Cylinder head temperature gauge (including tachometer)
by Had Robinson
updated October 1, 2020
A cylinder head temperature gauge (CHT) is an essential tool used to monitor the running temperature and health of a (2) stroke engine. It is also good to have an hour meter/tachometer so you will know if your engine is running at peak output and when maintenance is required. Would you own a car that did not have an odometer nor a temperature gauge? You may never need them and could guess when maintenance is due, but then....
NOTE: In extreme cases of fuel starvation, a CHT may not be helpful because there is so little fuel reaching the engine that it will not even run at or near full throttle. In this situation, the engine does not overheat. Always do a thorough test of the fuel delivery system before assuming there are ignition or mechanical problems with the engine.
An unmodified Top 80, for example, will have a cruising temperature between 120°C - 170°C with the TTO CHT gauge, depending on ambient conditions and altitude. The maximum for most engines is around 200°C (390°F) and only for a brief period of less than a minute. Some of the bigger engines can safely run at 250C (482F). I have yet to see any Top 80 get even close to running the max temperature. Having an engine run too cool (rich) will not do any harm. Running too hot (lean) can quickly destroy an engine.
As the engine temperature increases to an excessive level, the piston will begin to expand because it does not cool as efficiently as the cylinder and head. As the temperature increases even more, the piston will expand enough that its diameter will begin to exceed the inside diameter of the cylinder. At this point, irreversible damage occurs to both the piston and the cylinder. If the temperature increases very quickly, the crown of the piston can melt at the center and form a hole. As things heat up even more, the piston will seize and the engine will stop.
This is a piston from an overheated Top 80 engine. It had seized in the cylinder while running. Note that the lubricating oil on the surface of the piston has burned. Burnt oil cannot lubricate engine parts. If the pilot had had a CHT, an expensive repair could have been avoided. In this case, the overheating was caused by fuel starvation from a clogged fuel inlet filter and a worn out fuel pump. The oil used in this engine was probably some inexpensive brand because the oil burned so easily.
Running the engine too hot will destroy the lubricating properties of the oil. Burnt/charred oil can cause the piston ring to bind to the piston lands. In addition, the piston, ring, and cylinder will experience much greater wear.
Bottom line: a cool running engine will last longer and have fewer problems. While you are at it, you should also install a tachometer/hour meter gauge.
Here are some photos of what can happen to an engine. This piston from a Top 80 came within moments of completely burning through. The piston ring lands got so hot that they separated and melted causing the ring to become loose (3rd photo). The melting piston began to hit the cylinder head, as well (2nd photo).
1. Parts and special tools needed
a. TTO Trail Tech Tachometer/Hour Meter Black #742-A00
b. TTO Trail Tech Cylinder Head Temperature Meter Black 14mm Sensor #742-ET3
c. TTO Trail Tech Temp. sensor extension lead (24") #V300-24 (The extension MUST be used on the Top 80 and most other motors!)
d. 4.5mm (3/16") wide black nylon zip ties (use commercial grade which is available in the electrical dept. at hardware stores)
e. Black electrical tape
f. #14 drill (11/64" or 4.5mm) or reamer to widen the mounting holes in the gauges
The TTO gauges and the CHT extension are available from Miniplane-USA. Be sure to check the owners manual of your engine for its maximum operating temperature!
Note: Experience has shown that manufacturers measure cylinder heat temperature differently. The maximum values in their specifications are often greater than what we see with the TTO gauges.
2. Cooling air duct modification
Depending on the engine, the cooling air duct may have to be modified in order for the CHT sensor to be mounted underneath the spark plug. It must be modified while it is installed on the engine or the cut will not line up with the cooling fins on the cylinder head. There is just enough room between the fins for the sensor to fit. The Thor engines do not require any air duct modification but you will have to remove (or modify) the rubber seal that goes between the spark plug and the air duct.
Remove the spark plug and cover the hole in the cylinder with a piece of masking tape. You do NOT want debris falling into the cylinder! Using a Dremel tool or a round file, make a cut in the duct by the spark plug 3/8" long and a width of the space between the fins, as shown in the photo. The cut is in line with the spark plug hole and perpendicular to the two duct mounting points. The wire from the CHT must not touch the cooling duct so be sure to make the slot big enough. It is better too big than too small. The CHT wire is delicate and should never be pulled tight at any point.
3. Gauge and sensor installation
The method used to mount the gauges on any Miniplane frame are the same. Other motors are similar.
a.) Mount the gauges Use nylon zip ties to mount the gauges on the left side bar. Do not drill holes in the side bar in order to mount the gauges because it could weaken important structural materials. A failure of the side bar while flying could be catastrophic.
I have tried many locations. The location shown here is the best and will result in the least wear and tear on the sensor wires. Having anything on the throttle increases the risk of catching the throttle on the risers, glider lines, or straps. If you are having trouble in the air, you do not want the throttle to catch on anything. The gauges are also easily visible on the side bar. If they are mounted on the upper sticks, for example, you will have to turn your head quite a bit to read them. Mounted on the left side bar, it is easy to glance at the gauges without taking your eyes off where you are going.
The holes in the gauges are slightly smaller than the width of the ties. Smaller ties could be used but they are likely to break which is why the 4.5mm ties should be used. Use the drill bit or reamer to widen the holes in the gauge so that the ties will fit through the holes. If you drill the holes without using a drill press, be careful because the bit can snag the plastic and split it. A reamer works much better than a drill bit.
b.) Attach the wiring For $10 USD or less on eBay, pilots can purchase a small roll of 1/2" HST (heat shrink tubing) which can be used to enclose the tachometer and temperature wiring. A typical Miniplane frame requires about 17". The wires must be fished from both directions with a thin line. 4.5mm nylon zip ties or a minimal amount of electrical tape can used to attach the sensor wires or HST assembly to the side bar as illustrated below up to the frame attachment point. If you use HST, it should not extend beyond the rear of the side bar.
The stock wire on the CHT is short so the (24)" extension must be used for most paramotors. The Minari, for example, has a horizontal engine so the extension is not needed.
Look at the last photo on this page and note how the sensor wire is run along the back of the frame. Keeping it close to the engine's center of rotation helps cut down on the intensity of engine vibration and its effects on the wires.
HST is the best way to attach the gauge wiring because it protects the temperature - sensor connection.
There must be a loop in the sensor wires (as shown) so they can flex with the up and down movement of the side bar. Stow the extra wire lengths in loops just below the side bar attachment point. The sensor wires are very durable but they cannot be stretched.
c.) Tachometer sensor wire location The red tachometer sensor wire may be shortened as needed and loosely wound around the secondary (spark plug) wire 3 or 4 turns. If you wind the pickup wire with too many turns it will overload the circuitry in the gauge and the gauge may give erratic data. The red sensor wire can be secured with a bag tie, as here, but this is not necessary. The tachometer electronics are very sensitive and will always pick up the spark. Because I remove and change out engines so often, I do not use a bag tie or a nylon zip tie but merely wrap the wire around itself, as illustrated (2) photos down. The excess black tachometer wire and the excess CHT extension wire can be secured to the frame with electrical tape of nylon zip ties.
d.) CHT sensor installation Unscrew the stock crush-washer installed on the spark plug with a pair of pliers and save it (in case you need to run the engine without the CHT connected). If you do not remove the stock crush washer on the spark plug it will not harm anything but your engine will have less compression and slightly less performance. Use large pliers to unscrew it. BE CAREFUL NOT TO DAMAGE THREADS ON THE SPARK PLUG!
WARNING: SOME CYLINDER HEADS E.G. MINARI HAVE SPARK PLUG HOLE SURFACES THAT ARE NOT FLUSH! THE SENSOR MUST BE ORIENTED SO THAT THE FLAT AREA OF THE SENSOR ENTIRELY CONTACTS THE FLAT AREA AROUND THE SPARK PLUG HOLE. The Minari will have the sensor at the 8 or 9 o'clock position.
e.) Spark plug reinstallation Re-install the spark plug with the CHT sensor between the 2mm thick spacer washer (if there is one) and the base of the spark plug.
While tightening the spark plug, hold the sensor so it does not touch any parts of the air duct (if there is one) or the cylinder at any point. Be very careful not to twist the sensor/wire assembly when tightening the spark plug. Always use a torque wrench to tighten the spark plug to specifications.
The completed installation should look like this on a Top 80. Other engines are similar.
Carefully follow the directions included with the gauges. The CHT can be setup with either Fahrenheit or centigrade. I recommend using centigrade because all paramotor engines are manufactured outside the U.S. and the temperatures specified are always given in centigrade. Someday, the U.S. will abandon the antiquated, difficult to use English systems of measurement and fully adopt the metric. I am not optimistic that it will be soon....