Secondary wire permanent replacement (old technique)
by Had Robinson
This repair has been thoroughly tested in wet, humid environments. It can be done repeatedly and is much easier to do than taking the front of the engine apart, replacing the finger screws, re-timing the engine, etc. If you just replace the whole coil assembly, it will fail soon enough, just like the OEM, and you will have to do it again ... and again.
First, you must check the coil to be sure it is good. Stick a sharp needle into the secondary wire about 3" from the coil in order to the resistance to the engine (ground). You should be about 10K Ohms (the resistance of a good coil plus the resistance of 3" of secondary wire). If it is less than 10% of this value (or open/infinite), the entire coil assembly must be replaced. Also, check the other parts of the ignition system, including the primary wire. It is rare that the coil is bad but running the engine with a faulty secondary wire or a faulty boot may ruin it.
Small two cycle aviation engines all have a low flywheel mass to save weight. Consequently they all experience severe and damaging vibration. This leads to early failure of engine components, including the secondary ignition wire that goes from the coil to the spark plug. Unfortunately, the Miniplane OEM supplier (IDM) uses an inexpensive secondary wire that burns out quickly compared to automotive grade wire. The thickness of the automotive core is about twice that of the Top 80 OEM core. The core itself is made up of a carbon impregnated sheath surrounding hundreds of tiny flexible synthetic fibers that give it strength. In tests done here it is easy to tear/break the Top 80 carbon sheath with gentle flexing compared to the automotive grade sheath. Premium grade secondary wire does not use a carbon core but has a high resistance spirally wound wire core. This grade of wire, though expensive, will last the life of the engine. The new fix I am working on uses this type of wire.
The structure of a modern secondary ignition wire.
The heavy duty automotive grade secondary wire compared to the OEM Top 80 wire.
In this photo, the barbed post for connecting the secondary wire is only visible because the surrounding extension tube has been cut away. I am working on a secure repair that removes the original secondary wire entirely back to this post.
If a temporary repair is preferred, go to this page for instructions.
This repair should take less than 45 minutes to complete and does not require advanced skills. Why not replace the entire coil assembly? Most pilots do not have the technical skills to replace the coil or to accurately check the ignition timing and then change it, if needed. Most of the engines that come in here have had coil replacements that were done incorrectly, especially not setting the coil – flywheel gap to the best value. If the gap is not perfectly set, both the ignition timing and spark output are affected, especially at low speeds. This repair also takes about 1/4 less time and costs much less.
Replacing the secondary wire
1. Get the following. (Harbor Freight has everything except the ignition wire and the nylon cable clamps.)
- Replacement secondary wire like the one shown below from a local auto parts store. The wire should be at least 24" long and have a permanently attached spark plug boot at the end. The other end has a fitting for the distributor cap. The caps are usually labeled. The cap for the plug is longer than the one for the distributor cap. The wire can be 7mm or 8mm (preferred) in diameter. The 8mm wire will last longer than the 7mm.
- 1/2" OD wire loom
- 1/4" nylon cable clamps – Ideal #772835L for 8mm wire only
- 1/2" & 3/32" heat shrink tubing (HST)
- 5 minute epoxy
- Nylon quick-ties, preferably those with a fastening hole – use only commercial electrician quality ties as the other kinds go bad quickly.
- ALNOX or an equivalent dielectric grease. This repair will not last if dielectric grease is not used.
- A wire stripper used by electricians – it must be able to strip #10 wire at a minimum (see photo below).
The only special tool you will need is a hot-air gun to shrink the HST ($15 from Harbor Freight).
One end of the replacement wire fits over the spark plug and the other plugs into the distributor. We only need the end that goes on the spark plug (right side of photo). It is important that the spark plug end have a 90 degree connector, as shown here. The photo on the right is that of an electrician's wire stripper which must be used to strip high voltage secondary wires.
2. Cut the original secondary wire about 2" from the end of the heavy sleeve that protects the wire as it goes into the coil (see red arrow in the first photo below). If the secondary wire is good less than 2" from the end of the heavy sleeve, you may not have enough of a stub of the original wire to make this repair secure. Then you must replace the entire coil assembly.
In the second photo below, I removed the heavy sleeve in order to check the wire condition but this should not be necessary. At this point, check the secondary resistance of the coil again, just in case. It MUST be within specifications. For example, if the stub is 3" in length from the coil, the measurement should be around 10.2K Ohms (8.5 + .57 + .57 +.57). If the values are greater than 10% less, I would replace the coil because it likely has some internal shorts that will only get worse over time.
The coil here has been removed from the engine for clarity. Using pins, it is easy to see the internal structure of the OEM secondary wire. The OEM carbon polymer sheath is very fragile.
3. With a razor blade, make a slice lengthwise 1/4" from the end in the outer black/white insulation of the OEM secondary wire just deep enough to expose the inner core. Use a block of soft wood, some straight pins, and a pair of needle nose pliers to pin back the insulation, exposing the inner core in order to lift it out. Once the core is exposed, it is easier to slice open the insulation another 3/4" if it is pinned open without damaging the inner core. Otherwise, it will take a bit more time if you can't pin it open.
Remember that the carbon core is very fragile. Leaving the outer black/white insulation connected and intact allows it to make a "sandwich" (like a hotdog bun over a hotdog) around the new joint of the two cores, making it stronger and more durable. Mostly importantly, it protects the OEM core, as any slight bending of it will break it.
The HST needs to squeeze about an inch of the two cores together. In the photo below, the arrow points to the barely visible slice.
4a. Coil installed on the engine: Connect the spark plug boot end of the new wire to the spark plug, install the (2) clamps, and dress the wire down to the coil, as in the last photo. Keep the wire tight in order to keep it from shaking and fragmenting the core prematurely. There must be enough slack at the plug end so the cap can be easily removed from the spark plug. Cut the new wire to an adequate length to make the connection at the coil end.
4b. Coil not on the engine: Cut the new wire so its overall length is 19".
5. Note: You may want to do a "dry run" of the next steps. Do not apply ALNOX, the epoxy, nor cure the HST in order to be sure you have everything the right length and the (2) pieces of HST pushed up on the new wire. The purpose of the next steps is to be sure that the new wire is strongly attached and sealed to the old wire from the coil.
Slide two 3" pieces of 1/2" HST over the new wire. Push them far up the wire towards the spark plug boot so they are far out of the way and unaffected by the hot air gun used in the next step. These will be used to strengthen the joint of the old and new wires and keep the joint from bending too sharply. Do not apply heat to the pieces of HST at this time.
6. Carefully strip 1" off the end of the new wire using the #12 wire size on the wire stripper. The stripper will not quite reach the core and prevent the cutters from nicking it. You might want to practice stripping a piece of the wire that was cut off before trying it on the final product. Coat both cores with some ALNOX. The ALNOX must be used to keep the cores from arcing between each other. If you have a syringe with ALNOX in it, just push the core into the syringe to coat it.
If it is not used, the cores will burn each other up after 50 hours or so. Put an inch of 3/32" HST over the original core (1/16" HST will work but it is a bit tight). Push the new carbon core into the HST that is on the original wire as shown below. This way, an inch of the cores will be in contact with each other. The pins are there to help make the photo clearer.
The most important part of this fix is to be sure the cores are firmly squeezed together by the HST.
Shrink the HST that surrounds the cores. You will have to lift out the core going to the coil a bit in order to do this. Let things cool off. The HST tends to stick to the wire insulation so you may have to carefully use a razor blade to free it from the black/white insulation in the slit. Next, push the joined cores into the slit made earlier, like a hotdog into a bun. Use a cotton swab to clean up an excess ALNOX that oozed out of the HST joint.
The OEM wire is "sandwiched" (below) but the new one is not – which is not a problem because the new wire is much sturdier and better made than the OEM wire.
7. Epoxy prep: The epoxy tubes should be put in ice-water for (10) minutes in order to slow down the curing rate. Coat the exposed HST that covers the cores and the area of the joint 1/2" in each direction with 5 minute epoxy. You have to work quickly. Slide the first piece of HST that is over the new wire down so that it covers the new joint evenly. Shrink it with your heat source. The HST will squeeze the epoxy over everything and out the ends. It will FIRMLY hold the new and old pieces together and tightly seal everything from water, oil, and dirt. If the epoxy is omitted, the wires may eventually shake apart. Epoxy must not be used with the first piece of HST because it will come in contact with the carbon cores and may insulate them from each other, despite the ALNOX. When you are done, things should look like the photos below.
If done on the engine:
If done with the coil off the engine:
8. After things are cured and cooled down, slide the third piece of HST over the joint an inch closer to the coil than the other piece of HST. The two pieces of HST will be staggered and this greatly increase the strength of the joint. Shrink it.
9. Cut a piece of wire loom long enough to go from the coil to at least an inch past the HST on the new wire. Butt the loom right up to the coil. It will help stabilize and strengthen everything and minimize shaking. Do not fill it with epoxy because you might have to remove the whole thing sometime in the future. Use small nylon zip ties to secure the loom on the new joint and wire. Use the nylon tie with the fastener hole to help secure things and make sure that things do not slide off. If this step is not done, engine vibration will eventually cause a break in the stub that comes out of the coil.
We are done.
Be sure to measure the secondary resistance again. The automotive grade wire and boot should last 10X the stock Miniplane wire and boot.
The clamps must be firmly fastened to the engine, just like the original. Failure to do this will ensure that the core will break and fail prematurely. It is best to use metal screws to fasten the clamps to the cooling shroud and engine frame rather than use the original plastic screw and a rivet. Note that the spark plug boot is bigger and more rugged than the OEM part.
If you make a mistake, remove the loom, slice off the HST with a razor, and start over. The engine below has already had another 50 hours of hard use with this repair and is working perfectly.
IMPORTANT NOTE: THE AUTOMOTIVE GRADE BOOT IS VERY TIGHT ON THE PLUG. USE SOME SALIVA (or "SPARKGUARD") ON THE BOOT AND PLUG TO INSURE THAT THE BOOT IS SEATED COMPLETELY DOWN ON THE SPARK PLUG TERMINAL WHEN INSTALLED, THE BOTTOM EDGE OF THE BOOT WILL BE ABOUT 4.5mm FROM THE METAL BASE OF THE SPARK PLUG. If the boot is NOT fully installed, the boot terminal will be damaged when the engine is running.
This modification greatly prolongs the maintenance schedule for the ignition system. However, even heavy duty automotive grade wire can still wear out. After 150 hours, the severe vibration caused a crack in the insulation near the boot but it should last another 50 hours. Check this secondary wire many hours later, it, too, had a cracked carbon core right at the boot. The wire had to be replaced.