This is a harmonic balancer for a 400 c.i.d. externally-balanced small-block Chevy. Note how the balancer weight is not consistent around the circumference. The extra weight is the external balance. A 400 c.i.d. externally balance flexplate would have weighs welded to the flexplate while a neutral balance flexplate would not have any added weight. (Image/Jeff Smith)

I have a Camaro with a 420 c.i.d. small-block Chevy that I recently bought. We pulled the engine to freshen it and the question came up as to whether my engine is internally or externally balanced. Since I know almost nothing about this engine, is there an easy way to tell or do I have to take my engine to a machine shop and have them decide. Thanks for your help.  — J.T.

Jeff Smith: On most engines, it’s fairly easy to tell the difference between internal and external balance.

The quick answer is that if the rotating assembly is externally balanced, it will have an offset weight on the harmonic balancer and the flywheel/flexplate. If the engine is internally balanced, the harmonic balancer and the flywheel / flexplate will be neutrally balanced with no offset weights.

For example, all small-block Windsor Ford small-blocks are externally balanced from the factory. Every 302 c.i.d. Windsor Ford engine used external weights on the balancer and flywheel/flexplate for balance. What makes that more confusing is that Ford changed the value of the external balance value in 1980. On the other hand, all small-block Chevys (except for the 400 c.i.d. and another that we’ll address in a moment) were internally balanced.

The longer version answer involves why there are externally balanced engines. With regard to the small-block Chevy, all production small-blocks built until the 400 c.i.d. engine were all internally balanced. Because the 400 engine used a longer 3.75-inch stroke, the factory decided to add the additional counterweight mass to compensate for the longer stroke and larger piston by adding offset weight to the balancer and the flywheel/flexplate. The farther the weight is added from the center of the engine (fore-aft) the more effect this mass has to counter-balance the effect of the longer stroke.

This external balancing is a simple solution, but it has consequences. As engine speed increases as in the case of race engines operating at 7,000 rpm and higher, this eccentric mass generates increasing force on both ends of the crankshaft that are not conducive to long engine life and is especially damaging to main bearings. Most aftermarket long-stroke crankshafts like your 3.875-inch stroke 420 c.i.d. crankshaft are almost always built as internally balanced assemblies. This makes the counterweights larger and heavier but also eliminates external offset weights on the harmonic balancer and flywheel/flexplate.

On some performance steel flywheels, instead of added weight, the manufacturer will drill a series of holes on the opposite side from where the weight should be for an externally balanced engine. That creates the same effect as adding weight to the opposite side, so keep that in mind if there is ever any question.

You will find some externally-balanced performance crankshafts that are available for direct replacement engines. Scat Performance, for example sells an externally balanced cast crankshaft, but all of its 4340 steel performance cranks are internally balanced. This just makes life easier on the main bearings for a performance engine to run an internally balanced crankshaft, so that’s the best choice if you’re faced with a decision between the two different styles of cranks.

Of course, there are exceptions to this rule. In 1986 GM upgraded the classic small-block 350 with a one-piece rear main seal crankshaft to minimize oil leaks. If you look at a traditional two-piece rear main seal crankshaft rear flange (we’ve included a photo), you will notice a small, offset weight on the crank flange. When GM converted to a one-piece rear main seal, this required moving this offset weight (still considered part of internal balance) to the flywheel or flexplate.

The one-piece rear main seal flange uses a different bolt pattern, so this unique flywheel/flexplate will also incorporate a slight offset weight. The balancer on the front of these engines is still a neutral balance and the engine is considered a neutral balanced engine even though the flywheel/flexplate has an offset weight.

Two gold stars for those of you who followed this without having to read it more than once!

Sometimes this tech stuff can get confusing.

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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.