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April 5, 2020

Gauges

I believe that all GT6 and Spitfire cars were delivered with just fuel and coolant temperature gauges.  This is a shame, since many people consider a full compliment of gauges sort of a hallmark of a real sports car.  Never mind that the Smiths gauges of the era were not really known for their precision, accuracy, or longevity.  In fact, much of the time, they probably weren't much more informative than idiot lights.  Still, we sports car enthusiasts have our expectations.

My gauges were pretty crusty.  I didn't know if they worked or not.




The bezels twisted off after a little coaxing, revealing under the glass a two-piece dial face.  Under the face is the actual gauge mechanism.  These gauges are actually thermal instruments.  A voltage presented to the gauge terminals causes a current to flow through some thin resistance wire inside the gauge.  The wire is wrapped around a bimetallic strip.  The resistance wire heats up in proportion to the voltage, and causes the bimetal strip to bend.  The motion is transmitted through a linkage to the dial pointer.  I'll show the restoration process for the fuel gauge, but the temperature gauge is identical.




The metal housing was a little rusty on the outside, and was painted blue on the inside, apparently to tint the light from the small incandescent lamp that is pushed in from the rear.  I blasted it clean...




...then powder coated the outside, and painted the inside with a tasteful blue pretty close to the original.




The actual guts of the gauge looked fine, except for the pointer.  It was pretty faded on the part exposed to the light of day.  The protected part looked like a fluorescent orange.  I found something that was pretty close.




The dial faces actually looked pretty good.  I just cleaned them, touched up a few areas, including the white strip along the top.  I presume this was a reflective surface to get light down onto the face.




Then put everything back together.




Before adding the glass and bezel, I thought it probably wasn't too soon to actually test the gauge to see if it even worked and was anywhere close to accurate.  It did work, and after some fiddling with the calibration slots on the back, I got it close to the way I wanted it.  I didn't have a tank sender to test with, so I just used known voltage inputs.  In operation, the voltage the gauge sees is some fraction of the ten volts from the voltage stabilizer.  The fraction depends on the position of the float in the tank sender.  I like to have a gauge that shows empty before the tank actually runs dry.  It's a little like having a reserve tank.




Then it was time for the repainted bezel.  I used some silicone rubber O ring stock as a seal.  A little petroleum jelly makes sure it won't catch and bunch up when twisting the bezel.




The final gauge.




These are the calibration ports in the back of the case.  They get covered by those little cork plugs that are almost always missing.




The temp gauge was a re-run of the fuel gauge.  Again, I didn't have a sender to test with, so I set some reasonable calibration points.  I may have to revisit this if it is too far off.




The twins.




What I said earlier about it being a shame that the GT6 didn't come with a more complete complement of instruments was more than idle moaning.  At some point, I decided that my car will have proper instrumentation.  So I put out some inquiries, and a week or so later, thanks to some kind folks on the 6-Pack forum, I had a Smiths vintage oil gauge and voltmeter.  Both of these gauges in general looked better than mine.




Smiths made quite a lot of variants of these gauges over the years, so getting a perfect match is not easy.  The new (to me) gauges have a different bezel shape, and different fonts.  I ordered new bezels to match my originals, but the fonts are something I may just have to come to peace with.

The gauges got the same treatment as the originals. The voltmeter had a green lens for the illumination that had to be removed.

 


It was also fairly out of whack on calibration, but I got it pretty close in the middle of the scale.  It's still a ways off at 10 volts, but I don't think that's very important.




The oil pressure gauge is completely different inside, since it is a mechanical gauge and not an electrical one.  The first hint of a difference is the nut holding the guts in place.  A sacrificed socket became the "special tool" required to remove it.




The oil gauge uses a Bourdon tube.  It is just a curled tube into which oil pressure is transmitted.  The pressure tends to straighten the tube, and the deflection is linked to the pointer.




The parts of the oil gauge ready for assembly




The four sisters.  I don't quite know where the new gauges are going, but I'm noodling it.




This was a fairly quick project.  I'd done it once before on my TR6, so I sort of knew what to expect.  Other than the cost of the "new" gauges, the outlay was minimal--pretty much just some consumables.

Comments to Ed at elhollin1@yahoo.com

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