Spark Plug Rotation

Have you ever wondered why you should rotate your spark plugs? And then how they should be rotated.

Basically there are two reasons.

The first is the electrodes wear faster either on the center post or the outer ring depending on the polarity of the charge from the magneto. By changing the plugs from the center post having a + charge to the outer ring having that + charge, (or the reverse) you can keep your plugs in workable condition longer. If you don't rotate them to change the polarity either the center post or the outer ring will wear beyond serviceable limits. You can look at each plug as you remove them and you will be able to see the ones on the same mag on the same side will have similar wear characteristics.

Each plug is powered through a magneto. Most engines have two mags so there is redundancy built into the system. The left mag will power the top plugs on one side of the engine and the bottom plugs on the other side. This also determines the polarity of the charge given to each plug. By using the rotation shown below you will be changing the polarity of each plug. That should increase the life of your plugs as one of the charges will wear faster than the other. If you look closely at your plugs as they are removed or after cleaning, you will see more wear on either the center electrode (as it becomes elliptical) or the outer circle.

This graphic shows the rotation for a typical 6 cylinder engine.

For a six-cylinder

1T with 6B

2B with 5T

3T with 4B

1B with 6T

2T with 5B

3B with 4T

Four cylinder

1T to 4B
2B to 3T
1B to 4T
2T to 3B

Don't forget a very small amount of anti-seize when reinstalling. Always use new or annealed washers. You can anneal the solid copper style washers by hanging on a loop of safety-wire and heat until red hot with a propane torch. Quenching with water will remove the scale and oxides so they will look brand-new. Remember that you are using an open flame. Do not use the torch around flammables.

If possible, use a torque wrench... 300 to 360 inch-pounds for 18 mm while only 180 to 240 inch-pounds is recommended for 14 mm. plugs.

Teledyne Continental engines 25-30 Ft. Lb (300-360 In. Lb)
Lycoming engines 35 Ft. Lb (420 In. Lb)

Slick Ignition lead

5/8 on the left and larger 3/4 on the right.

5/8 - 24 plug takes a 3/4 wrench on the lead nut. 3/4-20 plug takes a 7/8 wrench on the lead nut.

Generally, if you are replacing the plugs and harness the 3/4 (all weather) is a better setup because the lead end of the spark plug is completely sealed from moisture.

REM designates a 5/8 barrel plug while RHM designates a 3/4 plug

Use anti-seize. It will allow a smoother seat as you torque them in and allow a smoother removal later.