internally balanced vs. externally balanced - Ford Muscle Forums : Ford Muscle Cars Tech Forum
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post #1 of 7 (permalink) Old 06-12-2005, 11:51 AM Thread Starter
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internally balanced vs. externally balanced

Could someone please explain the difference?? Does internally balanced mean that the rotating assembly is balanced? What balance factor should a flywheel and dampener have if the engine is internally balanced? This is for a 289. Thanks
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post #2 of 7 (permalink) Old 06-12-2005, 12:26 PM
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internally balanced vs. externally balanced

With a internally balanced crank the damper and flywheel need to have zero offset weight.
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post #3 of 7 (permalink) Old 06-12-2005, 01:03 PM
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internally balanced vs. externally balanced

Balancing Act

The process of balancing begins by equalizing the reciprocating mass in each of the engine’s cylinders. This is done by weighing each piston on a sensitive digital scale to determine the lightest one in a set. The other pistons are then lightened to match that weight by milling or grinding metal off a non-stressed area such as the wrist pin boss. The degree of precision to which the pistons are balanced will vary from one engine builder to another, and depends to some extent on the application. But generally speaking pistons are balanced to within plus or minus 0.5 grams of one another.

Next the rods are weighed, but only one end at a time. A special support is used so that the big ends of all the rods can be weighed and compared, then the little ends. As with the pistons, weights are equalized by grinding away metal to within 0.5 grams. It’s important to note that the direction of grinding is important. Rods should always be ground in a direction perpendicular to the crankshaft and wrist pin, never parallel. If the grinding scratches are parallel to the crank, they may concentrate stress causing hairline cracks to form.

On V6 and V8 engines, the 60 or 90 degree angle between the cylinder banks requires the use of "bobweights" on the rod journals to simulate the reciprocating mass of the piston and rod assemblies. Inline four and six cylinder crankshafts do not require bobweights. To determine the correct weight for the bobweights, the full weight of a pair of rod bearings and the big end of the connecting rod, plus half the weight of the little end of the rod, piston, rings, wrist pin (and locks if full floating) plus a little oil are added together (100 percent of the rotating weight plus 50 percent of the reciprocating weight). The correct bobweights are then assembled and mounted on the crankshaft rod journals.

The crankshaft is then placed on the balancer and spun to determine the points where metal needs to be added or removed. The balancer indexes the crank and shows the exact position and weight to be added or subtracted. The electronic brain inside the balancer head does the calculations and displays the results. The latest machines have graphical displays that make it easy to see exactly where the corrections are needed.

If the crank is heavy, metal is removed by drilling or grinding the counterweights. Drilling is usually the preferred means of lightening counterweights, and a balancer that allows the crank to be drilled while still on the machine can be a real time saver.

If the crank is too light, which is usually the case on engines with stroker cranks or those that are being converted from externally balanced to internally balanced, heavy metal (a tungsten alloy that is 1.5 times as heavy as lead) is added to the counterweights. This is usually done by drilling the counterweights, then press fitting and welding the heavy metal plugs in place. An alternate technique is to tap the hole and thread a plug into place. Drilling the holes sideways through the counterweights parallel to the crank rather than perpendicular to the crank is a technique many prefer because it prevents the metal from being flung out at high rpm.

After drilling, the crankshaft is again spun on the balancer to determine if additional corrections are required. If the crank is for an externally balanced engine (such as a big block Chevy), the balancing will be done with the flywheel and damper installed. On internally balanced engines, the flywheel and damper can be balanced separately, or installed on the crank and balanced as an assembly once the crank itself has been balanced.

New machinery has been introduced that both simplifies the balancing process and increases the accuracy of the job. Electronic equipment that allows accurate measurement of not only the amount of unbalance force, but also accurately reports the unbalanced vector position is now available to engine rebuilders. Typically, balancing machines have assumed that the unbalance force was equally opposed, so they would require the technician to correct the excessive amounts of unbalance on the excess side to the point of making them equal. Technicians have had to ‘stair-step’ the corrections equally until the final tolerance was attained.

Hope this helps...

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post #4 of 7 (permalink) Old 06-12-2005, 01:36 PM
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internally balanced vs. externally balanced

Just as easy to link to your source for that...........
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post #5 of 7 (permalink) Old 06-12-2005, 02:20 PM
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internally balanced vs. externally balanced

That is a good article which explains it. To add something else to all that, internal balance can make a few more ponies than external balance, because the weight is closer to the center of rotation with an internally balanced crankshaft that it normally is on an external balance. Think of putting a rock on the end of a 4' long string, and swinging it around. It's a little hard to get it started spinning, and when it finally is in motion the out-of-balance is pretty severe and it's kind of tough to keep it going. Now, shorten the string to 6-8" and spin it around again. MUCH easier to get it started and keep it maintained in motion. This is where internal balancing helps. IT's not worth 100 HP (maybe 10 in all actuality) but it's just another trick to get a few extra out of a given combination.
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post #6 of 7 (permalink) Old 07-28-2011, 01:09 PM
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Re: internally balanced vs. externally balanced

I have also found a ton of information on balancing at
Engine Balancing and the Performance Rebuild

I think they are the one's who originally wrote the artical in Stroker heaven as well.
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post #7 of 7 (permalink) Old 07-28-2011, 02:25 PM
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Re: internally balanced vs. externally balanced

2005?????????? Hello??????

Grabber Green \'70 Mach I 351C 4V 12.32 at 111mph 1.63 60ft. time
Robin Egg Blue \'79 Fairmont 408C 4V 10.50 at 127mph 1.38 60ft. time
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