The capillary tube on the original Bourdon tube temperature gauge had ruptured, rendering it useless. Evidently, mechanical temperature gauges can be repaired, but I opted to convert my 122 to an electrical gauge setup.
original gauge, with weird Bourdon appendix squiggling around in foreground
Figuring I was just going to tear it apart anyway, I bought the cheapest electrical gauge I could find at my local auto supply shop. For $16 plus tax, I got the gauge, sender, and 4 fittings with different threads.
Unfortunately, none of the fittings had the right thread for my B18 head, so I ended up drilling out the fitting from the original sender, and tapping it to 3/8" NPT for the new sender. (Supposedly, electrical senders from B20 heads will also fit, but I decided not to chance it.)
new sender unit on left, original fitting on right drilled and tapped with 3/8" NPT thread
new sender installed
I was able to remove the old temperature gauge while keeping the rest of the instrument cluster in place. I unceremoniously disassembled and discarded the guts of the old gauge, retaining the backplate, instrument face, and needle for the retrofit.
It felt a little weird ripping a perfectly good new gauge apart, but a pair of pliers and some brute force were all that were necessary to free the internals for "re-purposing."
old and new gauge parts meet on my workbench.
Hacking old and new together was relatively straightforward, but required some trial and error fitting. I was hoping to use the standoffs from the original backplate, but they were not spaced far enough apart, so I had to drill them out. I elongated the remaining holes, and made some new standoffs from aluminum tubing. Bolts through the backplate, standoffs, and into the flanges on the new gauge (tapped to receive the bolts) hold it all together. The bolts are a tight fit next to the screws that affix the gauge face, but it all comes together, albeit with minimal room for error. Wires exit through existing holes in the backplate (enlarged slightly).
original standoffs can be used to determine correct distance between backplate and face
In order to mount up the original needle to the shaft of the new gauge, I drilled it out using a miniature wire-gauge bit. In my haste, the hole is neither centered, nor the correct diameter. Because of the loose fit, I had to use some silicone adhesive, rather than friction fit it onto the shaft. I also accidentally drilled through to the front of the needle. Doh. Luckily, the bottom third of the gauge is hidden when the assembly is installed in the instrument cluster.
"Calibrating" the needle to the correct angle and whatnot was a complete shot in the dark. Accuracy will not be a forte.
managed to only slightly damage the original gauge face.
Back in the car, I pulled +12 volts off of the fuel gauge, grounded under the nearby turn signal flasher bracket (where another ground is already located), and ran the sender wire back into the engine compartment along the same route as the original capillary tube.
back in the dash and looking stock
It was surprisingly satisfying to power it up and see the needle jump to action! And now I can tell (relatively speaking) when the car is warm.