Capillary temperature gauge and fuel pump

The early NK cars had a capillary, or mechanical, style water temperature gauge.  I think it changed to electric around ’66 or ’67.  The capillary gauge works on a principle similar to that of a thermometer and bulb where a fluid, ether in this case, expands when heated and acts on a Bourdon gauge which is still found on your basic $20 “mechanical” temperature gauge found at Autozone.

Capillary gauge bulb and receptacle

The temperature sensor, or bulb (top element on the picture above), on these cars is screwed into a hollow cylinder (above picture, bottom) receptacle which itself is screwed onto the cylinder head at the thermostat housing.   The thing missing from the picture above is the capillary tube connecting the bulb to the Bourdon gauge.  When removing these bulbs from the head one must unscrew the bulb off the receptacle by turning the knurled ring since this avoids the capillary tube twisting as it would if you simply unscrew the whole receptacle.  Mine was almost ready to snap having been twisted too many times in the past and when I tried to remove it it snapped.  Immediately you could smell the sweet smell of ether.

The capillary tube is very small and the orifice within it even smaller.  It’s incredibly small.  I have no idea how you’d manufacture such a small-orificed tube.

Can you see the orifice in the capillary tube stump? Click on it to enlarge...

Some folks have detailed online how to go about repairing these by grafting a new  tube and bulb from an off-the-shelf mechanical gauge ( ) I tried to do it *twice* but it didn’t work.  The new gauges’ capillary tube is much larger in ID than these are and although I was able to mechanically match everything as they describe, I don’t think the pressures from the new gauge/bulb combo have enough force to activate the old BMW gauge.  I was able to source an identical gauge on Ebay with a working bulb/tube.  It was also stamped “1965”, something which makes me feel all warm and fuzzy that I’m doing right by my car.

Capillary tube


Below are some pictures of the mechanical fuel pump in various stages of disassembly as I was in the process of cleaning it up and visually inspecting it.   This pump is the same ubiquitous design of seemingly all cars of the era that I am familiar with (Fiat, Citroen and BMW).  It has a built in screen filter plus the carburetor also has a fine filter in it which makes me wonder if a third external fuel filter is needed as these cars were never fitted with one.


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