You can use an MC18 rev counter and temp guage as long as you also have the MC18 sub harness for it. You can then unplug the 21 wiring and plug in 18 stiff into the rev conter. As for the temp guage, unscrew the 3 wires going to the back of it and fix them to the 18 guage. They both work the same. This way you have a nice neat and much better (like the 18 all round really) set of clocks. Hope that helps
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If I have to take the carbs off once more...