two smaller forces become timed such that they make one large force. The damper on the front of the crank is there to manage/dampen these forces on the front of the crank and the flywheel-clutch or flexplate-torque converter dampen these moments on the rear of the crank. Obviously when running the crank out of balance there are additional twist forces that wind and unwind the crank in addition to the cyclic forces, this will lead to breakage of the crank if the damping action on either end is insufficient. Balancing once again doesn't remove all these forces but when done removes the twist forces the crank has to deal with from the imbalance. Additionally, these forces on the nose of the crank cause the timing set between the crank and cam to wind and unwind. This causes changes in cam and thus distributor timing.

This gets so critical in pro race engines that you will often see that where the factory gets away without counterweighting the center two throws because they balance each others weights. But many NASCAR type engines add counterweights here to remove the twist in the center main from these forces being offset to that main which results in whats called a rocking couple thru that bearing and journal. So when running flat out at 8-9000 RPM they take the hit on additional crank mass in order to increase the durability and reliability of the center main. Drag engines don't usually do this since they only touch those RPMs for moments rather than hours and the usual pro drag engine is rebuilt after a weekend's worth of runs having only a few seconds of total running time on it.

So you puts your money down and you takes your chances based on your decisions. If you think all you've put into the engine to date and your mental comfort isn't worth a couple hundred dollars. Then just put it together and run it. Maybe you'll get lucky, maybe you won't.

Bogie
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