The Farm Machinery Digest Encyclopedia: Part 2, Engine Terms for Tractor Collectors.June 10, 2021
This is the second installment in a two-part series reviewing important terms for the antique tractor collector. Please use my Idle Chatter podcast as a supplement to Collectors’ Corner. Most if not all topics covered in the podcasts are applicable to any machine or engine.
Spark plug heat range, reach, and gap
The precise arcing the of the spark plug is required not only to start a gasoline engine but to occur on every ignition event.
As collectors, we do not use our tractors as their original design intended. Because of this, there are many engines that run poorly due to a misunderstanding about spark plugs.
The gap of the plug, which is defined as the separation between the side and center electrodes, is there to create an arc. If there were no gap, there would be no arc to ignite the fuel and air mixture since the electricity would go directly to ground.
The gap is the measurement of this space and varies from 0.025 inch to 0.035 inch. It is a specification and not a descriptive term.
The gap (measurement) creates the ionization window, where the mixture is ignited. The wider the gap (more clearance) the larger the ionization window, which is ideal, if examined in that respect alone.
The gap is a balancing act of creating an aggressive ionization window while allowing the ignition coil to jump it and keep an arc there for as many degrees of crankshaft rotation as possible. The ideal is to keep the plug arcing for an extended time, measured in crankshaft degrees.
The potential energy of the ignition system determines the magnitude of the ionization window.
An engine misfires when the gap is too wide (the coil cannot keep the spark plug arcing) or too narrow (an insufficient area for the air fuel mixture to enter and be exposed to the arc).
The reach of a spark plug is the distance from the beginning of the threads to the sealing surface. The reach along with the tip design will impact how far into the bore the ionization window will be placed.
Heat range of a spark plug is often misunderstood. In simple terms it describes how much heat will remain at the tip and not move into the casting of the cylinder head.
For a spark plug’s tip to remain clean and not collect carbon, it needs to maintain a tip temperature of a minimum of 932 F degrees. Most spark plugs are designed to operate reliably up to 1,472 F degrees.
If the tip does not exceed the minimum temperature, over time, it will become fouled.
The heat range is a metric to identify the temperature that remains at the tip and is not moved to the cylinder head.
Thus, what we call a hotter plug will transfer less heat and a colder plug will move more temperature to the cylinder head.
A hot plug is slower to transfer heat while a colder one moves it from the tips quicker.
The length of the porcelain insulator cone that is exposed above the casing of the plug determines the heat range.
A hot plug (slow heat transfer away from the tip) will have a long insulator cone. The tip will stay hotter and the deposits will burn off.
Conversely, a cold spark plug will transfer heat from the tip quickly due to a short insulator cone. The tip will stay cooler and be more prone to deposit formation.
This term describes the mechanical clearance between the camshaft and the valve, measured at the juncture of the rocker arm and the tip of the valve stem.
It can be considered the free play in the valve train. It is used to compensate for the expansion (growth) of the valvetrain from heat.
The lash is extremely important. If it is too loose (increased free play) the valve will not open far enough and will open later in the camshaft’s rotation. When excessively tight, the valve has the possibility of not seating fully.
Either too loose or tight will impact engine performance. A tight exhaust valve is more prone to failure (burning) since the exhaust gas has a reduced window to exit the cylinder.
When the lash is excessive, the impact of the rocker arm on the valve stem is amplified and will over time, mushroom the tip.
Breaker points and condenser
The breakers are the switch the allows the coil to be energized. When the breakers are closed, the coil is being charged. When they open, the field in the coil collapses and the spark plug arcs.
The gap of the breakers impacts the dwell. It is defined as the length of time in distributor rotational degrees that the breakers are closed, and the coil is being charged.
As the rubbing block of the breakers wears from contact with the cam, the gap is decreased, and the ignition timing retards.
As the breakers separate, the condenser is used to absorb the excess energy in the coil. This is necessary so that the breakers do not arc. If they do arc, the coil will not fire the spark plugs.
Coolant thermostat rating
The numbers on an engine thermostat identify the crack open temperature. This is when coolant flow begins to the radiator.
For example, a 180 F degrees thermostat will start flowing coolant to the radiator at that temperature. Most thermostats are designed to be fully open 10 to 20 F degrees above the crack open temperature.
The thermostat is designed to be spring loaded closed (no flow) and then a wax pellet called a wax motor, melts and expands, operating a push rod that works against spring pressure and opens the flow path. When the coolant temperature drops, the wax solidifies, and the spring closes the flow path.
Cylinder head temperature versus coolant (liquid temperature)
The hardest part of the engine to keep cool is the combustion chamber surface and the area around the exhaust valve.
During engine operation, especially under a high load, the coolant around the combustion chamber is exposed to a level of heat much greater than elsewhere in the engine.
The sending unit for the coolant temperature is traditionally found in a region that would represent an average of the stored heat for the bulk coolant. It is an excellent overview of the liquid temperature.
The cylinder head temperature reading (if so equipped), is measuring the surface temperature of the combustion chamber. On most engines, it is in a passage that is not exposed to any liquid but in some applications it is. You need to determine this for your tractor.
When examining the readings, the liquid temperature is considered an overview of the engine temperature (along with engine oil temperature) while the cylinder head temperature is much more telling and critical.
A hard-working engine may see a dramatic rise in cylinder head temperature with little or no perceptible change in that of the bulk liquid. For this reason, the cylinder head temperature is more authoritative than the liquid temperature.