Dan:
Inside the gauge is a curved copper tube. One end is connected to the
input line and the other to a gear train, rack and pinion, if I remember
right. The rack is on the curved tube, and the pinion on the needles
shaft. There might be an intermediary gear to amplify the movement.
I've seen some use a pull wire and return spring. Pressure inside the
tube causes the tube to want to straighten out. As the free end moves,
its movement is transfered to the gauge needle. It works like those
curled up New Years Eve party play things with the feather on the end
that you blow into. You might even have gotten one at a circus.
It's not necessary for oil to get into the tube. That's why a thread a
while back suggesting bleeding the oil line to the gauge is not going to
help a non functioning gauge unless there is a restriction in the line
going to the gauge.
Blake
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