CHT Gauges for Air Cooled Engines

First of all, this is a guide that expresses an interest I had in CHT's and how it could apply to a riding lawnmower engine.  Basically, it's a guide to do something fun.  You will find various names for the CHT on Ebay.  The proper name is Cylinder Head Temperature (CHT) Gauge.  There are several abbreviations of the name, but I will try to use CHT (gauge) without trying to confuse anyone.  Most air cooled engines (especially on riding mowers) do not provide enough information on the dashboard for the operator to know the condition of the engine.  I thought a 'temperature gauge' would be an interesting addition to my riding mower. 

Understanding a CHT -

Normally, tempertaure gauges measure the liquid coolant of an internal combustion engine.  These numbers vary from 165 degrees Fahrenheit to 250 degrees Fahrenheit.  Water boils at 212 degrees (sea level), but coolant can be hotter due to the pressures exerted via the closed cooling system.  But, cooling system theory is another story for a later time.  Air cooled engines do not use a liquid to cool the engine.  Air flow around the block, cylinder, and cylinder head removes the heat from the engine.  Since these parts are in close proximity of the heat (fire), temperatures can seem a little more extreme.  Hence, my research concerning what would be an acceptable CHT reading.  The reason for not using a CHT is that the temperature can lag behind the actual engine conditions.

For example, the Cylinder Head Temperature (CHT) is pretty far away from the crankshaft.  If the engine was overheating due to low oil or low oil pressure, there will be severe damage to the engine until the CHT registers this heat load.  In another words, the crankshaft, piston and upper cylinder walls will have to get really heated up to transfer that heat to the cylinder head spark plug area.  Also, if there is too much effective cooling at the cylinder head, the probe won't readily react to the internal temperatures of the engine.  Hence, this is one of the con's of using a CHT.  There has also been discussion of using an oil temperature gauge (for the engine) to possibly get quicker information.  The concern is, "where do you get the best reading" on any particular engine. 

CHT's are expensive if purchased new.  There are various brand names of CHTs.  The most common CHT uses a "thermocouple" that attaches under the sparkplug of the engine.  The thermocouple is a bi-metal piece that develops a Direct Current (DC) Voltage that is sent back to the CHT gauge.  The CHT converts the voltage into a measurement.  It's a very simple process, but requires some thought and planning.  Because the voltage is low, wire resistance can readily effect the readings.  Therefore, the correct wiring harness must be used.  Connections are critical.  There are drawbacks to using a thermocouple CHT.  First of all, most of them are ambient temperature (outside air temperature) calibrated.  That means the 'air' surrounding the gauge is the basis point for converting the DC Voltage into degrees.  Hence, a temperature inside a vehicle could cause an inaccurate reading of the gauge.  The overall idea, is that I really didn't care what the CHT read, but how consistently did it read the temperature.

Purchasing a CHT

I purchased my CHTs off of Ebay.    These two CHTs were used for snowmobile racing.  It took several weeks to bid and buy, but I was being particular.  I didn't want to spend a lot of money for a project that would be a waste of time.  I picked up these two gauges for $41.00 (on Ebay to include S&H).  They were used and did not come with any thermocouples (probes).  After I received them, I used a small Volt Ohm Meter (VOM) to 'diode test' the gauges.  The VOM can forward bias the gauge and you will obtain a reading.  I would recommend being extremely careful in this check.  Just momentarily touching the leads will get the needle to jump.  Both CHTs worked.  I needed to obtain a probe. 

The picture above is the thermocouple (probe) that is placed under the spark plug.  You should remove the spark plug gasket prior to installing the probe.  Also, be careful with the probe.  Even though it can take heat; it won't take abuse.  This picture also shows the name of the gauge (WestLine).  I performed a huge google search and could only find WesTach.  I called and talked with the owner of the business.  On back of the CHT were the model numbers....270-6.  Because I needed a probe, the owner took

my order over the phone....and for another $20.00 (including S&H), I was into the CHT for a probe.  What was really nice about WesTach was the customer service. 

WesTach and Installation

I spent some time talking about the history of the company and how CHTs operate.  I also asked about the operating temperatures of air cooled engines.  Overall, I was informed that most air cooled engines (land based) really don't like getting above 400 degrees Fahrenheit.  Also, WesTach rebuilds older CHTs and sells newer models/styles.  They have a nice web site and it can easily be found by searching.  Even though my CHTs were originally going to be used for snowmobiles, there shouldn't be any major problem connecting them to a riding lawn mower engine. 

I connected the probe to the CHT.  Because the probe and CHT are polarity sensitive, I had to check if the gauge was operating properly.  You can get the positive and negative leads connected backwards and this will cause the CHT to read a negative number.  Basically, you'll peg the CHT to zero degrees.  I used a small lighter and heated the probe very slowly.  Soon, the CHT was reading a temperature higher than 250 degrees Fahrenheit. 

I connected the probe to the sparkplug of my riding lawn mower.  The MTD brand (Huskee SLT 4200) uses a Briggs and Stratton 18.5 horsepower, single cylinder engine.  I removed the spark plug and installed the probe.  I then positioned the wiring and CHT on the frame of the tractor.  Since the spark plug is towards the right side of the unit, I placed the CHT on the right side of the frame. 

Getting measurements (temperature/readings)

The picture above shows the CHT with a chrome gauge bracket installed.  The red needle is indicating just below 100 degrees Fahrenheit.  This is the ambient temperature calibration factor I previously mentioned.  The horizontal line on the CHT's glass is a reflection.  The CHT shows a major 100 degree line and a mid-point line (50 degrees).  During normal operation of the mower, engine temperature will indicate a little more than 350 degrees.  This is with the engine running wide open (in accordance with the owner's manual), cutting/bagging grass, and moving the unit in a forward direction.  At idle, the temperature will come down to about 250 to 275 degrees. 

End of Project

I am very pleased with the addition of the CHT to my riding lawn mower.  Thus far, the temperatures have been consistent with the operation of the tractor.  I enjoyed the hunt of finding a CHT and probes.  I also enjoyed learning about CHTs and how they operate.  As a side note, I have thought about installing an EGT (Exhaust Gas Temperature Gauge) on my mower!  So I have upgraded my riding lawn mower to an active oil pressure gauge and a cylinder head temperature gauge.  More seriously, I plan to install a volt meter on the unit, but that's another guide.

If you enjoyed reading this guide - - or - - learned anything about CHTs - - or you just thought it was a neat guide, please vote YES.  If you didn't care for it, please drop me a line.  Thank you.