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Rotating lower plugs to extend life

C

Captain Avgas

Well Known Member
I have a Lightspeed EI firing my 4 upper auto plugs on my IO 360. I do not rotate them because I am not aware that there is any benefit in doing so.

However I have a Slick 4371 Magneto firing my 4 lower massive electrode aviation plugs and it is my understanding that rotating those bottom plugs will provide extended plug life. The goal is to rotate the plugs to reverse the polarity and hence even out the electrode wear.

The question is what is the correct sequence of plug rotation if only the bottom plugs are being rotated. My cylinder firing order is 1, 3, 2, 4.

I’ve had a really good look through the archives on this one but cannot find an answer. I am imagining that this might be of interest to a lot of other RV builders because many have EI on the top plugs and a magneto for the bottom plugs.
 
Captain Avgas said:
I have a Lightspeed EI firing my 4 upper auto plugs on my IO 360. I do not rotate them because I am not aware that there is any benefit in doing so.

However I have a Slick 4371 Magneto firing my 4 lower massive electrode aviation plugs and it is my understanding that rotating those bottom plugs will provide extended plug life. The goal is to rotate the plugs to reverse the polarity and hence even out the electrode wear.

The question is what is the correct sequence of plug rotation if only the bottom plugs are being rotated. My cylinder firing order is 1, 3, 2, 4.

I’ve had a really good look through the archives on this one but cannot find an answer. I am imagining that this might be of interest to a lot of other RV builders because many have EI on the top plugs and a magneto for the bottom plugs.
Click to expand...

On a mag, the polarity reverses on each spark event. 1 is norm, 3 is reversed, 2 is normal, 4 is reversed. That is if my recall on the firing order was correct. Norm and reverse is illustrative, not factual.

Correct that any 12 VDC driven coil always fires the same polarity, unless you wired one of the coils with reverse polarity from the other. Most EI systems won't fire the coil w with reversed polarity though, as the mosfet is on the negative side.
 
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Captain Avgas said:
I have a Lightspeed EI firing my 4 upper auto plugs on my IO 360. I do not rotate them because I am not aware that there is any benefit in doing so.

However I have a Slick 4371 Magneto firing my 4 lower massive electrode aviation plugs and it is my understanding that rotating those bottom plugs will provide extended plug life. The goal is to rotate the plugs to reverse the polarity and hence even out the electrode wear.

The question is what is the correct sequence of plug rotation if only the bottom plugs are being rotated. My cylinder firing order is 1, 3, 2, 4. I’ve had a really good look through the archives on this one but cannot find an answer. I am imagining that this might be of interest to a lot of other RV builders because many have EI on the top plugs and a magneto for the bottom plugs.
Click to expand...

I also have my Lightspeed firing the top plugs and a Slick 4370 firing the bottom plugs. Since massive plugs will typically last about 400 hours and they typically need servicing (cleaning and re-gapping) every 100 hours, I keep spark plug rotation pretty simple. I move 1 to 2, 2 to 3, 3 to 4, and 4 to 1 every hundred hours. That way, each plug has been in each cylinder once. This has been working out for me for nearly 2000 hours.
 
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lr172 said:
On a mag, the polarity reverses on each spark event.
Click to expand...

Hi Larry, I thought that was the case and in fact I have been swapping my massive plugs to follow the firing order of 1, 3, 2, 4. ie 1 to 3, 3 to 2, 2 to 4, 4 to 1.

But just wanted to check.

But in future it might be easier for my ageing brain if I just rotate the plugs on one side ie 1 and 3 are always rotated, and 2 and 4 are always rotated. That should work just as well and simpler to remember and execute.

I have read that rotating the plugs to reverse the polarity can often double the life of the plugs.

Anyway thanks to Larry and Jerry for their advice. Much appreciated.
 
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