bbc INTERNAL OR EXTERNAL BALANCE? - Chevelle Tech
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  #1  
Old Jan 11th, 08, 3:00 PM
dgrobels dgrobels is offline
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Default bbc INTERNAL OR EXTERNAL BALANCE?

I havent bought my crank rods or pistons yet and have seen post here about internal and external balance

price difference between the two and advatnages disadvantages between the two...

this question is based on a street/strip engine that will see less than 1000 miles per yer and hoping to build this one time and keep it alive for as long as possible with minimal maintenance besides checking/adjusting valves once a year. and on pump gas my compression will be under 12 to 1 and hoping to get around 10.5 to 1 or as close to 11 to 1 as possible
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  #2  
Old Jan 11th, 08, 3:20 PM
SWHEATON SWHEATON is offline
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Smile Re: bbc INTERNAL OR EXTERNAL BALANCE?

396/402/427 BBC are internal ballance & 454 and larger BBC are externally ballanced .

But with that said i feel its a little $ very well spent in the scheme of things meaning motors cost a lot of money to rebuild and the few hundred to ballance isnt that bad .

I would do it,any motors i have rebuilt that were ballanced (recipricating assembly) felt/ran very smooth but when someone would chince out and not want me to have it done when doing a rebuild for them i have noticed those motors didn not run/feel as smooth.

Ask our resident engine builders/machinests experts like MikeWolfplace/billK/gofast/Carl from CNC what they think.

You could try a search titled ballancing becausee i beleive there was the same question asked/posted here a while back and these experts already chimed in with their good expericned answers.

Just my 2 cents.

Scott
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1969 CHEVELLE SS396,ORIGINAL #'S MATCH,GOT IN 1978,(In 2001 rblt/bored original 396 .030 to 402)/M20/12BOLT/3:31'S
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Last edited by SWHEATON; Jan 12th, 08 at 7:22 AM.
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  #3  
Old Jan 11th, 08, 3:28 PM
RB69SS396Conv RB69SS396Conv is offline
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

Internal balance is easier to find parts for, and to interchange.

As stated, the short-stroke motors are internally balanced, and the 454 is externally balanced, from the factory.

The reason the 454 is externally balanced is that there is not enough room between the crank centerline and the bottom of the pistons when they're at BDC, to fit a large enough counterweight, without moving the wrist pin up and thereby compromising the ring package (in the factory's opinion, at the time it was originally designed in 1970 or so), either by spacing the rings too close together, using narrower rings, making the oil ring overlap the pin bore, or a combination of those.

A 454 or larger can be internally balanced by using longer rods and an appropriately designed crank. Obviously the wrist pin must be MUCH higher up in the piston; and the piston made MUCH less tall. With modern rings and care during assembly though, the ring sealing issues are minimal. People do it all the time, especially in small blocks, where the 400 has basically the same situation.
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  #4  
Old Jan 11th, 08, 11:21 PM
wills70 wills70 is offline
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

Hi! have a Q on this had rebuilt my 396- .060 over steel crank cast rods -heads installed 2.18/1.88 valve new guides and sprng,vlvs shimmed. 7/16 psh rds. anyway did have it balanced G.M. balanced shop says checked # of 1 rod & # 1 piston set to get bob # and I seen were he added weight to the crank in a couple of the counter weights. and got a print out of all the weight and reciprocating factors. He stated that would be good for what I was doing running the streets. Ths is better than doing nothing I would think!
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  #5  
Old Jan 11th, 08, 11:29 PM
pdq67 pdq67 is offline
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

If the sucker is Detroit Balanced right, regardless, it will be a dandy street engine, imho and my 496 is balanced like that now!!

I use my good truck thumb rods and I am NOT going to rpm her above 5,700 to MAYBE 6,000 b/c of their 3/8" rod bolts.

pdq67

Last edited by pdq67; Jan 12th, 08 at 9:49 AM.
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  #6  
Old Jan 12th, 08, 1:05 AM
Bob West Bob West is offline
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

Mine is externally balanced, and sees plenty of track time, 450-500 passes between freshens isnt too bad.
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  #7  
Old Jan 12th, 08, 1:20 AM
Wolfplace Wolfplace is offline
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

Quote:
Originally Posted by wills70 View Post
Hi! have a Q on this had rebuilt my 396- .060 over steel crank cast rods -heads installed 2.18/1.88 valve new guides and sprng,vlvs shimmed. 7/16 psh rds. anyway did have it balanced G.M. balanced shop says checked # of 1 rod & # 1 piston set to get bob # and I seen were he added weight to the crank in a couple of the counter weights. and got a print out of all the weight and reciprocating factors. He stated that would be good for what I was doing running the streets. Ths is better than doing nothing I would think!
=
Not even close in my world
As soon as you change parts in an engine there is no such thing as a so called "Detroit balance"
GM has specs they use for FACTORY PARTS
Now even though the tolerances are very loose by performance standards they do have a tolerance
What do you suppose happens when you install aftermarket parts that are not somewhere near the original weights?
And what is the point in calling something balanced if you do not check & match all the parts not just one
I am sorry but to me this should not even be called balanced

I do not care if you buy a $300 crank or a $3000+ billet from me
If I am balancing your rotating assembly the same rules apply or I don't do it.

As for internal or external
If at all possible I balance everything internally.
While there are many engines out there externally balanced that appear quite happy if you understand balancing you will understand that the closer you balance to the point of unbalance the happier the part will be

If you do not believe this take your trick shiny wheel & static balance it by adding weight to the inside when it wants weight to the outside
On a static balancer it is fine
Now stick it on your car & go drive the thing & tell us what happens,,,
This is only 8 inches or so from the point of imbalance

What do you suppose a crank does when you hang the weight off the end
You may not feel it but your bearings can
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  #8  
Old Jan 12th, 08, 8:43 AM
bracketchev1221 bracketchev1221 is online now
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

I would go internal. I've had both but I think it works better when you put the balance weights closer to the imbalance. Also it makes it easier if you every damage a flywheel. You don't have to worry about balance, just go neutral.
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  #9  
Old Apr 7th, 08, 1:40 AM
71 chevy 71 chevy is offline
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

I have a question. David Vizard in his small block book says that an unbalanced Rotating assembly does not reduce the longevity of the engine. is this true?

I just balanced my rotating assembly then decided to change the pistons.
these are 640 + 150 pin = 790
the sheet from the old balance says those were 893.1 for piston and pin.

will I reduce the life or performance of my engine if I just stick the new pistons on there?
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  #10  
Old Apr 7th, 08, 2:08 AM
Wolfplace Wolfplace is offline
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

Quote:
Originally Posted by 71 chevy View Post
I have a question. David Vizard in his small block book says that an unbalanced Rotating assembly does not reduce the longevity of the engine. is this true?

I just balanced my rotating assembly then decided to change the pistons.
these are 640 + 150 pin = 790
the sheet from the old balance says those were 893.1 for piston and pin.

will I reduce the life or performance of my engine if I just stick the new pistons on there?
=
I do not agree with that at all but what do I know

Considering even the factory has tighter tolerance than 100 gms by a bunch
I don't think I would just stick them in.
The factory is 15 or so grams as I recall
I don't really remember as I don't care what the number is but I know it is not 100 grams, that is a bunch
They even change the balance factor on the same casting crank for different weight pistons
Like the 366 truck vs the 396/427
Or the 305 vs the 350
Same crank forgings or casting, different counterweighting
I don't think they do this because they have nothing better to do with their time
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  #11  
Old Apr 7th, 08, 2:15 AM
FourEightyNine FourEightyNine is offline
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

Quote:
Originally Posted by dgrobels View Post
I havent bought my crank rods or pistons yet and have seen post here about internal and external balance

price difference between the two and advatnages disadvantages between the two...

this question is based on a street/strip engine that will see less than 1000 miles per yer and hoping to build this one time and keep it alive for as long as possible with minimal maintenance besides checking/adjusting valves once a year. and on pump gas my compression will be under 12 to 1 and hoping to get around 10.5 to 1 or as close to 11 to 1 as possible
You will be fine either route you go as long at it is a balanced assembly.

My builder runs a externally balanced LS7 motor(12.5:1) and bracket races it. He does a rebuild(new rings/bearings) every 5 years and it looks good everytime.
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Old Apr 7th, 08, 7:19 AM
71 chevy 71 chevy is offline
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

Quote:
Originally Posted by Wolfplace View Post
=
I do not agree with that at all but what do I know

Considering even the factory has tighter tolerance than 100 gms by a bunch
I don't think I would just stick them in.
The factory is 15 or so grams as I recall
I don't really remember as I don't care what the number is but I know it is not 100 grams, that is a bunch
They even change the balance factor on the same casting crank for different weight pistons
Like the 366 truck vs the 396/427
Or the 305 vs the 350
Same crank forgings or casting, different counterweighting
I don't think they do this because they have nothing better to do with their time
maybe they do it for the vibration reasons. I think I will go ahead and do the balance. what was to be a 2k engine slowly became 5k, and we are not even done yet lol

do you by any chance have any data or articles on the subject?
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  #13  
Old Apr 7th, 08, 7:58 AM
10secBu 10secBu is offline
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

My 461 is externally balanced still. I picked up a factory GM 1053 crossdrilled steel crank that was like $300. It just didn't make sense to spend upwards of $300+ to internal balance a stock GM crank. If I was gonna spend that kind of coin, I would have purchased a lower end aftermarket crank that was already setup for internal balancing.

You need to consider what parts you have and how much tungsten it's gonna take to intenal balance as the slugs aren't cheap.

My little 461 has been running 7600+ rpm for several seasons now and knock on wood holding up fine.

In an ideal world, yes, internal balance for sure. But with my setup, I don't regret the decision I made. Even if the crank cracked down the road, I still doubt you could blame the balance method, rather the material, lack of journal radius and the crossdrilled configuration which is sometimes thought to weaken the crank.
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  #14  
Old Apr 7th, 08, 3:07 PM
Wolfplace Wolfplace is offline
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

Quote:
Originally Posted by 71 chevy View Post
maybe they do it for the vibration reasons. I think I will go ahead and do the balance. what was to be a 2k engine slowly became 5k, and we are not even done yet lol

do you by any chance have any data or articles on the subject?
=
This should keep you busy for a while

http://www.chevelles.com/forums/showthread.php?t=125573

http://www.ghbalancer.com/techinfo/norm_auto_precision%27.htm

http://www.centuryperformance.com/balancing.asp

http://www.aa1car.com/library/2003/eb10330.htm

http://www.babcox.com/editorial/ar/eb10330.htm



As for internal vs external
What I said was if at all possible I internal balance an engine & I explained why.
I also said there are many engines out there very happy external balanced so no, I would not recommend spending a fortune on internal balance for a stock crank for most applications.

And internal is considered the better of the two ways.k
In fact as the stroke goes up it has been found the bearings are much happier if you add center counterweights.

The only reason the OEM's ever went to external was cost & packaging, much simpler to add weight externally than to redesign the engine to accept the weight internally.
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  #15  
Old Apr 7th, 08, 3:44 PM
Schurkey Schurkey is offline
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Default Re: bbc INTERNAL OR EXTERNAL BALANCE?

One more thing about "external" balance:

The factory chooses a certain amount of counterweight to add to the flywheel (flexplate) and/or damper. That weight is the same for every engine of that type--it's a standard amount of offset weight.

Each engine gets balanced at the factory, and they carve additional weight out of the crank counterweights as needed for that particular engine. They do NOT screw with the "standard" out-of-balance weight at the damper and flywheel.

Some "engine balancers" weld or grind weight off the flywheel or damper rather than doing it right by adding/subtracting at the crank counterweights. That upsets the "standard" weight--and makes it impossible to replace the damper or the flexplate with another unless you pay to have the new one matched to the old one.

DO NOT allow the people doing your engine balance to screw up the damper/flywheel!
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