on my 64 v4 75 hp it is saying there is a different style low voltage ignition is there a certain kind of tach that they should use or is any marine tach ok for the ignition
Normally outboard tachs run off the charging system, not the ignition, so your ignition voltage shouldn't affect the tach.
I am not familiar with your engine. I would suggest you go to a marine store and look at the tachs they have. For each tach the installation instructions will have a list of all the engines that it will work on. Check to see if yours is on the list. If it is, that's the type of tach you need.
Does that motor have a magnito ignition? On our old 59 Johnson 75hp V4 we used an automotive tach that ran off the points in the magnito, just set it for an 8 cylinder four cycle engine.
1964 75hp motors with mechanical shift had magnetos and DC generators
1964 75hp motors with electric shift had battery ignition and alternators.
Neither one will work with today's alternator driven tachs
The battery one will work with an automotive tach that gets it's signal from points. Except cars don't have points anymore, so watch what you are buying.
The magneto one uses a special tach for mags. Hard to find except for used ones.
You can also use tachs that get their signal from a spark plug wire.
I ended up going this route, since it's cheap and super simple to install--just run one wire, coiled 4 times around any spark plug wire, up to a little digital tach. I bought the PT15E2 listed here: http://www.enmco.com
