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Cylinder head temperature gauge (including tachometer)

by Had Robinson

Introduction

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....

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.  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 other than foul the spark plug and, to a lesser extent, other parts of the combustion chamber.  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 cheap brand because the oil burnt so easily.  100% synthetic oils retain their lubricating properties at higher temperatures and is why they should be used, but only with the correct types of fuel.

burnt Top 80 piston

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).

overheated Top 80 piston

overheated Top 80 cylinder head

overheated Top 80 piston

overheated Top 80 piston

1.  Parts needed

a. TTO Trail Tech Tachometer/Hour Meter Black #742-A00 (optional but highly recommended)
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 wide black nylon zip ties (commercial grade available in the electrical dept. at hardware stores)
e. black electrical tape

The TTO gauges and the CHT extension are available from Miniplane-USA.  Note: Other brands and types of CHT's may give different values than what appear on these pages.

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.

Top 80 engine showing cutout in air duct for CHT senir

3.  Gauge 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 be weakened.  A failure of the side bar while flying would be catastrophic.

Unfortunately 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 a grinding wheel or file to make them just wide enough to go through the holes in the gauge.

I have tried many locations, including mounting at the throttle.  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, and 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.

b.)  Attach the wiring  Use a minimal amount of electrical tape to attach the sensor wires to the side bar every (6) inches up to the frame attachment point.  The stock wire on the CHT is too short so be sure to add the (24)" extension.  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 during use.

Top 80 side bar showing CHT gauge mounting location

Top 80 side  bar showing CHT location detail

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 at any time.

c.) Tachometer sensor wire location  The red tachometer sensor wire can be loosely wound around the secondary (spark plug) wire 3 or 4 turns.  It can be secured with a bag tie, as here, if desired.  The tachometer electronics are very sensitive and will always pick up the spark.  Because I remove and change out engines so often, I do it the former way.

Top 80 detail of CHT and tachometer sensor wire location

d.) CHT sensor installation  Unscrew the stock crush washer installed on the spark plug with a pair of pliers (not the 2mm aluminum spacer washer) and save it (in case you need to run the engine without the CHT connected).  If you do not remove the stock washer it will not harm anything but your engine will have less compression and less performance.

e.) Spark plug reinstallation  Re-install the spark plug with the CHT sensor between the spacer and the spark plug.  While tightening the spark plug, hold the sensor so it does not touch the air duct 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.

Top 80 CHT sensor location in air duct

4. Testing

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.

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