If heard of guys using an aftermarket tach but not sure the brand and how it ran. The posts above are correct the tach runs off a sensor that is plugged into the timing gear housing.
When installing it clean up the tip of the sensor because you can get metal fragments and other junk on it and that will cause the tach to not read correct. Also before you install it make sure the 2 wires coming out of it are not messed up, sometimes the wires will fray and break right at the base of the sensor and thus will also cause the tach to not read correct.
See the oil fill tube? The tach sensor goes just to the right of it.
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