Jim Smart
January 13, 2016

How many of you have rebuilt a 289/302ci small-block, fired it up, and discovered a horrible dashboard chatter from unacceptable engine vibration? If so, you undoubtedly have the wrong flywheel and/or harmonic dampener. Small-block Fords are externally balanced, meaning offset balance weight gets hung on the flywheel/flexplate and harmonic dampener. It means the dynamic balancing process includes flywheel/flexplate and dampener. This is an easy mistake to make because it tends to go unnoticed at times. Small-block Fords from 1963-1981 are 28-ounce offset balanced meaning you must have a 28-ounce offset dampener and flywheel. From 1982-up, small-block Fords are 50-ounce offset balanced to allow for additional connecting rod mass and weight. The larger 351W and 351C engines are 28-ounce offset balanced. Offset balance is determined during an engine rebuild—a builder can go 28- or 50-ounce during the balancing process. They must have matched components going in.

The difference in offset balance is more apparent in these harmonic dampeners. On the left is a classic 289 dampener (left, 28-ounce offset). On the right is a 50-ounce offset for the 5.0L High Output. By the way, people call this a harmonic balancer. However, it doesn’t balance anything—it is a harmonic dampener. It dampens crank twist.

Whenever you replace a flywheel or harmonic dampener pay close attention to which type you have. Offset balance weight size is easy to see on the flywheel, flexplate, and dampener.

Man…is that your Mustang shaking?