From 3W on temperature (HFE engine?)

Checking of Cylinder Head Temperature ( CHT)
To be able to check the cylinder head temperature ( CHT ) a thermo element has to be installed which
fits under the spark plug ( M 10 thread ). Optimum operating temperature range : 180°C - 220°C. In
this range piston, spark plug and combustion chamber will be free of remains. Maximum temperature
of 270°C should not be exceeded. During a certain time at full speed the temperature could be 250°C.
Colour of the spark plug should be : light or middle brown. A grey / gray colour is a typical sign for
over heating. Temperatures above 270°C are critical and over 300°C, piston can seize and could
completely damage the engine.
Temperatures below 180°C create remains ( carbon ) on the piston crown. Increasing remains hit the
cylinder and cause abnormal running noises ( knocking ) which will increase the load of the needle
bearings and can destroy them. Under extreme conditions the crankshaft can break and a complete
damage of the engine will be the result.