Spark Plug Position

Geico266

Geico266

Touchdown! Greaser!
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Geico
I have always wondered why airplane mechanics insist on putting the same plug back in the same hole after cleaning. Rotating plugs (top to bottom) every couple hundred hours seems like a good thing also.

So why do we put the same plug back in the same hole? What difference does it make? What is the reasoning? :dunno:
 
Per 8083-32 you should rotate them to the next in the firing order. I'm not going to search through the manual for the reference. Long story short, the theory was to even out the wear because the firing polarity would be opposite if you rotated in that manner.
 
I have never seen a mechanic(s) do that...they have always rotated the plugs for even wear
 
It's a good theory, but we all know the gap on the electrodes widens no matter what polarity is. When the electrodes are eaten away the plug is tossed, does it matter which electrode is gone ?
 
Tom-D said:
It's a good theory, but we all know the gap on the electrodes widens no matter what polarity is. When the electrodes are eaten away the plug is tossed, does it matter which electrode is gone ?
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In the "old" days when we could still get Auburns that didn't fail internally like Champions, plugs lasted a long time. If the plug was never rotated, one electrode could erode down to something pretty thin and maybe shorten the life of the plug.

But plugs don't last like that anymore, unless folks are finding Unison/Tempest plugs lasting 1000 hours or more. Auburns had a 400-hour warranty at a time when Champion actually recommended plug replacement at 50 hours. I kid you not. I saw that in print in one of their publications. I might still have it; I'll have to look.

Dan
 
Tom-D said:
It's a good theory, but we all know the gap on the electrodes widens no matter what polarity is. When the electrodes are eaten away the plug is tossed, does it matter which electrode is gone ?
Click to expand...

I'm unclear.

The side electrode is always grounded.

Is it being put forth that top plugs and bottom plugs differ in whether the center electrode is getting positive or negative voltage from the magnetos? And, finally, that it would matter where the spark was originating from re: wear?

Not sure how that would work. Not saying it's not possible, but I've never heard of it before.

Finally, if it is a real "thing", would it also apply with electronic ignition? In my ROTAX I've been putting the NGK's back in the same location, though as cheap as they are they get replaced fairly often.
 
FastEddieB said:
I'm unclear.

The side electrode is always grounded.

Is it being put forth that top plugs and bottom plugs differ in whether the center electrode is getting positive or negative voltage from the magnetos? And, finally, that it would matter where the spark was originating from re: wear?

Not sure how that would work. Not saying it's not possible, but I've never heard of it before.

Finally, if it is a real "thing", would it also apply with electronic ignition? In my ROTAX I've been putting the NGK's back in the same location, though as cheap as they are they get replaced fairly often.
Click to expand...

The quick answer is that it makes a difference and the plugs should be rotated for even wear. If the plugs aren't rotated then it's pretty easy to see the differences in wear at 500 hours.
 
I have rotated them as a matter of habit and because that was what I was taught to do. OTOH, mags produce alternating current so what's the point of rotating them?
 
Clark1961 said:
The quick answer is that it makes a difference and the plugs should be rotated for even wear. If the plugs aren't rotated then it's pretty easy to see the differences in wear at 500 hours.
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500 hours is the expected life of a aviation plug. If properly rotated, you might 500 hours and 10 minutes.

the rotation is a theory that the magneto will reverse the polarity to the next plug to fire. north to south , south to north. that means the spark will remove the molecule of metal from the electrode it originates from.

so when you remove the plug from a negative sparked hole, you would place it in a hole where it gets a positive spark. this in theory gets the two poles of the electrodes to decrease in size equally.

my question who cares ? once the electrodes have lost the amount of metal off either or both electrodes the plug will be tossed.
 
FastEddieB said:
...Is it being put forth that top plugs and bottom plugs differ in whether the center electrode is getting positive or negative voltage from the magnetos?...
Click to expand...

No, it's the next plug in the firing order of the same magneto since the rotating magnet has a North and a South pole so it doesn't matter what you consider to be "ground" the polarity of the potential is opposite to the previous firing. This is a characteristic of magnetos, not electronic or coil based ignition systems.
 
Silvaire said:
No, it's the next plug in the firing order of the same magneto since the rotating magnet has a North and a South pole so it doesn't matter what you consider to be "ground" the polarity of the potential is opposite to the previous firing. This is a characteristic of magnetos, not electronic or coil based ignition systems.
Click to expand...

You'll run into similar behavior with the coil packs on auto ignition systems. The factory plugs are single plats with the appropriate electrode coated. When changing the plugs, double plats should be used since we can't buy single plats with the correct coating.
 
Do the rotation ,of the plugs every annual seems to work . On the twin I used fine wire on the bottom and regular plugs on the top
 
Silvaire said:
No, it's the next plug in the firing order of the same magneto since the rotating magnet has a North and a South pole so it doesn't matter what you consider to be "ground" the polarity of the potential is opposite to the previous firing. This is a characteristic of magnetos, not electronic or coil based ignition systems.
Click to expand...

But.... are both mags on the same polarity?

IOWs the right mags is firing a negative spark to the top/front row, while the left mag is firing a positive spark on the bottom/rear row. If not, what sense does it make to shift the top to the bottom?

You can set the internal (e-gap) setting to be either at #1 cylinder. that will place the next plug in the firing order to be opposite.
So, if both mags are #1 + and both mags fire the same cylinder at the same time what good will it do to go from top to the bottom? When you wanted to change polarity you would be required to move both spark plugs to the next cylinder to fire, and it would not matter if you put them in either hole.

This brings up the question how do you tell if the mag is sparking + or - at #1?

Does this rotating of plugs need to be done on a odd number of cylinders of a radial engine?
 
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You know I don't try to over think it too much Tom, I just rotate to next in firing order and swap top to bottom so at next servicing it won't be the same plugs that get the vibrator crammed in them to clean the lead out.
 
Tom-D said:
But.... are both mags on the same polarity?

IOWs the right mags is firing a negative spark to the top/front row, while the left mag is firing a positive spark on the bottom/rear row. If not, what sense does it make to shift the top to the bottom?

You can set the internal (e-gap) setting to be either at #1 cylinder. that will place the next plug in the firing order to be opposite.
So, if both mags are #1 + and both mags fire the same cylinder at the same time what good will it do to go from top to the bottom? When you wanted to change polarity you would be required to move both spark plugs to the next cylinder to fire, and it would not matter if you put them in either hole.

This brings up the question how do you tell if the mag is sparking + or - at #1?

Does this rotating of plugs need to be done on a odd number of cylinders of a radial engine?
Click to expand...

its a free country if you dont want to roatate them then dont.
 
taters said:
its a free country if you dont want to roatate them then dont.
Click to expand...

When I ask questions, it's not to be argumentative or contrarian.

I really want to tease out the rationale.

Maybe it's the (ex-) instructor in me, but if I'm going to follow or recommend a practice, I like to be able to articulate the reasoning and/or physics behind it.

As an aside, is there supposed to be sound with that Mike Busch webinar?
 
FastEddieB said:
I'm unclear.

The side electrode is always grounded.

Is it being put forth that top plugs and bottom plugs differ in whether the center electrode is getting positive or negative voltage from the magnetos? And, finally, that it would matter where the spark was originating from re: wear?

Not sure how that would work. Not saying it's not possible, but I've never heard of it before.

Finally, if it is a real "thing", would it also apply with electronic ignition? In my ROTAX I've been putting the NGK's back in the same location, though as cheap as they are they get replaced fairly often.
Click to expand...


Think of a simple electrical circuit. Battery, Conductor, Load and Conductor back to battery. Now replace those with Magneto, High Tension Leads, Spark Plug and Conductor back to Magneto (usually engine block). Now unlike the battery example the Magneto is Alternating Current. So one plug fires from positive pole to negative pole. The next plug fires from negative pole to positive pole and so on and so forth.
 
Clark1961 said:
Why do you think the "theory" doesn't work?
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Where did I say it didn't work? That reference was a question.

No matter the polarity, is a molecule of metal being removed?

How do you know the plug is being moved from a + to a - hole if you don't know the polarity of the mag at #1?
 
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FAA guidance is to rotate top to bottom and advance one hole forward in the firing order. Really why waste time debating just ****ing rotate the plugs.
 
Tarheelpilot said:
FAA guidance is to rotate top to bottom and advance one hole forward in the firing order. Really why waste time debating just ****ing rotate the plugs.
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Because some would like to know the theory and why it does or doesn't work?
 
Tom-D said:
Because some would like to know the theory and why it does or doesn't work?
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Yes while thats nice and all it is still best to use the FAA accepted guidance unless there is something more relevant published by the manufacturer. Even if you conclude it is really not that important to rotate the plugs fact is there is no harm in rotating and that is the accepted data recommendation,


Rotate your ****ing plugs
 
Tarheelpilot said:
Yes while thats nice and all it is still best to use the FAA accepted guidance unless there is something more relevant published by the manufacturer. Even if you conclude it is really not that important to rotate the plugs fact is there is no harm in rotating and that is the accepted data recommendation,


Rotate your ****ing plugs
Click to expand...

Show me the FAA guidance.
 
Tarheelpilot said:
Rotate your ****ing plugs
Click to expand...

When the mag has a two pole rotor, and the engine has a odd numbers of cylinders, doesn't the polarity change every time the engine completes a rotation?
 
Tom-D said:
Where did I say it didn't work? That reference was a question.

No matter the polarity, is a molecule of metal being removed?

How do you know the plug is being moved from a + to a - hole if you don't know the polarity of the mag at #1?
Click to expand...

Maybe you don't know the polarity but the manufacturer does. Maybe the manufacturer's recommendation should be followed because it results in more uniform electrodes and better spark.

For certain you are making up the statistic which you provided that rotated plugs last 10 minutes longer than un-rotated plugs. The fact is that you don't know so you just made something up to suit your whimsy.
 
FastEddieB said:
How is it that alternating current has polarity?
Click to expand...

http://www.allaboutcircuits.com/vol_2/chpt_1/1.html

Its polarity changes. Instead of the electron flow being in one direction all the time (direct current), it shifts back and forth. In your house the electrical supply is likely 60 Hertz, meaning that it goes from neutral to positive, then through neutral to negtive and back to neutral, 60 times per second. When you tune your Comm to 126.7, the current to the antenna is making that cycle 126.7 million times per second. AC is fun and useful.

The magneto generates an alternating current in time to the RPM of the engine. When one phase is generated, the sparkplug gets its zap in one direction while the next plug gets it in the opposite direction.

Dan
 
Dan Thomas said:
http://www.allaboutcircuits.com/vol_2/chpt_1/1.html

Its polarity changes. Instead of the electron flow being in one direction all the time (direct current), it shifts back and forth. In your house the electrical supply is likely 60 Hertz, meaning that it goes from neutral to positive, then through neutral to negtive and back to neutral, 60 times per second. When you tune your Comm to 126.7, the current to the antenna is making that cycle 126.7 million times per second. AC is fun and useful.

The magneto generates an alternating current in time to the RPM of the engine. When one phase is generated, the sparkplug gets its zap in one direction while the next plug gets it in the opposite direction.

Dan
Click to expand...

So does the polarity change or stay the same in a plug position? :dunno:
 
Tarheelpilot said:
see
Rotate your ****ing plugs
Click to expand...
Thank you Tarheel.

Like the OP, I initially thought plug trays were used to insure the same plugs were re-installed into the same positions. Doing my own maintenance I discovered they are used to simplify proper manufacturers' recommended rotations.

I've pulled and rotated them 3 times now over 500 hours and the pattern of wear that drives rotation is quite apparent. I haven't bothered to fully understand the mechanics though it looks quite simple. But seeing the wear pattern quickly convinced me that rotation was the optimal procedure.

Carry on
 
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