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So the EGT gauge on the 172 (and other cessna singles?) doesn't have a numerical scale on it, just markings every 25 degrees. I know that one is supposed to use the EGT for leaning operations (ROP, LOP, what have you), but it's always stymied me that there are no actual numbers on it. Any reason why this is so?

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up vote 13 down vote accepted

The simple EGT gauges do not have numerical scales because you never lean to a temperature. You find the peak temperature, set the movable red needle to that temperature, and then lean to x degrees lean (or enrichen to x degrees) of peak temperature.

Set the peak temperature at full throttle, full rich at a sea level airport.

This system eliminates the concerns of calibration: Is the gauge calibrated, did the mechanic install the probe in exactly the right spot, etc. etc.

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I'm not sure about Cessnas, but on my plane the EGT uses 2 probes - one in the exhaust, and one in the engine compartment. The gauge simply shows the difference between the two. –  Dan Pichelman Jan 1 at 22:21
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Dan P., I'd like more information on your second temperature probe. Is it measuring ambient temperature in the engine compartment? Something else? By the way, some planes have a carb temperature gauge, and this second probe might be the source for that gauge. The PA-28s that I fly have only one probe, into the exhaust pipe. –  Skip Miller Jan 2 at 20:28
    
yep, it's measuring "ambient". Now that I think about it, it's not actually in the engine compartment, but behind it on top of the fuel tank. Sorry for the misstatement. –  Dan Pichelman Jan 2 at 22:16
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