Autometer 2698 tachometer conversion

 The only part of all this that requires a lot of modification is the grey plastic housing for the original tach - you're going to chop it up real good. If that bothers you, don't read any further. 

When you receive the new Autometer tach, the tach movement will be contained within a sealed plastic housing - you're going to cut off the faceplate using a Dremel too and a cutting wheel. Remove the faceplate and the nuts from the two studs attaching the tach movement to the housing. Keep the nuts and the grommet, you're going to re-use them. 

After removing all of the guts of the old factory tach from the grey plastic housing that is attached to the back of the gauge faceplate, you will now whittle away at the plastic housing using a combination of a cutting wheel and a sanding disc. It's plastic, so take a little off at a time until you get it to fit like what is shown in the photos. You will also elongate the holes for the original threaded electrical connections as shown in the photos. 

Carefully remove the pointer needle from the Autometer tach (it pulls straight off) and glue it (I used super glue) to the old needle that you cut off the factory tach's pointer needle hub. Also, you may have to enlarge the hole in the gauge faceplate a little, but test-fit first and go easy. Note that the silver 'button' is part of the gauge faceplate and is not actually attached to or part of the OEM tach pointer needle. You will need to carefully bend it out of the way using a pair of needle-nose pliers. If it breaks you can repair it with some solder (I did this with mine).   

Fit the guts of the Autometer tach into the modified plastic housing and the studs should go through the holes that you elongated earlier. I put a little blue Loctite on the threads and then snugged them down. The grommet that came with the Autometer tach can be re-purposed as a pass-through for the three wires from the new tach - green (signal, connected to the negative - side of the ignition coil), black (ground) and red (connect to a switched and fused 12VDC source.) If you want to get fancy like I did, I bought a repro tach harness and hooked up the tach red wire to blue and have it under the dash going to the fuse panel, and the green wire to yellow and going through the firewall to the ignition coil. The ground can be run to the firewall, the interior lighting, etc.  

Attach the new tach and housing assembly to the gauge faceplate using the two original screws, and then push the tachometer housing through the printed circuit board. You can also use the 3 original screws to attach the tach housing to the circuit board. Rig up a connector between the new tach wiring and the factory tach harness (if you went that route) and you're done.