Why Rotate Spark Plugs?

Oct. 1, 2001

Why Rotate Spark Plugs?

Merits of spark plug rotation

By John Schwaner

October 2001

Editor’s note:
The following discusses the advantages of rotating spark plugs to positions of opposite polarity. It was written by John Schwaner of Sacramento Sky Ranch. For more information you can contact him at (916) 421-7672, or visit their website at www.sacskyranch.com.

Why Rotate Spark Plugs?

Here is a method of plug rotation that not only swaps plugs from top to bottom, but also swaps from short lead to long lead, and reverses spark plug polarity. This method works with most horizontally- opposed engines.

For a six-cylinder engine move plugs:
1T to 6B
2B to 5T
3T to 4B
1B to 6T
2T to 5B
3B to 4T

For a four-cylinder engine move plugs:
1T to 4B
2B to 3T
1B to 4T
2T to 3B

Where T=top and B=bottom and the number is the cylinder position number.

Spark plug life can be just about doubled by proper plug rotation. Look at the spark plug electrodes the next time you remove the spark plugs from your engine. One electrode is usually worn more than the other. On one plug, the center electrode may be worn, while on another plug, the ground electrode may be worn. Swapping plug position evens out the wear between the center and ground electrode, thereby preventing any one electrode from incurring excessive wear.
The spark plug’s firing polarity causes uneven electrode wear. The magneto generates energy by means of a rotating magnet. As the magnet rotates, the magnet’s north and south poles generate positive and negative electromotive force. The magneto sends a positive voltage down one lead and a negative voltage down the next lead and so on. Each ignition lead always fires the same voltage (on horizontal-opposed engines), but the voltage alternates among leads. (D-2000/D-3000 magnetos on 4-cylinder engines always fire at the same polarity). Current flow, from the spark plug’s cathode electrode to the anode electrode, causes the cathode electrode to wear.
To equalize wear, move the spark plug to a different position so that it fires at the opposite polarity. Do this by swapping the spark plugs with worn ground electrodes with the plugs that have worn center electrodes. You may also move the spark plug to the next lead position coming out the back of the magneto. Spark plugs also wear unevenly due to differences in lead salt deposits and ignition lead capacitance. The higher wear rate on spark plugs operating on avgas than on autogas is due to lead salt corrosion from the lead in avgas. Swapping plugs from top to bottom helps equalize wear caused by lead salt corrosion.
The greater the ignition lead’s capacitance, the more current flows across the spark plug gap and the more the plug electrodes wear. Shielded spark plug leads, having an insulator separated by two conductors, act as linear capacitors. Approximately 25 percent of the energy sent to the plug from the magneto is used to charge the lead’s capacitance. Once the lead is charged, the voltage across the spark plug electrodes increases until a conductive path forms between the ground and center electrodes and the plug fires. The conductive path completes the electrical circuit between the lead’s center conductor and the ignition lead shielding. Capacitance energy stored in the lead then discharges across the spark plug gap after the original arc. Usually, the spark plug has already lit-off the fuel/air mixture, so this capacitance energy is of no use except that it causes electrode wear. The longer the lead, the greater the capacitance, and the more the electrode wears. Swap spark plugs between short and long leads to equalize wear caused by lead capacitance.