We've used a temp gauge once or twice just to see what we had. We've never used one for tuning an engine. We tune by sound and performance. A tach can help you find peak RPM, though you don't need one.

One time, we were running an O.S. Max .40 FP engine with an APC 10 x 6 propeller. The engine was tuned to peak RPM by sound and backed off a bit to insure it was running rich of that peak. We then put a contact temp probe against the cylinder head at the rear of the engine, where airflow was at the lowest velocity. We measured 375 degrees F. The engine was running on 10% Omega fuel. It would run "all day" at that needle setting.

As far as temps go, if you rely on them to find the "correct" needle setting, you are setting yourself up to destroy your engine. They are not quite reliable because everyone's engine will have a slightly-different installation and setup. Different fuels and propellers will also make quite a difference. One person't "good" temp will destroy another person's engine....unless the temp suggested is "stupidly low".

When it's running normally, a model airplane engine will become hot enough to cause a severe burn if you touch the cylinder head...that's a normal thing. You just don't touch the engine until it's had a chance to cool off after it's been run for a while.

Because they've become available, too many people think that using a temp sensor of some kind will make setting the engine easy...like using a formula: "Do this, then that and you'll be OK". Not going to happen.

Basically, at full throttle, if you give the fuel line a quick pinch and the RPM goes up, you can lean a bit. Repeat the process. If the RPM stays the same or goes down, you richen the needle until you get a small RPM increase with the pinch. You're ready to fly. Listen to the engine in the air to see if it leans more once airborne...usually it does, and you may have to richen the needle a bit more. Eventually, you'll get the hang of getting the engine set correctly.