Inline Tube - Technical - 68-72 GTO Hood Tach Install
There are a few manufactures that have reproduced this option and it comes with the tach, wiring harness and instructions for easy installation. We chose Classic Industries for our unit. We have verified the mounting location off the instructions and also made a template off an original hood, 12 ½” from the back edge and 10” from the side will locate the big hole. The hood area where the hole will be cut is first covered in tape to protect the hood. The template is traced on to the tape to locate the big hole and the 2 small mounting holes. We have also located the front rivet hole. We have now taken a small 1/8” drill and drilled four pilot locating holes.
We are using a 3 5/8” metal hole saw that was bought at Home Depot. The size is a bit smaller than the final hole because saw teeth angle out and as the saw moves around the hood the hole gets larger. This is a metal cutting HOL-MUL brand made by Lenox. This is the 5th time this tool has been used and it is a high quality tool that is still sharp. We have found others burn up before they penetrate the hood. Locate the small pilot hole and drill through the hood.
Most battery drills may not have the power to cut this big of a hole so we are using an electric drill. The first few bits into the hood give a pretty good grab on the drill so make sure you have a tight grip. Once you have penetrated the paint the drill must be slightly rocked in every direction to start cutting the metal away. Once the drill starts breaking through the hood it will want to grab so have a good solid grip. Once you are through the outer skin there is a small part of the under skin that will be cut. Keep the saw going until you are completely through.
Once the hole is cut there will be a bur on the edge of the metal. A half round file is used to clean the bur off. File until the edge is smooth and you have worked the bur off. The file will have to be angled to work the bur completely off.
The big hole is now done and the small holes are drilled to the correct size and the tach fitted to the hood. If the holes are off they can be slightly filed for a perfect fit. From the factory these holes were punched in the hood for a clean bur free hole. With a few minutes of work with the file and some sand paper the edge can look like it was factory cut.
The Tach is set in the holes and fits great. On factory cars a small hole was drilled to hold the front of the tach to the hood and held in it with a pop rivet. We have drilled this hole in the hood and marked the tach from underneath. We now drill an oversized dummy hole in the bottom of the tach so we can put a fake rivet in from underneath. This gives us the factory look and also allows the tach to be removed in the future if necessary.
You can see the rivet in the front and mounting nuts installed. We also added the tach sticker and the clip that holds the wiring to the edge of the hood lip.
The completed hood tach looks just a good as a factory - installed unit and works even better, thanks to the revised internals.
All cars with a hood tach, under hood light or hide-a-way headlamps had an accessory grommet and 1 ¼” hole in the firewall just above the fuse block. It is located with a dimple at the upper right of the fuse block. If your 68-69 GTO had hide-a-ways the same hole is used for the vacuum headlight lines and the accessory wires but uses the three hole grommet. If you just had the tach or hood light it is the single hole grommet.
The wiring runs in the existing clips and under the dash. The brown wire with the red end plugs into the accessory lead wire that is already part of the existing under dash harness, it has the mating red end. The black wire is grounded to the screw of the horn relay and the pink stripe wire goes to the Neg (-) Terminal of the coil. It is just a matter of plugging it all in.
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