I don't think anyone mentioned the early Cherokee 180s. They have (or
should have) a red arc on the tach from 2150 to 2350 and a placard
stating that continuous operation should be avoided in that range.
The reason was harmonic vibrations developed between the Sensenich 76"
prop and the hollow cranked O-360-A3A. There were some prop tip
losses in the late 60s that prompted the restriction.

The restriction was removed in the '68 and later models, with the
introduction of the solid cranked O-360-A4A engine.

John Galban=====N4BQ (PA28-180)


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JGalban
Posted at www.flight.org