Is the an STC to put an electric, dual needle, tach in a pa-30/39. Vans sells a unit that attaches to the accessory case at the mechanical tach drive that acts as a "tach generator". You screw the unit on to your mechanical tach drive housing and the shaft goes where the origingfal one was and it drives some sort of transduser that provides rpm infor for the tach. Most 6 cylinder beech/cessna/piper twins have a pad for a tach generator on the accessory case but not the 4 cylinder O-320's.
bernie