Solid spark plug wires for magneto?

I've been told that when the tractor has a magneto you need to use solid spark plug wires. Is this true? Thanks, Mark

the way they came from the factory was with solid wire, Interesting to drive by the house looking in the window at the TV when it is running, you will then understand why all modern vehicles have carbon center wires.

This might sound strange but a friend of mine always makes his own solid wire spark plug wires. He takes the ends off the old set and puts them on coax TV wire. seem to work fine.

Mark in Iowa wrote: I've been told that when the tractor has a magneto you need to use solid spark plug wires. Is this true? Thanks, Mark

By "solid wires", I presume that you mean plug leads with a copper conducting core! If so, then you are right - magnetos do not like carbon cored leads. Copper cored plug lead is still easily available.

Nobody mentioned the reason why so I will... but be forewarned... it involves physics... and it IS Saturday...

The reason why solid-core wire is used for magnetos... is to protect the magneto against damage from extremely high secondary voltages.

A Kettering-type (coil/battery) discharge ignition circuit fires because a PRIMARY in the ignition coil has gone from a current-carrying state (points closed) to a non-current carrying state (points open).  In the current-carrying state, the primary has built a magnetic field around the coil core and secondary winding (high-voltage side).  As long as primary current does not change, there will be no VOLTAGE appearing on the secondary, but the moment that primary current stops, the secondary's magnetic field collapses, and the collapsing action induces voltage on the secondary winding.

The secondary winding starts building voltage, and voltage rises very, very quickly.  The only thing that STOPS voltage from rising... is if the windings of the secondary become shorted... at that point, voltage developed across the secondary winding is subject to Ohm's law... meaning that if the resistance of the short is high, voltage will be higher, but if the short resistance is very low, voltage will be low.

Or in simpler terms... if the plug arc is wide, and compression is high, voltage will be higher... and if the plug arc is narrow, voltage will be narrow.  Why?  Because resistance across a plug gap is related to the distance it has to jump, and the amount of atmospheric pressure it's jumping through.  (if this interests you, start reading about corona-effect).

  am I going too fast?  Is it stuffy in here?  Lemmie open a window...

There... much better.

So you've got two pieces of metal, separated by some distance, and the gap is filled with... a gaseous element... like... argon or something... doesn't matter, as long as it's pure.  If you take these two pieces of metal, and put some voltage 'tween 'em, and move the pieces close together, you'll eventually find a DISTANCE where an arc develops.  Likewise, if you set the distance, and bring up the voltage, you'll eventually find the point where an arc develops... this is called 'breakover voltage'.

Now, if you put a voltmeter and ammeter on the wiring, you'll be able to see not only the 'breakover voltage', but you'll see a little 'leakage' that occurs as you approach 'breakover'... and once 'breakover' occurs, you'll see LOTS of current flow.

This is because the gas is CONDUCTING electric current... but...  not very well... at least... not 'till you hit such a high voltage that the gas becomes 'excited'... it glows... the arc forms!  At that point, the gas's atomic layers 'jump', and the resistance of the gas becomes impressively low... it becomes VERY conductive... LOW RESISTANCE.

If you work with equipment that uses signal and power supply wiring that needs to be hooked up over long, long distances, you'll see devices in protective circuitry called 'gas discharge tubes'... these are just what they're named... tubes where stray high voltages get shorted to ground, to protect sensitive stuff on either end.  HAM radio guys used to use old spark plugs... core connected to the antenna, and the threads shorted to ground stakes... to protect their radio receivers from static electricity.

Time for a potty break- be back in your seats in 5 minutes.

Okay...  where were we?

Ah... okay.  Now, the next piece, is what Lou was alluding to... and the reason why we use CARBON_CORE wires and resistance-type plugs...  it's called HARMONIC OSCILLATION... and observed in what we call Radio Frequency Interfereance.

Take a hammer, hit a bell.  Bell rings.  Bell rings at whatever note it wants (based on elasticity, mass, and shape of the bell).  That's an oscillation.

Put a child on a swing, give 'em a push.  Child swings, and you push each time... your push, and the child's swing rate correspond.   Now shorten up the ropes, give the child a push, and she swings three times, so you push again... you're only pushing once every three times... the swing rate is 3x the push rate.

Imagine hitting the bell with a hammer... it's oscillating several hundred, or several thousand times per swing.

Got that?  Back to electricity...

The coil builds voltage, until it hits the point where breakover occurs... at which time, the spark plug, carrying it's arc, is darned-near a dead short.  I say darned-near, because there's STILL a little resistance... not much, but you may have 'tweaked' your plug gap a little tight, or a little narrow... and of course, if the fuel/air mix is a little leaner or richer (choke knob doesn't stay pushed in, eh?) the amount of resistance across the gap (both before, and after breakover) will change.

The amount of current flow and arc voltage will vary (they'll trade off) based on plug gap, compression, and fuel mix/quality, ALTITUDE... and finally... any oil, carbon-soot, or other crud that may be fouling the plug.

Back to oscillation...

When that collapse occurs, voltage builds... eventually, hits the 'breakover point', arc forms, discharges coil current, then the arc stops, and everything's done, right?

No.

The coil STARTED it's part of the program by having a magnetic field built up... but when the discharge across the plug stops... it doesn't just 'stop'.  Current flowing through that plug wire, is like rolling a bowling ball down a lane... just 'cause you swung and let go of the ball, doesn't mean it's over... the electrons flowing through that plug gap are CURRENT... and anytime you have CURRENT flowing, you have a MAGNETIC FIELD... and the firing of that plug means there's a magnetic field built around the plug wire...

And like pushing that child on the swing... the child swinging through the BOTTOM of the arc is like current flow, and top of the arc is VOLTAGE.  The child will swing in one direction, but once stopped, she'll swing back and hit you.

What's happening, is there's power rattling back and forth through those plug wires, and it's occuring at a frequency MUCH HIGHER than the snap that occurs when the points open... millions of times higher.  If you turn on your AM radio, you'll hear it.

IF you replace the wires with carbon core, or plug in resistance-type plugs, you'll be doing the same thing as if the child was dragging her feet each time she swung.  You'll have to push harder to make her swing as high, but if you don't push, she'll stop pretty quick.  This is why resistance plugs and wires get rid of the AM radio noise... you're suppressing the harmonic generation that occurs in that ignition system.

The reason why Lou referred to the G-men...

Is because the earliest radio transmitters didn't have semiconductors or oscillator circuits.  They used coils, capacitors, and spark gaps to create high-frequency oscillations that were conducted to long wire antennas.  The resulting racket was detected on crystal sets hundreds, and sometimes thousands of miles away.

Unfortunately, they weren't very 'selective'... they transmitted about as wide as your whole AM radio band...  and they were outlawed many, many, many years ago.

Magnetos, however, are still legal.  ;-)

Many thanks, Dave.  That took more than a few minutes of your time.  It is just about the best description I have seen on the physics of ignition systems and their acting as an RF noise source.  Even a Civil Engineer can understand it now.

73,
WJ3K/Gordon

If the CB1 still works it WILL hear the WC running.

Back in the late 50s or early 60s my dad and I tried the RG-8 braid over the ignition wires on  a car. Don't bother. It did quiet the radio noise a little, but it changed the timing from the added capacitance of the shield and it destroyed the ignition wire because the inside of the braid wasn't smooth and tight against the insulation which made an air gap that broke down before the solid insulation and the heat burned the insulation. To keep the car running we had to take the shields off and replace ignition wires.

Good quality Packard copper ignition wire is a stock item in shops doing marine and outboard engines.

Gerald J.

Thanks Dave, most of that was good review from school days plus a little extra. I find it quite common for mags to have coils that get weak when hot or are just plain weak and wonder how much is damage from previous years of running with resistor wires. I bought a FMJ mag recently and it came with auto wires. Coil is the old black one and its soft and gooy. Haven't run it on the mag tester yet though.

I know the HW's well, and have a Shawnee transciever and a Seneca 6'n'2 transmitter, a bunch of Single-Banders, an SB-220, one of the general-coverage receivers, various test tools, keyers, and other stuff... but not a single CB...

But if the CB1 is a direct conversion RX, Charlie ought'a be able to hear my STEREO through it from his house... but only if he can hear over the noise from those big power lines around his house.  We call those receivers 'barn doors' 'cause any signal can just walk right in...

Yeah, I certainly wouldn't expect a Lucas mag to last long with any resistance in the secondary path... many of 'em would self-destruct after losing an electrode... a buddy told me a story about him running a streamliner Triumph out in Wendover back in the early '70's, started pinging on one cylinder, burned the ground electrode right off, and it killed the mag's coil... he said it was a long walk back across the salt...  having driven through there many times, I imagine it WAS...

I think the CB1 is like the sixer and the twoer, a superregen receiver, hence its lack of selectivity. Lots of hiss in the audio.

I used a twoer long ago. Still have it but it quit working after a lightning strike got into the basement hamshack.

K0CQ, Gerald J.