Ok so you want to take apart you tach. First the tach is a Magnetic flywheel inside a cone. When the Magnet spins it trys to spin the cone but the cone is held by a spring so it doesn't spin freely. So the faster the magnet spins the harder it pulls on the the cone which turns the needle.
Removal. First you have to remove the gauge from your bike. To do this simply remove the two acorn nuts behind the tach and unscrew the tach cable. Now unplug the Light and take the tach somewhere safe (a dirty workbench is suitable).
To dissasemble the tach all you have to do is remove the front cover by folding up the lip. See picture. becareful not to bend it too much because you can kink and tear it and then you'd have an ugly tach. After the front cover is off just unscrew the two screws behind the tach and pull out the assembly. Thats it. Becareful with the face because it is just paint on glass so dont rub it off. (not too easy to do but be careful).
Next up is going to be installing LED's into your gagues to give them a cool GLOW.