Identifying rear counterweights is not an easy task. There are very few features that differentiate the various counterweights from one another. The best way to insure that you know which counterweight you are dealing with is to scribe, or otherwise permanently mark the counterweight as soon as you take it off the motor. This only applies if it is the correct counterweight for the motor that it came on. If you are in doubt about which counterweight you are dealing with, or in doubt that the counterweight that came on your motor is the correct one, you will want to take your rotating assembly to be balanced, or purchase a new counterweight, in order to insure that you have the correct counterweight for your specific application.

If you plan to “mix and match” different parts, or use lightened rotors, you will need to have your rotating assembly balanced. Using the wrong counterweight can result in the motor shaking or vibrating, and in high rpm applications, can result in engine failure. High frequency vibrations can cause parts to shatter, especially if the motor is solid mounted to the chassis. For this reason, all high rpm motors should be properly balanced.

When we refer to the “front” of the counterweight, we are referring to the flat face of the counterweight that faces forward when the motor is in the car. “Rear” refers to the part of the counterweight that bolts to the flywheel, and faces rearward when the motor is in the car.
Front Side Rear
Front Side Rear
Measuring The Counterweight
With a good micrometer, measure the lip at the bottom of the front face. Measure only the flat section, as shown, do not measure the chamfer. We measured several counterweights, new and old. These measurements are meant as a guideline only, to help in identifying counterweights. If you are at all in doubt as to which counterweight you are dealing with, have your rotating assembly properly balanced, or purchase the correct new counterweight.
Measure the Flat Section, Not the Chamfer Click to Enlarge
Measure the Flat Section Avoid Measuring the Chamfer
Year, Engine, Part Number Description of Distinguishing Features Image 
Click To Enlarge
79-82 12A
No real distinguishing features.
No Real Distinguishing Features
83-85 12A
Has a circular cut in rear face, where the nut goes.
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76-85 13B
Early ones have “29”, “1029” or “1029A” stamped on them. Not all are stamped.
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86-88 13B
Sometimes has a groove on the lower part of the rear face.
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89-95 13B
No real distinguishing features.
No Real Distinguishing Features
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