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Ask Away! with Jeff Smith: Out of Balance

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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  1. Pingback: Ask Away! with Jeff Smith: Out of Balance

  2. Jeff Engine development has been my livelihood and I knew about all of what you said. But I still had to read one paragraph twice, you were right. So keep the gold star.

  3. Daniel Wilson says:

    Jeff, I have a solid understanding of all of the potentially confusing Ford Windsor and Cleveland engine balancing strategies so I was able to make sense of your well written article without having to do a double take for clarification. You mentioned some of the advantages that can be realized with internal balancing. In my opinion, when rebuilding any engine, and especially one that’s going to be used in a performance application, internally balancing the rotating assembly should be included as standard procedure. A good example is my 351 Cleveland build with the extremes of drag racing in mind. Working within a budget didn’t allow for an aftermarket crank and rods. To achieve a proper internal balance specification required the use of several expensive Mallory (heavy metal) slugs in the crankshaft. The solid roller camshaft I chose will require around 7500 rpms to realize its full potential so the additional cost of internal balancing, especially when the engine will see higher rpms, should payoff in smoother operation and longer main bearing life. I’m also using a quality ATI “zero balance” damper from Summit Racing for the best results.

  4. Fred McGarvey says:

    Hi Jeff, thanks for this info. I have a 383 stroker came out of an automatic car, now has a 4-speed. I originally used the flywheel from my 350 that was in the car, had a horrible vibration. Switched to a flywheel for a 400, vibration still there, but not a bad as before. It starts to be noticeable around 3500 rpm and get progressively worse as rpm’s increase. If you could steer me in the proper direction it would be greatly appreciated! Looking forward to more of your articles!
    Thanks in advance!

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