I have a '78 1400 on a '75 Glastron...the original thunderbolt tach does not work, it acted like it wanted to...it went to about 500 RPM's and then slowly sank to zero...also, from my observation, the previous owner had the wires on the wrong terminals...maybe the tach quit and they just tried swapping wires or something...it is the "square faced" Stewart Warner style just like the early seventies tachs, and it is a 6000 RPM tach and it has a plastic body...NOW, i have another thunderbolt tach, it is just like the previous tach, but it has a metal body, it was built in 1970 and is supposedly NOS...looks brand new too...BUT when i hook it up, it pegs the tach needle past 6000 RPM's....could this newer tach i have, be for a mercruiser with inboard engine with an alternator (i thought all the 6000 RPM tachs were for outboards)...could i be wrong here...AND if i wanted to buy a new tach, what tach do you buy to replace the thunderbolt tachs??...I even hook the tach's straight to one of the yellow wires from the stator at the rectifier...same results all the way around...Got any ideas folks?...