Installing CHT Sensors




I initially installed the CHT sensors as described in the Rotax installation manual bending the sensor ring at a right angle to the sensor lead. Installing by this method has been problematic from the very beginning. The problem is that the spark plugs fit into a machined recess in the cylinder head. The depth of this recess varies (see photo below) and is deeper on the intake side than on the exhaust side. This recess, plus the need to have clearance for the wrench socket means that a simple right angle bend as illustrated in the Rotax manual will not work. I tried to make a complex 'z' bend in the sensors, but these have not worked out over time. There are too many clearance issues between the recess and the socket. The result is that the sensor rings bind on the plugs and get mangled and stretched. The binding makes changing plugs an unnecessary ordeal. The stretching means that the hole in the sensor becomes oval instead of round. This makes it impossible to get a good seal which results in dark, oily leakage onto the fuselage under the engine and probably some loss of compression and/or air suction into the cylinder - neither of which is a good thing. The photo shows the old, mangled sensor along with a new replacement. The new sensor has just a slight offset bent into it for the new installation.







Credit for the idea for this installation goes to Bob Robertson. The sensor leads are routed inside the cooling shroud and through the fins of the cylinder head from the exhaust side. This photo shows the hole in the shroud just behind the fan on the exhaust side. A rubber grommet is intalled in the hole and the sensor leads are run through the grommet (only one lead is shown in this photo). The leads should be run through the bottom shroud (top shroud on an inverted engine such as the Challenger) so that the top shroud (bottom shroud on an invert) can be removed for service - such as cylinder head retorque - without disturbing the sensor leads.



The sensor leads emerge from the fins right at the plug and lie nearly flat instead of running down alongside the plug and making a difficult bend within the plug's recess. The sensor is totally out of the way of the spark plug socket. In addition, the crimp fitting on the sensor lead bears on the adjacent cooling fin. This prevents the sensor ring from trying to spin as the plug is torqued down. The sensor is installed against the head; and the compression gasket on the spark plug gets a light coat of permatex high-temp anti-seize lube. This also helps keep the torque on the plug from being transferred to the sensor ring. The result of all this is no distortion of the sensor ring, a good seal, and easy installation and removal of the plugs. Much better; thanks Bob.





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