# Magneto Ignition Wires

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#### cliffrosa

I can’t seem to find absolutely anything about magneto wires. I have a slick magneto and I don’t feel like spending $240 for 4 wires for a magneto. There has to be a how to or is it possible that I can use other magneto wires that say fit a different motor. Are magneto wires specific to its application and is it specific to the actual brand of magneto? I find wires all day on ebay for a lot cheaper for what I would buy online. FYI this is for a Type IV VW motor. #### wally ##### Well-Known Member From what little I know about magnetos, there is nothing special electrically about the ignition wires. The typical aircraft mag has wires which are shielded with a braid over them. The basic wire is actually just copper wire with maybe 7 or 8mm or so high voltage insulation. The shielding over the insulation has a threaded adapter at each end to connect the shield to the mag on one end and to the spark plug at the other. The shielding became needed when people started using radios in planes. It is the threaded adapters and the shielding which makes them special and costly. If there is no radio anywhere around, you could get along just fine with plain copper core automotive spark plug wire. Or maybe even resistor wire. The newer car wire has a core of resistance material which reduces the radiated electrical noise. The only problem you will have is getting the wire secure with a good connection at each end and not shorted out. Now for the "P" lead. That is the terminal and wire thich controls the mag being on or off. Grounding the P lead terminal = no spark. With the P lead open, sparks happen whenever the mag is turned fast enough. It is usually an 18 gage shielded low voltage wire going to the cockpit ignition switch. Open is on, grounded back to the mag is off. It is shielded to prevent noise in the radio and (I think, don't know) because if it is pinched or cut, it is more likely it will short the inner wire keeping the mag from sparking instead of the wire going open and possibly firing the engine if it is turned. Wally #### Joe Fisher ##### Well-Known Member As far as I know you can by plane spark plug wire at tractor supply. The auto parts places only sell the resistor type. #### Dan Thomas ##### Well-Known Member Modern aircraft shielded ignition leads have a spiralled center wire. It's not just a stranded wire like we used to see in old automotive plug wires or the unshielded plug wires found on old airplanes. The spiralled wire (it's like a long, stretched-apart spring), along with the sparkplug's resistor, help prevent the "ringing" caused by the magneto's coil discharging. That ringing can caused burned points and other troubles in magnetos. I believe the ringing is a capacitive product of the shield interacting with the spark signal. Unshielded wires didn't suffer that. Or something like that. Dan #### fly2kads ##### Well-Known Member HBA Supporter Now for the "P" lead..... It is shielded to prevent noise in the radio and (I think, don't know).... The P-lead shielding does prevent radio interference. I found this out the hard way when an old and tired Cessna 172 that I used to rent had its P-lead shielding come loose in flight. It started off making a constant hiss like a breeze flowing across a stuck mic. After a few minutes of that, it started getting progressively worse and eventually turned into a very loud and extremely obnoxious hum. What a god awful noise that was. At normal volumes, it completely drowned out all radio chatter. I found that if I cranked the volume way up, the noise got louder, but I could just make out the radio transmissions. I made plans to divert to an uncontrolled field if I found I couldn't hear ATC well enough to get back home, but I was able to make it back okay. When I got back on the ground, the FBO mechanic diagnosed it as the P lead right away. Not that I could hear him after all that racket. "EH? SAY AGAIN? OH, THE P-LEAD? WHAZZAT?" My ears rang for a couple of hours afterward. As luck would have it, I was a student pilot making my first solo cross-country flight. Who knows, maybe it was a good thing...I really had to focus on what I was doing and I didn't come close to getting lost. #### fadec ##### Well-Known Member I can’t seem to find absolutely anything about magneto wires. I have a slick magneto and I don’t feel like spending$240 for 4 wires for a magneto. There has to be a how to or is it possible that I can use other magneto wires that say fit a different motor. Are magneto wires specific to its application and is it specific to the actual brand of magneto? I find wires all day on ebay for a lot cheaper for what I would buy online. FYI this is for a Type IV VW motor.
Are you refering to the wire itself or the whole harness? If a whole harness is required then only a slick harness cap will fit on a slick magneto. If you are using automotive sparkplugs then a slick harness will need to be reterminated at the plug end to fit them. Or you could reterminate an automotive harness with a slick harness cap but I would not recommend carbon ignition wire in an aircraft application as it will likely die young. Also the springs on the cigarette ends in a slick harness cap require spiral conductor wire for the retaining pin to screw into.

Resistive sparkplugs should be used with shielded ignition wire to prevent premature electrode erosion, though this is more of an issue if you are using expensive aircraft plugs. The central conductor, insulation and shield form a long skinny capacitor which the magneto has to charge up before it can fire the plug. When it does fire there is more energy than is required for combustion, but the resistor in the sparkplug disipates some of this excess energy from the 'capacitor' as heat, thus reducing electrode wear.

Your local aircraft maintenance/engine overhaul shop will likely have a box of old harnesses lying around which they could use to make a serviceable harness for you. But be aware that insulation regularly subjected to high voltage will break down over time so get it tested first.

A quick google search turned up the following pre-made harness using automotive parts- note price is for 2 (L&R) harnesses
http://www.g3ignition.com/order.html
There are also instructions for diy
http://www.g3ignition.com/magmod.html

##### Well-Known Member
Again with the double post!

#### SHIPCHIEF

##### Well-Known Member
A pull from MSD:
MSD's 8.5mm Super Conductor wire sets have a special helically wound core that has just 40-50 ohms of resistance per foot--but with an RFI suppression equal to a 1,500 ohm wire. In fact, each foot of finished wire features 40 ft. of tightly wound copper for superior conductivity. A tough, 8.5mm synthetic/silicone jacket over an extra-heavy glass braid and dielectric insulator keeps the current guided to the plugs. Underneath the high-temperature boots, dual crimp stainless steel terminals feature snap-locks to ensure a secure fit. MSD's 8.5mm Super Conductor wire sets come available in custom or universal-fit applications
Other brands offer spiral core wire as well.
I wonder if this is the way to go when replacing the plug wires on your mag, especially if you use auto spark plugs?

#### Dana

##### Super Moderator
Staff member

Modern aircraft shielded ignition leads have a spiralled center wire. It's not just a stranded wire like we used to see in old automotive plug wires or the unshielded plug wires found on old airplanes.
But shielded ignitions have been around for a lot longer than spiral wound wire... presumably they used stranded wire?

Here's my dilemma: The Slick mag on my 1/2VW engine had unshielded wires. The new (1980s vintage surplus) mag came with a shielded harness, which I'd like to use with shielded plugs but it may be too short. I don't want to pay the going rate for a certified 4 cylinder harness, so the idea is to redo the harness using the end hardware, unshielded wire, and shielding braid. Or do I need spiral wound wire? The springs on the ends of the unshielded mag don't screw into the wires; they're just a pin which pokes into the wire.

Dana

#### N8053H

##### Well-Known Member
I use non shielded everything and a handheld. No problems with the handheld using the little antenna. I want to use a remote antenna for longer range and an A&P friend gave me shielded antenna wire. I can mount the antenna far away from the engine using this shielded antenna wire. I spend pennies on spark plug wires and spark plugs. No need to go shielded on these little airplanes. Waste of cash in MHO. These planes do not have the systems needed to run shielded ignition. What is the benefit?

Tony

#### Dana

Staff member
Tony, the benefit is not hearing the loud ignition noise in my headphones, and more robust aircraft style connectors. Cost is low as I already have the shielded ignition wires (if they're long enough; in case they're not is why I'm posting here), and the plugs were quite reasonable at under $10 each. My radio can't use the little antenna with an external headset, and the plane already has an antenna mounted to the center section of the top wing. Dana #### gearhead ##### Well-Known Member Dana, the leads that came with my 4220 are 21" and 31", nut-to-nut, two of each. All are long for my application, so I can trade the 31s for shorter. #### Dana ##### Super Moderator Staff member Thanks... looks like I got a 21" and a 31" with my 2220. The 21" may be long enough; it's a couple of inches shorter than the old leads but I'm not sure how much slack there was with the old wire. It would have been a lot cleaner with right angle connections at the spark plug ends, though. I'll know when I go to put it together on Friday or Saturday. Dana #### N8053H ##### Well-Known Member Tony, the benefit is not hearing the loud ignition noise in my headphones, and more robust aircraft style connectors. Cost is low as I already have the shielded ignition wires (if they're long enough; in case they're not is why I'm posting here), and the plugs were quite reasonable at under$10 each. My radio can't use the little antenna with an external headset, and the plane already has an antenna mounted to the center section of the top wing.

Dana
Never had the noise in the radio problem. So I know not what you speak. My plugs cost under 3 bucks each. My wires around 20 for the pair. They will last longer then my airframe.

Tony

#### Anders

##### Member
Ok here we go again with the mags I just bought a new 4220 Surplus Slick mag from Standardmagneto.com for my Vw 1834cc conversion.

I want to use automobile sparkplugs and the sheilded ignition leads that came with the magneto. What to do? Could I use NGK No.BP7HS(BP7HIX IRIDIUM TYPE Plugs and 1K shielded plug cap?

Powerdynamo, shielded sparkplug cap 70 10 500 00
NGK No.BP7HS(BP7HIX IRIDIUM TYPE)

I guess the shielding needs to be connected to the engine head?

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
Use an ohm meter and you will see that the shielded plug wires and ends are grounded through the mag housing, same as the shielded wires to the p-lead to the mag switch. Could always run a ground wire from the grounding lug on the mag to the engine just to make sure.

Dan

#### Dana

##### Super Moderator
Staff member
On my Mosler I briefly used those wires with Autolite 2304 spark plugs but those are 3/4" reach plugs which won't work on a standard VW head, which I believe uses 1/2" reach plugs. The surplus wires will not fit the Great Plains shielded plug covers. You can probably adapt the wires to the NGK shielded covers, but if you do, make sure you don't use resistor plugs (use resistor plugs or a resistor cap, not both). Or you could replace the wires with a Slick M2266 harness and use the Great Plains spark plug covers (has to be a genuine Slick harness; the Aircraft Spruce clone won't work), which is what I ended up doing when I put on a Slick 4316 magneto.

Are you going to modify the impulse pawls? If you don't, hand propping will by all accounts be... exciting...

Dana

#### lake_harley

##### Well-Known Member
What method(s) have been used to deal with the magneto threaded end when using 7MM, 8MM or other "automotive" spark plug wires? I had a 2A042 with a F-M mag and had considered putting it on a diet. One possible weight savings would have maybe been non-shielded wires but wasn't sure how the mag end could work out with the rubber nipples like normally used on auto stuff.

Lynn

#### Dana

##### Super Moderator
Staff member
The Slick wires have a tapered ferrule and sleeve deal that clamps on the wires. A pin is stuck into the center conductor, which attaches to a spring which makes contact with the magneto terminal (or the spark plug on the other end in a "standard" setup, though I had rubber automotive caps on the spark plug ends). When I bought my plane, it had a Slick 2316 mag with unshielded wires attached to the threaded connections on the magneto. I would probably still be flying with that setup if the A&P who started doing my annual hadn't tried to open up the mag to check the points, with the result that it was fried after being reassembled incorrectly.

Dana