We did that a few weeks ago when it first started reading high. We found
that on the ground, it was consistently 10% high. However, in the air,
particularly this time, it was reading considerably higher and bouncing
around. I understand that one failure mode is for the shaft bearing to wear
and let the rotating magnet touch the aluminum drag cup. When this happens
it reads high and bounces around. Ergo, order a new tach.

--
Bob (Chief Pilot, White Knuckle Airways)


"BTIZ" wrote in message
news:sTGrg.11095$6w.10302@fed1read11...
have the current tach calibrated before replacement.. those cheap hold in
your hand and look at the prop works pretty good.

BT