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True Statement?

British Slave

British Slave

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Gonzales, Texas... The home of Texas independence.
All Chevy 350 engines prior to 1986 were internally balanced. In 1986 the 350's changed to a center bolt valve cover, a one piece rear main seal and a different crankshaft. These engines are externally balanced at the flywheel. If you look at the late flywheels you will see a pad of extra metal cast in. The harmonic balancers continued to be neutrally balanced.
 
Obviously I don't know, but I did find this article which makes it sound more complicated than that statement above....​

Internal Balance & External Balance Explained​

When the counterweights alone can be made to balance the crankshaft, the crank is said to be "internally balanced". If the counterweights are too light by themselves to balance the crankshaft and more weight is needed, an "external balance" can be used. This involves a harmonic dampener or flywheel that has a weight on it in the same position as the counterweight that effectively "adds" to the weight of the counterweight on the crankshaft.
external_flexplate.jpg
external_dampener.jpg


Since the harmonic dampener (front) or flywheel (rear) play a part in the balancing of the assembly, they must be installed on the crankshaft when it is balanced. This is unlike an internal balance configuration where the harmonic dampener or flywheel do not contribute to the balance of the crankshaft and are not required to be installed when the crankshaft if balanced. Both methods are used from the manufacturer. An example of some factory internally balanced engines are Chevy 305 and 350 (2 piece rear seal only!), Chevy 396/427, GM LS-series, and Ford "modular" 4.6. Some examples of factory externally balanced engines are Chevy 400 and 454, Ford 302 and 351W. Some engines are even a combination of both: being internally balanced in the front and externally balanced in the rear! The most common example of this is the Chevy 350 (1 piece rear seal) including LT1. Regardless of how an engine is balanced from the factory any balancing method is acceptable as long as the required harmonic dampener and/or flywheel is available.​
 
yes technically they are externally balanced. But it's not like a 400 or 454, it's not much weight on them....different bolt pattern too, so can't swap older flexplates/flywheels to the newer engine...
 
unless motor was built and balenced with that plate it will make about .0000001 diffrence in the real world.
 
If it's a stock motor then really aint' gonna make or notice a difference. A performance built motor, then should be balanced with the rotating assembly. Im pretty sure any weight is with the crank, so long as it's neutral it would work.
 
British Slave said:
All Chevy 350 engines prior to 1986 were internally balanced. In 1986 the 350's changed to a center bolt valve cover, a one piece rear main seal and a different crankshaft. These engines are externally balanced at the flywheel. If you look at the late flywheels you will see a pad of extra metal cast in. The harmonic balancers continued to be neutrally balanced.
Click to expand...

All is correct except that the centerbolt valve covers did not happen until 1987.
 
I dropped the 406 sbc into my truck and noticed the flex plate was a bit oblong. It rubbed on the side of the trans case. I sent the flex plate back to my engine builder and he ran that plate and then a new plate on a test assembly to determine if both flex plates were the same. They were, so it was an easy swap. I now have 20k miles on the engine and it is doing fine. It is built to 420hp/525tq and has been spun up to 6k rpm one time and has only spent a matter of seconds at 5500 rpm. It lives between idle and 4500 rpm for the most part.
 
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