Cylinder Head Thermostat

Most modern day cylinder heads are all made from alluminium alloys, although most diesel engines still come with cast iron castings for the cylinder heads. Heads don’t like the heat to soar over the desired operating temp limit when driving so make sure your thermostat is replaced at regular intervals. Years ago I discovered and pointed out a faulty batch of thermostats supplied by Landrover.

We fitted one to a 4 cylinder turbo diesel that we had remanufactured the (head, block and sump), for a Landrover dealership. The vehicle overheated and had to be towed back from Central Africa. I couldn’t believe that a new engine we had remanufactured could overheat and seize. My gut feeling told me to remove the thermostat on inspection. If you place a cold thermostat in a kettle you will see it start opening before the kettle reaches boiling point, this one remained closed. The result was, we were off the hook and Landrover SA discontinued the batch of thermostats.

When overheating an engine not only do the cylinder heads become stressed, but the pistons also made from aluminium alloy collapse, distort and shrink slightly. The change cannot be detected with the naked eye but when measured with a micrometer these changes are magnified. An example would be a new piston measuring 91.88mm to fit a std bore of 92mm should not collapse more than anything under 3 hundredths of a millimeter. With a standard piston to bore clearance of 2 hundredths of a millimeter this would mean the piston to bore clearance is now borderline at 5 hundredths of a millimeter, providing the bore is still standard and ‘on size’, (give or take another 100 th of a millimeter for bore wear), and its over the top.

This is the limit and experience has show that any more than this, results in an engine that uses oil and will show signs of loss in power. In most cases severe overheating will cause the rings to glaze the bore and collapse, thus loosing power due to the tension loss in the spring qualities left in the piston rings. Rings also tend to seize within their ring grooves of the piston due to the lack of spring tension forcing themselves outwards against the bore of the sleeve and sometimes break the ring land. You will immediately notice a nice constant stream of smoke emitting from your exhaust as you drive and guess what, your sub assembly will now have to be removed to replace the pistons, which ultimately means it’s time for an engine change or complete engine overhaul.

My advise is not only replace your thermostat on a regular interval but change all your water hoses when they have reached the 100,000 km mark and always fill the radiator with at least 50%, good anti freeze-anti boil additive.

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