I monitor both CHT and EGT on my bikes. When I researched the topic of temperature monitoring about 15 years ago, the consensus seemed to be that EGT was useful for tuning (carb jetting etc.), while CHT was better for running.
I set up my thermocouples the same way on each bike; The CHT thermocouple is attached to a ring terminal under the spark plug seat, while the EGT thermocouple is screwed into a welded-on boss on the exhaust header pipe, within 100mm of the piston skirt. As Pokey mentioned, putting the EGT thermocouple closer to the piston skirt will give a higher temperature indication. However, it takes some experience to interpret readings.
For CHT, I have adopted an absolute limit of 250degC. If it looks as if the CHT will reach that figure, I change riding style or let the engine idle until the temperature reduces. Of course, some lower limit may apply if the thermocouple is located in a different position. However, the ring-under-plug-seat location seems to be fairly standard and seems to allow the most plausible case for comparison between different engines. I have found that my air-cooled engines generally operate in the 125-200degC range for varied types of riding, whereas the water-cooled ones generally operate in the 100-130degC range. The usefulness of CHT on a water-cooled bike is that the gauge should respond a lot quicker than a water temperature gauge (although obviously a lot of off-road bikes don't even have those!).
I have found EGT to be a much less definitive parameter than CHT and it seems difficult to apply an absolute limit. There are a lot of factors which will affect EGT (although not necessarily indicating a problem), so the best suggestion I can offer is to get some experience with it on a known 'good' engine and see how it varies with carb jetting, fuel and oil types/mixtures, riding conditions etc. Once you have a good general idea of how it varies and the figures which are obtained with normal running, it will be a lot easier to spot trouble from air leaks, dodgy fuel and the like. Although the thermocouple is mounted within 100mm of the piston skirt on each bike, I haven't been able to achieve exactly the same distance in each case, due to the proximity of frame tubes, radiators, slip joints and other obstructions.
All of my experience is with two-strokes - temperatures normally obtained from four-stroke engines (particularly EGT) may be different.
Most of my bikes have a Westach 2DC1-2 dual CHT-EGT gauge. These aren't perfect, but are fairly robust and the system is self-powered (i.e. no battery - the analogue gauge just responds to the voltages generated by the thermocouples). I would probably find a digital electronic gauge easier to read, but these are frequently more fragile and need batteries (or rectified supply from a magneto) to operate. Also, I haven't yet found one which offers the simplicity of the Westach system.
Admittedly, one problem with monitoring temperatures and other engine operating conditions is that you may have to take your eyes off the road/trail/track/course to look at the gauge(s). This is not usually a problem on roads or smooth trails, but could spell disaster in other circumstances. Mounting the gauge(s) in an appropriate position may lessen the risk.
With some experience and care, monitoring CHT and/or EGT should give sufficient early warning of impending melt-down to allow large-scale engine damage (due to overheating, whatever its cause) to be avoided.
Regards,
James
