Spark plug information and installation

by Had Robinson

(see "B. Installation" below for the how-to.)

A. General Info

Loose plug  You can tell if your spark plug has not been torqued down correctly by looking for the presence of oily goo around the base of the spark plug that has leaked from the combustion chamber.  A correctly torqued plug WILL NOT LEAK OIL!  If you tighten the spark plug to specification (16 ft. lb/21 Nm) and you still see oil on the base or the washer, tighten the plug to 17 ft. lb/22 Nm and see if it still leaks.  An engine that has a leaking spark plug will have loss of power at full throttle.

Dirty cylinder head threads  NGK notes that the spark plug holes should always be cleaned prior to installation, otherwise you may be applying torque against dirt or debris and the spark plug may actually end up under-torqued, even though your torque wrench says otherwise.  A spark plug hole chaser should be a part of every pilot's tool collection.

Hot running conditions means heavy pilots (180 lb+/82 kg+) who run their engines at full power for more than a few minutes at a time, especially at sea level with ambient air temperature above 30C/85F.  "Cold running conditions" means everything else.  If you have any doubts, go with the B10 heat range.  If it runs "cold" the plug will foul more quickly.  If your spark plug is consistently black/sooty in color it mean that you need a hotter running plug, e.g., the B9.  Ignition and fuel system problems can also cause the spark plug to foul.  An overheating plug can also be caused by fuel starvation.

Iridium plugs  Pilots may use NGK's Iridium line of plugs. They are better quality and last longer but the cost difference is at least 3X the stock spark plug.  Some claim they give better performance.  This is probably true over the life of the plug.  Nonetheless, they must be gapped correctly and will also foul just the same as the stock spark plug.  Some non-NGK iridium plugs have a special ground electrode that retards wear.  If a pilot has to take the plug out every 10 hours or so to check the gap and clean it, why not spend a few dollars and replace it with a new one?  It will not have to be cleaned.

Resistor plugs  Miniplane does not recommend the use of resistor type plugs (BR9ES, etc.) for the Top 80.  Some pilots report that they have less radio interference caused by ignition noise when using resistor type plugs but these plugs may result in lower performance.  You will have to experiment with your engine.

Poor performance, poor idle, or a weak top end is most often caused by a worn out spark plug, incorrect spark plug gap, or a failed secondary wire (see below).

NGK has a general information sheet on spark plug installation.

The spark plug below is in normal condition after about 20 hours of use.  Lead deposits (from AVGAS without the TCP additive) are a light yellow whereas unleaded fuel deposits are brownish/black.  If deposits anywhere in the engine are black and gummy, pilots should use higher quality fuel and/or oil and be certain that the engine is not overheating.  The plug is clean (no soot).  Nonetheless, this plug is worn out and should be replaced.  This can be observed by the rounded edges of the inner electrode.  Plugs will soot up very quickly at idle which is why pilots should keep idling to a minimum.  In order to check the general condition of your ignition system, run your engine at full power in the air for a few minutes, kill it at full power, and then land "dead stick".  This is the only way you can get an accurate idea of the condition of your fuel and ignition systems.  Read this article on why using gasoline with ethanol is not a good idea.

 healthy spark plug from an engine using AVGAS

Contrary to rumors that circulate on the Miniplane blogs and elsewhere, lead monoxide or lead phosphate from using aviation gasoline (AVGAS) does not cause piston ring sticking.  The engine from which the plug below was removed had no problem with a stuck piston ring.  It just needed the routine cylinder head and piston cleaning that must be done more often on engines that use AVGAS.  However, if an engine overheats, it does not matter what kind of fuel or oil is used: the ring may stick because of the presence of burned oil.  Red Line, a manufacturer of 2 stroke synthetic oil lubricants, notes what happens in one of their technical documents,

"The time indicated [in the graph] is the time required for the lubricant to decompose to a sticky mass capable of sticking a two-cycle piston ring."

Below is a spark plug from an engine that used AVGAS without the TCP additive.  This plug was way overdue to be changed but still was in good enough condition to start the engine.  It shows how lead monoxide deposits build up.  To a much lesser extent, the top of the piston is covered with these deposits but they are easily removed.

 spark plug fouled by the presence of led in aviation gasoline

The spark plug below was run in an engine that used premium unleaded, ethanol free, gasoline.  It is a tiny bit fouled from many hours of use but still works just fine.  Note that the electrodes have sharp edges which means the plug is not very old.  Always check the gap.

spark plug used with unleaded premium fuel

Here is a spark plug from an engine using AVGAS with the addition of the TCP additive.  There is a small amount of harmless lead phosphate present on the white insulator.  The plug has about 10 hours on it.

AVGAS spark plug with TCP additive

Performance of the engine at the top end is everything per evaluating its condition.  If anything is wrong with any part of the ignition or fuel system, the engine will not achieve maximum power output.  Generally, always be careful running your engine at full throttle for long periods of time, especially if you are near sea level.   Install a CHT so that you can have some idea of the engine's normal operating conditions.  Monitoring the running temperature of the engine can alert pilots of potential disaster.

Here is the perfect spark plug color and condition.  It is a few hours old and is in the right heat range (inner insulator is off-white not bleached white).  AVGAS was used with the addition of the TCP fuel additive (almost eliminates lead deposits).  Note the the sharp edges of the center electrode.  This is how you know the plug has not been in the engine long.  The center tip gets more rounded as the plug is used.

spark plug in perfect condition from a Top 80

Remember that a cooler running engine will last longer.  Heat kills small engines!  Keep this in mind when tuning the Top 80.

B. Installation

Tools required

K-D Tools 165 Spark Plug Gap Wire Gauge tool (you cannot correctly adjust the gap without a wire-type gauge)
Lisle #20200 Spark Plug Hole Thread Chaser (optional but highly recommended)
3/8 in. drive 13/16 in. sparkplug socket
3/8 in. torque wrench
spark plug dielectric grease
red threadlock for the plug terminal


1. Remove the old plug and observe it.  If it is any color than the end photo in the first section (above), than you should find out why?  Is there is a fuel system or ignition problem?  Note any presence of oily goo at the base of the plug.  If you find some, the plug was not torqued correctly due to the presence of these deposits in the threads of the cylinder head.  For the picky: Store your old plug in the original little pack with the date and hours of the motor.  That way, you can get a timeline and have a good idea how your engine is doing over a long period.

2. Remove the compression washer (it is not needed) with a pair of Channel Lock pliers or equivalent.  Note:  If your engine does not use an aluminum spacer washer or has a CHT installed, the compression washer must be kept in place – do not remove it.

3. Check the gap of the new plug.  You cannot accurately measure the gap without using a wire-type gauge e.g. a K-D165 gauge.  Gauges in auto parts stores are for car spark plugs which have a much greater gap instead of the small gap in most paramotors.  Set the electrode gap to the minimum value because the gap always widens as the plug is used.  Too much or too little gap will cause the engine to miss at high loads and make it harder to start.  It is a good idea to check the gap every 7-10 hours and adjust as necessary.  Pilots would be surprised at how fast the gap changes on high performance engines like a paramotor.

4. Unscrew the top terminal with pliers and put red threadlock on the threads.  Note: some spark plugs have welded terminals which do not require threadlock e.g. the NGK BR10EG.  It will always loosen if threadlock is not used, even if tightened with a pair of pliers.  If the upper terminal gets loose, the internal threads on the aluminum terminal will be destroyed, the terminal will come free, and then become embedded in the end of the boot.  A loose terminal in the boot will not contact the plug properly and will cause missing at high loads.  Pilots would not believe how often I find this problem when working on motors.

5. Chase the hole with Lisle tool.  Put a few drops of cutting fluid on the tool.  If you do not have cutting fluid, use some WD-40 or very light machine oil.  Combustion deposits have a way of working up the spark plug threads in the cylinder head.  These deposits, as NGK notes in their installation manual, prevent the spark plug from being torqued properly.

6. CHT sensor probe – if the engine has a CHT, screw the plug into the probe.  If you do not have a CHT, be sure to get one!  It's like having a car without a radiator gauge/light.  You do not need gauges most of the time but when you do....

7. Washer spacer – Do not forget to place the spacer/washer underneath the spark plug, if the engine manual specifies one.  The washer lowers the compression and engine running temperature to an acceptable level.  Failure to use the washer will damage the engine.  Use a thicker washer if you must use low octane/poor quality fuel.  These washers can be purchased from Miniplane USA.  Remember that a cooler running and lower compression engine will last longer.  Heat kills small engines.  The higher the output of an engine, the shorter its life.

8. Screw the spark plug into the hole with your hand.  Be certain that it goes smoothly all the way in and does not bind.  Binding is caused by combustion deposits that were not removed and why you must have a chase tool.

9. Set the torque wrench (if the clickable type) to 21 Nm (16 ft. lb.) and torque down the plug.  If you have a CHT installed, be sure that the sensor probe is not touching any part of the engine where it come up past the spark plug.  If the plug is not torqued down properly it will LEAK at high loads.  You will notice this by reduced top end power and oily gunk around the base of the plug.  Be extremely careful to not over-torque the spark plug as this will damage the cylinder head!

10. Boot installation – Smear a small amount of Spark Guard or equivalent inside the boot.  The dielectric grease will permit the boot to be firmly seated and lessen wear of the boot, a common problem.  Push the boot firmly down on the plug as far as it will go.  Twisting it gently as you push down will help.  If the boot is not fully pushed on, the spark will fail under high load and the engine will miss and/or experience a loss of power.    As the boot wears, pilots will notice this by an abundance of black soot (pulverized rubber) inside the boot and on the terminal of the spark plug.  If pilots replace the secondary wire and boot with our kit (an automotive-quality boot and heavy duty wire), the secondary wire and boot will last much, much longer.

11. Test run the engine.  Use a Sharpie pen and write something like this on an open aluminum part of the engine, "Spark plug xxx hrs."  Doing this will greatly help you to remember when you should replace the spark plug.  When I change my plug, I also change the redrive/clutch oil at the same time as they both have about the same maintenance interval.

Turkey Vulture