Spark Plug Caps

It’s that time of the month again, and here I go with another little maintenance tip for your Rotax powered bird. This month’s tip is about an often over looked item of extreme importance to our powerplants well being. The spark plug cap! Now some of you may be saying why the spark plug cap who ever looks at those??? That is exactly my point, even on complete rebuilds this little electrical item is often overlooked and not replaced.

A brief recap of the two types of caps that are currently being used on our engines is in order. Originally Rotax engines were equipped with breaker point ignitions and Bosch sparkplug caps. A breaker type ignition is not the hottest spark producer at low cranking speeds and at low RPM’s. Because of this a spark plug cap with a 1000 ohm resistor built inside was used along with a sheet metal type shield on the outside of the cap. The intention of these two items was to reduce EMI, “electro-mechanical interference” or RFI, “Radio frequency interference”. This is necessary to reduce the interference our engine’s electrical system imparts on our aircraft radios, Gps’s, and for you not so politically correct low flying guys, even our neighbor’s TV sets. Later engines equipped with stronger dual electronic ignition systems may use a 5000 ohm NGK resistor cap, these may be either black or magenta in color, mechanically these are identical. Along with these resistor caps the newer engines also are equipped with resistor spark plugs. All of this just to reduce that annoying hum or whine you may hear in your headset while your engine is running.

OK, so now we all know more than you wanted to about this resistor spark plug cap, what ‘s the big deal? Well I have include a few photos below of a cutaway of these caps for your viewing pleasure, Notice the resistor built right inside these caps, take care when removing them from your engine never pry these caps off with a screwdriver etc as they are fragile and can break easily. The other key item that we need to be aware of is the connector inside of the cap that actually makes contact with the spark plug top. This connector on the Bosch caps is a simple metal tap that only makes contact on one side of the plug top. The newer NGK caps have a machined brass insert that fully surrounds the plug top and it also has a small metal wire spring to ensure it “snaps” into place on the spark plug.

The normal high frequency vibration our engines produce while running will eventually wear out the connectors in either type of cap and both the electrical and mechanical connection will be compromised. It is extremely important for the people with inverted engines to check these regularly. As for you upright guys and gals, these should still be checked at least annually and replaced whenever wear is detected. Also watch for corrosion between the cap and plug top. These can be coated with dielectric grease, as all connections on our planes should be. Do not forget to use solid top spark plugs whenever possible and if using threaded top plugs at least Loctite or ding the threads to ensure the tops do not vibrate loose.