At 08:21 AM 8/4/98 -0700, Simon Matthews wrote:
>.... On the MGA, there is, in essence a very large alcohol-filled
thermometer.
>
>The tapered pipe fitting pushes the bulb into the space below the
thermostat and (here is the key issue) through a hole in the centre of the
screw fitting runs the pipe back to the guage -- it is around this that I
wonder about leakage.
On the MGA the temperature gauge is essentially a mechanical pressure
gauge, much the same as the oil pressure gauge. It is driven by the vapor
pressure from the liquid in the thermometer bulb screwed into the cylinder
head. The line between that bulb and the gauge is a small pipe with a
spiral wrap wire around it to protect it. The pipe is soldered at both
ends to the bulb and to the gauge, so the circuit is completely pressure
tight. If the gauge can read anything like half scale, then there's plenty
of liquid inside to make it work, so you don't have to worry about a leak
where the small line attaches to the bulb. If the gauge doesn't move, or
barely moves at all when the engine is hot, then you've probably lost the
fluid and the gauge needs to be repaired.
Where the bulb is installed in the cylinder head there should be no sealant
or gasket required. The bulb is made of soft brass (copper) and seats
against a shoulder in the head. The threaded fitting that holds it in has
a straight thread and pushes the flange of the bulb tightly against the
shoulder in the head to affect a pressure tight seal. Just put a little
anti-seize compound on the threads so it's easy to disassemble the next
time, and don't over tighten the fitting.
Barney Gaylord
1958 MGA with an attitude
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