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Installing a Tach

by Irwin Arnstein
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To Tach or not to Tach?

      When I got my 1972 truck, it had neither engine nor transmission -- which was fine by me as it was originally equipped with a 350 V8 and a turbo 350 automatic transmission. Compared to my 1959 1-ton, this truck would be a real Cadillac with coil springs, an automatic, and factory air. I also noticed that the truck had space for a tachometer in the dashboard. But when I started looking at the prices for obtaining one, I decided that the $350-550 the conversion would cost (depending on whether I just added one or replaced the instrument cluster) was too much.  I re-powered my '72 truck with a preferred Stovebolt 6 (230ci) and a 700r4 automatic.

      My 1959 1-ton’s speedometer has never worked, so I added a small Sun Super Tach II 2-5/8 “  tachometer (pn# CP79606). I then used a GPS unit to figure my speed and created a card that has engine revs versus truck speed in second and third gear. Once I got used to that, I never bothered to fix the truck’s speedometer. I thought to myself that this tach should easily fit in the instrument cluster of my '72 and the $35 cost was a lot more attractive than solutions at 10 times the price.


Now, on to the "How to"

      The '66-'72 instrument cluster has a backing plate that mounts the instruments, a gauge cover or mask that partially covers the instruments, a plastic lens with the indications on it for the gauges (like speedometer numbers and ticks for the fuel gauge), and a front fascia which is the shinny bit that holds the switches for lights and wipers. You can get a new fascia pretty cheap from most suppliers and doing so will make you feel like you have a new truck. You might have to modify it slightly though, or the plastic lens to make everything fit back together. On the back of the backing plate you have a flexible printed circuit board that hooks up all the lights and gauges to the truck’s wiring.

      By removing a few screws, you can easily pull the cluster apart. The instrument panel itself is only held in with a few screws and there is only one logic connector on the back that hooks the panel to the truck’s wiring. Once the cluster is disassembled you can put the front fascia on the instrument mask and use a soap stone or magic marker to note the location of the outside of the tach’s original hole or circle. (See figure 1) Then lay the lens on the mask and eyeball the location of where the tach hole will have to be cut in the instrument mask. The hole in the mask helps secure the tach and is required so that the tach will poke through to the lens.  The hole doesn’t have to be cut with surgical precision -- either as a big o-ring from tractor supply can be laid on the outside of the tach housing to clean things up.  You can then fit the mask, lens, and fascia together and see how you did. (See figure 2)

      Remove the tach’s original fascia or benzel and the clear glass lens. They are no longer needed. There is an internal plastic gasket on the tach that you should keep to prevent light leak through from its internal lighting.

      I opened up the Sun Super tach to see what made it tick and to see if it could be mounted on the backing plate, but it’s just a bit too long by my calculations. This means that once a hole is cut in the mask, join the mask to the rear backing plate and cut the same hole the backing plate. But before you can do that, you’ll have to cut the printed circuit board on the back of the backing plate. That’s an easy task which can be done with a big pair of scissors. You can be a little smarter than I was and only cut what you need to out of the printed circuit board. I have a spare cluster and if things don’t work, I’ll swap the circuit board and try again. (See figure 2a) You can also number the lans or wires built into the printed circuit board so you know what to jumper to what when you get ready to finish the installation. 

      Once the circuit board has been cut, you can then cut the hole for the tach in the backing plate. (See figure 3) To do these modifications you can use a good saber saw, a dremel with a cutoff wheel, an air nibbler, or air grinder. I used all of them. The backing plate is a decent gauge metal so it can be a bit of work getting the holes cut. In this project, I took a lot of time thinking about it before I did it -- and then the execution went quickly and went well. In computer programming, this means 80% design, 10% implementation, and 10% debugging/repair. My holes are pretty ugly, but with the mask and fascia and o-ring on the outside of the tach housing, you can’t see the ugly bit.

      I then mounted the tach in the mask and backing plate and prepared to solder jumper wires for all the lans that were cut on the circuit board when I cut the board to provide a hole for the tach to slide through. Note the number of the lines so I know what goes to what. (See figure 4)

      The plastic circuit board is pretty simple. A plastic backing has the copper lans laid on it which feed power to the lightlbulbs and gauges, and this is followed with a thin clear plastic coating to insulate the lans and fix them to the plastic back. Carefully cut the top clear layer off and remove it so you can solder directly to the copper lans. Use a razor blade found on any high school student, or an Xacto blade

      For soldering I used one of the new ‘cold heat’ battery powered soldering irons that only heats up when a contact is made between the split points on the soldering iron. These things only get hot enough to melt the solder and make soldering on the circuit board easier because they don’t get too hot. From there it's pretty easy to connect 1 to 1 and 2 to 2, etc. Cut a wire jumper the needed length, and solder to the cleaned off copper lans on the circuit board. At the most there are only 10 jumpers required using about 3 feet of light gauge wire. If a chimp like me can do it, then anyone can. (See figure 5). Note that on my board, the lan 4 had already been burned up and I just had to go a bit further up the line to fix it. Later on, I added a paper gasket under the soldered bits to insure no shorts would occur. Duct tape or black tape will also work.

      Once the soldering is done, then you are done, and the panel is ready to be reassembled and remounted in the truck and you can hook the tach wires up to your coil, wiring, etc. as needed. (See figure 6) As you can see, the tach looks good in its new home and has its own built-in lighting. Use the tach without its benzel and glass. It also has an internal gasket which you keep, and put an o-ring around its perimeter in the mask to make it look nice. Bolt the instrument cluster back together with the fascia, lens, mask, and backing plate in that order. (And it only goes together one way .)

Conclusion

      If I had a criticism, it would be that the tach reads to 8000 rpm and my 6 will be lucky to see 5000 rpm so a portion of the travel is wasted. The 6 will be working really hard to get past the middle of the tach’s sweep. The original tach has a better sweep and is also larger. So you get what you pay for, but, just putting in a stock tach, where’s the fun in that? And the stock tach ain’t $300 better neither (or at least in my book). I have also heard that a late '70s model Camaro or Firebird’s tach fits perfectly and maybe worth looking for instead of the Sun tach. However, any parts store (O’Reilley, Autozone, Pep Boys) has the Sun 2 5/8s tach. So it's very easy to come by and very easy to replace.

      Then again, I do have a Holley 390 cfm carb on a Clifford ram manifold and Clifford headers so if I just do something with the cam and con-rods maybe 6000 rpms isn’t … too … high … turbo … nitrous…..

Irwin Arnstein
"Arnswine"
1959 Chevy 1-Ton
Bolter # 2139
Garland, Texas (Dallas)

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v. August 2006


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