The International Cessna 170 Association

Bruce, this IS Tom. I have successfully balanced under a skull cap, but as you guessed, there usually is not enough thread to allow for much weight. I keep a pile of "modified" AN970 washers which are trimmed to allow skull cap clearance, but the number one can install is still dictated by...

Just a few interesting notes. When researching sources for dynamic balance, ask if they can provide a spectrum analysis. As George mentioned, a spectrum analysis can reveal an "unhappy" engine. If everything looks good, at least you will have a baseline with which to compare in the future....