The International Cessna 170 Association

All,

What am I supposed to do with the CHT ring sensor when I clean/rotate/gap the spark plugs. Do you replace it or try to anneal that thing while on the little wire?

I'm (obviously) not an A&P, and my daughter and I have been doing this stuff in the grass at our tie down. Anything exotic will likely be beyond us. First plane, and we've got about 400 hours on the engine in our first 2 years, so it does not come up that often.

Thanks for the help.

You do nothing to it. You do not anneal it. Reinstall it as you found it on the new or rotated plug, rather than the normal copper washer.

Yep. What Bruce said.

Copper washers are used on spark plugs because they conduct heat (and electricity) so well. The spark plug gets hot due to the ignition process (and remember that spark plug part numbers relate to their heat-range) and that copper washer helps transfer the heat out of that plug to the cylinder head where cooling fins can pass it to the air. It also is soft (when new) and helps seal the base of the plug to the cylinder to avoid leaks.
But copper hardens in-use (disrupted by bending or clamping) and cannot be counted on to re-seat and seal when re-used. That is why they are annealed if they are re-used.... so that they will be returned to a soft condition so they can mate with the plug and spark-plug “boss” on the cyl head again. (Annealing them requires they be heated to the point of glowing and then cooling them again before re-use.)
The CHT sensor is made of iron-constantin (or other in some designs) and is not pure copper. It doesn’t harden like pure copper and heating it to the point of annealing it might also destroy its ability to generate the current necessary to energize the CHT gauge.

Thanks!

I changed my plugs and reinstalled the ring sensor to the same position it was in before, now I do not get any reading higher than 50-60 C....

Bryan
Sounds like your ring has failed for some reason, either broken wire or thermocouple failure?

bgiesbrecht wrote:I changed my plugs and reinstalled the ring sensor to the same position it was in before, now I do not get any reading higher than 50-60 C....

Make sure the replacement sensor is of the same type as the old one. Most I’ve seen are type K (chromel-alumel) thermocouples, but I’ve heard of some being type J (iron-constantan). One clue will be the color of the lead wires going back to the instrument (lead wire must be the same metals as those in the thermocouple itself). Type K will be yellow paired with red. Type J will be white paired with red.