Harmonic Balancer Write-up by Dane Deal:
"I have a Lycoming O-320 with the old-fashioned conical mounting with a fixed pitch metal prop. Like all Lycomings it likes to shake, especially around 1'200-1'300 rpm's.

First, I had the prop dynamically balanced for $250. This seemed to cut the "felt" vibration in half and gave me an increase of about 20-30 rpm when at full throttle on the ground. While it was a dramatic improvement, I still was not satisfied. It was pretty smooth in cruise, but when climbing at full power there was still enough vibration to get your attention.

Second, I purchased a harmonic balancer from:
Mark Landoll Starters & Alternators
1205 Redbud Lane
Newcastle, OK 73065
405-392-3847
[email protected] 

The balancer was $375 and is of very high quality. It is a machined aluminum ring that is hollow and contains a steel ring suspended in a silicone fluid. It weighs 12 pounds and bolts to the front of the starter ring. It was very painless to install and required no modifications to my 6A.

I could feel the difference the minute I started the engine. Even sitting on the ground idling it seemed to smooth out the engine's firing pulses, and it was even better in the air.

Over the first several hours of flying I could feel and hear the engine's vibrations changing and disappearing. It takes a couple hours for the balancer to "break in". During the break in period I would feel a new, minor vibration slowly appear then disappear 15 minutes later. This repeated for about 2 hours. Each time the vibrations became less and less noticeable.

After flying with the balancer for about 4 hours I noticed no further changes. The vibration was dramatically reduced at all rpm's, and the engine idles very well down to 700 rpm. This was definitely money well spent. Dane Deal  vaf.htm

O-320 Lycoming  160 hp with a fixed-pitch wood prop

Jim, who reports having carefully measured before and after, attributes 7 mph more speed to the harmonic balancer.