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I was always led to believe they weren’t necessary, so whenever I replaced a thermostat housing, I would throw the thermostat over my left shoulder. We all heard stories of thermostats jamming closed and letting no water circulate through the cooling system and motor with terrible consequences so why not throw this useless thing out.

Well later in life after owning many Valiants, and a short spell drag racing an AP6 in the early 1970’s and learning to build my own motors, I came to realise those little thermostats are there for a reason and now would not run a car without one. Why? Well I will explain.

Every internal combustion engine is designed to operate most efficiently at a certain temperature and it is desirable that this temperature be reached as soon as possible after starting and then maintained.

Warm up period should be as short as possible because until normal temperature is reached, engine wear can take place due to unequal expansion of different metals.

An engine operating at its optimum temperature makes most economic use of fuel and lubricating oil is maintained at correct viscosity reducing friction to a minimum so the function of a thermostat is to control the rate of coolant flow through the radiator.

There are two types of thermostats – they are known as wax pallet type, or bellows type.

Our cars are normally fitted with a wax pallet type.

Which is better? they both do the same job, however, the advantage of bellows type is if it fails it normally jams open not closed, whereas the wax pallet type will jam closed.

I must make mention here that to prevent localised overheating there must be some circulation of coolant around cylinder block when first starting engine and thermostat closed, and this is why we have a by-pass hose fitted. That is the little hose we all curse at some time when replacing it. The hose runs from the head to the water pump.

Handy hint, when replacing, push the top end on first making sure it’s up as far as possible and fitting correctly, slide the two hose clamps on then bend the hose in the middle and slide onto water pump ensuring the hose is now straight and with no bend, if it is not straight then it is too long and will need trimming. Once all good tighten up the two clamps. Be particular about where screws are on each clip and position them so you will be able to get to them with a screw driver next time you want to undo them.

Thermostats are very easy to replace and you will need a 180-degree Fahrenheit Thermostat.

If your thermostat does pack it in out on the road away from home then allow radiator to cool down, carefully remove top radiator hose from thermostat housing, remove 2 bolts holding housing and very carefully remove housing so as not to tear gasket, remove thermostat and replace housing, tighten 2 bolts and replace top hose, refill radiator, run engine with cap off and as coolant level in radiator drops keep topping up, replace cap and continue journey. Providing housing gasket has re-sealed there is no urgency to replace thermostat.

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