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Herald 1200 Water Temperature

Jem 1of3

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Hi All,

I recently purchased Henry Herald from Liversedge near Leeds. Every time I ran it only for short distances it blew water from the radiator. I have replaced the Head Gasket, fitted a 75 degree thermostat (there wasn't one in it) I drained and refilled the cooling system with non water coolant, and that seems to have cured it, though I haven't been very far in it. Has anyone fitted a water temperature gauge to a 1200 system?  Has anyone fitted an expansion tank? what can anyone advise please?

Jem 1 of 3

Edited by Jem 1of3
I forgot the script
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A temperature gauge would be relatively simple, you’d just need a sender and a gauge. It’s quite likely you already have a blanked off port for a temperature sender either on the top of your thermostat housing or on the water pump housing. Buying a sender specced for a Mk1/2 Spitfire would be easiest as it’s the same engine. Gauge won’t really matter so long as it’s a stabilised 10V one (the fuel gauge is stabilised on the 1200 right?)

There’s no need for an expansion tank, they were designed without one, just need topping up fairly regularly. If you’re desperate to do so it’d require fairly significant work. Have you given the cooling system a good flush out? Using washing soda is often recommended. In clean condition the standard system is perfectly good for what it needs to do!

I would first though get yourself an IR thermometer and measure the temperature at various points on a trip. It’s non invasive and easy, and will give you an idea if you really need to bother doing anything more immediately!

Edited by Josef
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oh I would be careful now because the beauty of water coolant is that it boils when the engine gets too hot so letting you know theres a problem before any damage occurs. With the stuff you got in there this wont happen and the engine could get cooked especially if you havent got a temperature gauge.

The other thing is you already have an expansion capacity as do all cooling systems because all liquid coolants expand a little with temperature and this must be accomodated somewhere in the system or it will be damaged by over pressurisation. If theres no external expansion tank then on heating the excess coolant is expelled by the rad cap and on cooling air is drawn back in to leave a void in the top of the radiator that then absorbs the expansion in the next run with no further discharges necessary.

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True what Johny says, it's a non-return system that requires regular topping up, there is always some loss over time. The coolant expands, runs out of the pipe by the cap, then when it cools you've lost some and now have a gap. Filling it up again was part of the regular service routine back in the day. There are some threads / posts already on fitting a gauge and sender, with some good photos, and it's a fairly simple job.


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yes agree an air gap in the rad header is quiet normal,     brimming the header will only expel excess coolant .

adding a overflow bottle wont help unless the overflow is sealed by the rad cap to ensure it is sealed to the filler neck and the atmosphere so a special cap is needed 


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  • 2 months later...

If it helps at all, my 60k mile Herald 12/50 shows around 70-75 deg C at this time of year on the capillary tube gauge I have fitted.
Temp rises to about 80-85 deg C when stood in traffic.

It has the standard alloy 4 blade fan only, and its original full width radiator which I had re-cored to original spec about 500 miles ago.



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