CHT Sensor Location
By: Jim Hayward
The normal location for the Cylinder Head Temperature sensor, or CHT as it's generally called, has traditionally been under a spark plug for a given cylinder. The problem with this location is that anytime the spark plug is checked or changed, the CHT wires have to be moved and this will sooner or later result in breakage.
There have been discussions from time to time about moving the CHT sensors from under the spark plugs to a nearby headbolt. Since I had never seen any reports of anyone's doing that, I decided I might as well do it and report whatever results I obtained to the Challenger list. I had a hundred hour inspection due in a few hours, so I decided to also relocate one of the sensors to a headbolt and compare the two locations on the same cylinder.
Deciding to use the MAG cylinder for testing, I removed the PTO CHT sensor from it's spark plug when I changed the plugs. I cut off the old terminal lug, getting one that would fit a head bolt stud. If the plugs were numbered 1 to 4 from the mag end, my senders were on the mag cylinder's #2 plug and the stud next to it with the large "coupling nut" on it, which also doubles for a shroud mount. I had the opportunity to fly with the new arrangement a few days later.
I fired up the engine and watched the two readings during warmup while looking for my 200º threshold temperature for takeoff. The plug CHT reached the temperature about 10º ahead of the headbolt CHT. This was my first surprise. A few more minutes brought the headbolt temp to what I wanted, and off I went.
Take-off was fine. The headbolt temperature stayed about 5º cooler than the plug temperature. About 8 minutes into the flight, I reached my cruise speed with a setting of around 5600 rpm. The headbolt temperature was about 3º cooler than the plug temperature. A few minutes later, both temperatures were within 1º of each other. Another few minutes brought them to identical readings where they stayed for the next 20 minutes or so. At that point, I decided to drop back to 4600 rpm and descend.
I wanted to check how fast the two sensors cooled off during a descent. The plug temperature cooled faster than the headbolt temperature. This was my second surprise, but they still stayed within a few degrees of each other.
During the entire flight, while in cruise, the temperatures were never more than 1 degree apart and usually were identical. I attributed the lag difference to the mass of the cylinder head vs. the thinner mass where the spark plug screws into the head.
To make the change, cut off the terminal ring used for the spark plug. Obtain some 1/4" rings and drill them out slightly to fit the head bolt. If you can find a metric terminal lug, so much the better. In some cases, depending on the manufacturer, a 5/16" ring can be used nicely if the ID vs. OD of the ring is thin enough to allow it to seat in the milled land where the stud screws into the head. Strip back the CHT wire's insulation about 1/4" or so, twist the wires together, and fold them over double.
There should now be about 1/8" of wire showing to crimp in the terminal. Folding them over gives more bulk for the wires to be crimped in the terminal lug. Crimp the wires and it's ready to install in it's new location.
Remove the large coupling nut on the headbolt stud nearest the spark plug. Place the CHT terminal lug on the headbolt stud and replace the coupling nut. Torque it to spec, and you're done.
This new location gives you the freedom to check your plugs as often as you like without fear of damaging the CHT terminals when the plugs are removed or installed. Also, the spark plug will extend into the cylinder just a bit more as it's designed to do. You will notice a bit longer warmup time for the engine, but you will also know that you're taking off with an engine warmed up as it's supposed to be.
The testing and modification was originally done in September of 2003. About 3 months later, I started noticing an intermittent fluctuation of the PTO CHT readings during engine warm-up. All was okay after reaching operating temps. They got so bad that they would jump around varying as much as 50º lower than the mag CHT. There were also some melted spots where the heatshrink tubing had rested up against the head fin edges.
I pulled the cooling shroud off last week (January 2004) expecting to find a loose CHT ring crimp causing the problem. Instead, I found the CHT wires chaffed thru where I chose to feed them out of the shroud. I also noticed the beginnings of rust on one of the wires in the PTO sender's crimp.
There was a thermocouple website posted by someone on the FlyChallenger list in December of 2003. An article there described thermocouples, the different types, and their operation. It also gave the types of materials used with the different thermocouples. Our J-type uses iron, therefore, the bit of rust I saw on my wires.
More interestingly, it stated that a third metal may be introduced in a thermocouple junction (such as soldering or welding) without affecting the accuracy of that junction. This is possible as long as the third metal is kept at the same temperature as the junction itself. Therefore, I decided to silver solder the ring crimp to protect the wires from rust and further secure them in the ring. Since I was going to solder them, I decided not to double them over in the crimp.
These pictures show how I've changed the routing of my CHT sender wires. I repaired the chaffed areas, drilled a hole in the mag end casing and routed the wires thru that hole. I put some high-temp silicone around them on the outside to help secure them at the hole. The wires had previously been routed out along the opposite side and under the shroud edge in the middle of the engine. Vibration was such that the insulation chaffed and exposed the wires to the cooling shroud thus causing my problem.