[SAJ]

That's great Keith. I just put another of Murray Horne's Dampers in Diane's Tudor which had a resonance at 55 mph. The harmonic has vanished too and the engine sounds more pleasant throughout the Rev range.
Both cars have counterbalance weights welded on the crankshaft and were dynamically balanced.
Since I don't hear a lot from others on the Barn about these vibrations (perceived more by sound than feel in my experience) I wonder if the welded-on weights shift the rev range lower into the range we drive at.
My Schwalms crank has harmonics at 1000, 1500 and 2100 rpm on my gauge, so as I traverse upwards through the gears the clatter from the timing gear was almost constant. And I had to avoid cruising at 2100 rpm which is about 57 mph.
Now It will sit there quite silently with no other changes but the rubber damper pulley.
SAJ in Nz