Spark plug rotation

[bob888]

I have electronic ignition on the bottom plugs and magneto on the uppers. I understand plug rotation is designed to change polarity and thus wear pattern on electrodes, at least for plugs fired by mags. How should plugs be rotated in my situation, if at all?

[BobTurner]

The reason for the polarity reversal is that the mags have a magnet with both a north and south pole, and both are used. Look up the firing order for your engine. The first, third, and fifth cylinders in the firing order use one polarity; the other tbree, the opposite. Move the plug from one polarity to the opposite. For a standard IO-540, you can move cylinder 1 to cylinder 2, 2 to 3, 3 to 4, etc.

[OkieDave]

Originally Posted by BobTurner

The reason for the polarity reversal is that the mags have a magnet with both a north and south pole, and both are used. Look up the firing order for your engine. The first, third, and fifth cylinders in the firing order use one polarity; the other tbree, the opposite. Move the plug from one polarity to the opposite. For a standard IO-540, you can move cylinder 1 to cylinder 2, 2 to 3, 3 to 4, etc.

I'm confused; wouldn't that have the plugs firing at the wrong time (i.e. 1 firing at the time 2 should fire, etc.)?

[ReidVaitor]

For a standard IO-540, you can move cylinder 1 to cylinder 2, 2 to 3, 3 to 4, etc.

well lets start with firing order for 540 1-4-5-2-3-6. Lycoming states to go outside to inside
1 to 6, 4 to 3, 5-2- spark plugs actually only spark on one side of the two, not both, so what happens is the spark plugs get worn out on one side (polarity) so switching them will allow for the other side of the spark plug to spark and wear out that side, by moving the plugs as Lycoming states it wears the plugs evenly over time.

now for - I'm confused; wouldn't that have the plugs firing at the wrong time (i.e. 1 firing at the time 2 should fire, etc.)?
your not moving wires your moving the plugs that spark in the proper order mentioned above so nothing changes as for the order of "timing".
Keep it simple- find out where the Mags wires go and change the spark plugs as close as mentioned above ONLY for the ones the MAGs provide spark to, for the EI, replace those spark plugs if they are automotive, otherwise stick to Lycoming plan in general.
Of course make sure you clean and regap the plugs to the proper range (.016-.021 for IO540) I use .018 since that is what my feeler is- also allows for my margin of error over or under tightening- but I ALWAYS need to regap it to this setting every year.

[OkieDave]

Originally Posted by ReidVaitor

your not moving wires your moving the plugs that spark in the proper order mentioned above so nothing changes as for the order of "timing".

Oh. Duh. I don't know how I misread that so badly, but it's pretty obvious now that you point it out.

[BobTurner]

Originally Posted by ReidVaitor

- spark plugs actually only spark on one side of the two, not both, so what happens is the spark plugs get worn out on one side (polarity) so switching them will allow for the other side of the spark plug to spark and wear out that side, by moving the plugs as Lycoming states it wears the plugs evenly over time.
.

This is a bit unclear, IMHO. The arc jumps across between the electrodes. Within the arc, you have electrons traveling in one direction, and ions (the metal atoms, minus an electron or two) traveling in the opposite direction. The side from which the ions are traveling is losing mass, atom by atom, and will get thinner. Reversing the polarity at plug rotation will even out the wear.

[Darin Watson]

Originally Posted by BobTurner

This is a bit unclear, IMHO. The arc jumps across between the electrodes. Within the arc, you have electrons traveling in one direction, and ions (the metal atoms, minus an electron or two) traveling in the opposite direction. The side from which the ions are traveling is losing mass, atom by atom, and will get thinner. Reversing the polarity at plug rotation will even out the wear.

Good high school physics lesson...and crystal clear.

[BillL]

Originally Posted by BobTurner

This is a bit unclear, IMHO. The arc jumps across between the electrodes. Within the arc, you have electrons traveling in one direction, and ions (the metal atoms, minus an electron or two) traveling in the opposite direction. The side from which the ions are traveling is losing mass, atom by atom, and will get thinner. Reversing the polarity at plug rotation will even out the wear.

Or emitter and target, the target will get much hotter than the emitter and erode faster (oxidize, evaporate, melt, go away). Like a TIG with forward and reverse polarity. You have a beautiful description of the ion movement, but the heat will melt things. I don't get a lot of metal transfer to my tungsten electrode, or do I?

[Kellym]

Originally Posted by ReidVaitor

For a standard IO-540, you can move cylinder
Keep it simple- find out where the Mags wires go and change the spark plugs as close as mentioned above ONLY for the ones the MAGs provide spark to, for the EI, replace those spark plugs if they are automotive, otherwise stick to Lycoming plan in general.
Of course make sure you clean and regap the plugs to the proper range (.016-.021 for IO540) I use .018 since that is what my feeler is- also allows for my margin of error over or under tightening- but I ALWAYS need to regap it to this setting every year.

You can not mix the plugs used for electronic ignition with the mag plugs without causing significant problems. The gap for all magneto fired plugs is the quoted .016-.021 regardless of engine. For electronic ignitions the gap will be at least .028 or larger. You do NOT want to change the gap between those values multiple times. In fact, aircraft massive plugs should NOT have the gap enlarged. Too likely to crack the ceramic on the center electrode.