My Tractor Forum banner

1 - 5 of 5 Posts

Daryl G
954 Posts
Discussion Starter #1
Recently I performed a small activity that greatly enhanced my Gravely’s performance quite noticeably. This action was very inexpensive yet concurrently delivered a great opportunity for me to learn more about an often-overlooked area of these fine machines.

I would like to share my findings with any interested (and like-minded) people here. My exploration will be documented almost like a research project fashion so I will be providing sources of where I obtained what I considered to be relevant information. This scientific-like approach serves a two-fold purpose: first, anyone interested in learning more can examine the sources I used and secondly, it will hopefully prevent any arguing about the validity of the statements that are to follow. Although I came across some creditable sources in several forums across the Internet, I am steering clear of using any of this information; instead, I am trying to use material from non-forum based sources. Also, I would prefer to minimize any information from manufacturers of some of the components as sometimes things are swayed/skewed a bit in their favor… so, I hope to stay true on course. Enough of that stuff so here goes a little background information:

Of my four Gravelys, I use one of these machines almost exclusively simply because of the front attachment – the rotary cutter with a 3/8” thick brush cutter blade attached. This machine is a 1974 C8 that came, at least to me, sporting a Fairbanks & Morris magneto. Although the machine ran nicely, I have been plagued with a non-snapping magneto for many months. It cranked and ran well enough for the multitude of hours that I used this machine but I knew that something must be awry because the magneto could be turned 360 degrees with absolutely no resistance so I purchased a used WICO magneto from Ebay to replace the F&M unit. After exchanging this magneto, I experienced some strange running issues which were later partially solved by replacing the condenser ( With the new condenser in place, I had a good strong spark when using a test wire and alligator clip to the magneto cap but the spark suddenly was lacking this ‘bbbbaaammmm’ when I reinstalled the spark plug wire to the system. This anomaly led me to investigate the spark plug wire itself so here is what I learned:

A Primer on Gravely (T-head) Spark Plug Wires

Spark plug wires are simply a conductor of electricity and transfer the current produced by the magneto to the spark plug. It seems as if this is a simple and straightforward objective; however, lots of undesirable things can occur within this relatively short length of insulated conductor. The environment in which the spark plug wire exists is hot, often oily, and full of vibrations - all of which can affect the performance and longevity of this wire. Couple these almost unfriendly conditions with any mishandling to the wire and the serviceable life of the conductor may be quite abbreviated[1].

The voltage produced by the magneto is transferred to the spark plug by using a special wire conductor. The most important requirement is that a magneto-based ignition machine use a solid core conductor wire[2]. This ‘solid core conductor’ terminology, at least to me, is a little bit misleading. A solid core sounds like it would be a single ‘solid’ strand of a highly conductive metal or alloy; however, in the world of spark plug conductors, solid core means wound stranded wires[3]. Tactilely, solid core wires are also stiffer and heavier than the other common types of spark plug wires. Most often copper and tin-plated copper are used as the conductive material in solid core sparkplug wires because of their high electrical conductivity (addendum: I did find references to both stainless steel and aluminum being used as the core wire but in examining the electrical conductivity reference tables it was readily apparent that neither aluminum nor stainless steel are anywhere close to the electrical conductivity of copper, therefore I purposely did not include these two materials). Since the solid core spark plug wire is equipped with materials with high electrical conductivity this makes the solid core design the most effective conductor of transferring the energy from the creation of the spark (i.e., magneto) to the spark plug versus other types of spark plug wires (spiral core wire, carbon core wire, and mag wire)[4] commonly found in today’s gas-powered engines. Solid core spark plug wires should NOT be used in modern engines and, likewise, do NOT attempt suppression wires on your magneto-equipped machine – also, be careful not to use ‘mag wires’ as, in this case, ‘mag’ means ‘magnetic suppression wire’ NOT ‘magneto wire’. Magneto machines are designed for solid core wires only! One bonus note, copper is an excellent conductor of electricity and is only second to silver in this attribute[5] but, of course, silver is way more expensive!

Using the right type of spark plug wire critically important – first check that the right type of wire has not been wrongly replaced with a suppression wire. Once this has been confirmed, the wire itself should be studied. The performance of the spark plug wires will degrade over time[6] and these do eventually require some attention and/or replacement. Visually examine the spark plug wires for cracks or brittleness in the insulation. Water can also be used to help identify problem areas(s) on the spark plug wire – using a spray bottle, coat the entire spark plug with water while the engine is running and look and listen for any arcing[7] – being in a darkened space can further aid with seeing any arcing. Machines can continue to run, albeit not very well, when voltage ‘leakage’ occurs from a small crack or break in the insulation…essentially, the motor is producing less power because of the decreased voltage[8] that eventually reaches the spark plug. If the spark plug wire passes those visual and auditory tests it is time to measure the resistance.

Resistance, in the case of ignition wires, is the “wire’s tendency to resist the flow of the electric current”[9]
therefore, any resistance to the flow of electricity should be minimized… all of the voltage that arrives to the magneto’s cap should, ideally, make it to the spark plug. Another Interesting note is that the voltage produced at the magneto during low rpms (i.e., cranking) is less than the voltage generated normal operating speeds. In other words, it may be concluded that the magneto produces more ‘sparking power’ at higher rpms to ignite the air/fuel mixture[10, 11] (this fact does not actually apply to the spark plug wire research but I found it noteworthy nevertheless).

There is a nice video on youtube demonstrating how to measure the resistance. Remarkably, the tools used in this video are just a common graphite pencil, a piece of paper, and a multimeter. To recreate this test yourself simply draw a thick line about six inches across on the piece of paper with a pencil. Then take your multimeter and set it to read Ohms and place the black lead at one end of the line you drew and the red lead to the other end. Jot down the value once your multimeter stabilizes. Next, move one of the leads closer to the other lead (but not so close that the leads touch one another). This second reading should be less than the first… less resistance exists. The pencil’s graphite conducts electricity and your multimeter passes a small current to allow reading of the voltage drop across the graphite pencil line. Watching this helped me understand what I was actually measuring since I am at total loss of all things electrically-related.

I could not find a definitive target Ohm value for a solid core spark plug wire. Instead, I saw some references of 0 Ohms while others gave values of readings of less than 1 Ohm; regardless, it should be really low – as a sort of comparison, some values of non-solid core wires (i.e., suppression-type wires) have an acceptable range of 3,000 – 10,000 Ohms per foot. Yes, there is a HUGE difference in the resistance of solid core spark plug wires versus the carbon core type of spark plug wire! Being quite curious, I was still not satisfied without an actual value to measure against therefore I investigated the gauge of wire used in 7mm spark plug wires (this is one of the most common sizes). Although many manufacturers only listed the outside diameter, I eventually did find a clearly stated ’18-gauge’ in a tinned copper solid core wire made by ACCEL[12]. Using this 18-gauge value, I searched for electrical resistance tables for stranded wire. Although these were easy enough to discover, most listed the resistance ‘per 1000 feet’ while I was looking for resistance ‘per 1 foot’. Since, again, I admittedly know nothing about electrical things, I had no idea if the resistance was linear, exponential, logarithmic, etc… thus I did not want to assume anything. Eventually, I found a table that contained both the ‘per 1,000 feet’ and ‘per foot’ resistance valves. This chart reveals that an 18-gauge stranded wire has a resistance value of 0.006520 Ohms per foot[13]. Finally, I have a value to measure/compare against and it appears that the resistance is linear so this is quite easy to calculate what the resistance for my Gravely machine’s spark plug wires should be! My plug wire was 26 inches in length therefore the resistance value I am using as a barometer against mine is 0.006250 Ohms/foot X (1 foot/12 inches) X (26 inches) = 0.013452 Ohms.

To measure the resistance, first disconnect the spark plug wire from the magneto cap and spark plug. As mentioned earlier, the solid core conductor offers almost no resistance which translates to getting the most spark to the combustion chamber[14]. Most guides for measuring the resistance of the spark plug wires recommend setting the multimeter at 20K Ohms; however, because of the low resistance of solid core spark plug wires that actually exist, this approach is not logical – set it on the lowest Ohm range (unless, of course, the multimeter automatically adjusts the range). Place the black lead on one end of the spark plug wire and the red lead on the other and wait until the readings stabilize. If the measured reading is above the calculated value it is probably time to replace your spark plug wire.

Another area of curiosity/confusion is the stated diameter/size of spark plug wires. The wires are generally identified by the outside diameter with 7mm and 8mm be the most common. Some manufacturers make spark plug wires as large as 10.8mm and often tout better performance that is unsubstantiated (hence my apprehension of including data from manufacturers!). The larger diameter offered are mostly thicker insulation whereas the actual core diameter of the wires are the same… whether it is a 7mm or 10.8mm[16, 17]. Benefits do exist as a thicker insulation can give a longer life because this added material insulates better and longer and decreases the likelihood of ‘arcing and sparking’ from the outside walls of the conductor. However, regardless of the thickness, this insulation, given the age, heat, oil, and vibrations of our Gravelys, can crack. This deterioration/damage can lead to the high voltage electricity finding an easier path to escape rather than making it entirely to the spark plug’s electrodes[18].

Basic spark plug wire maintenance – use dielectric grease (or silicone grease) as it helps to keep moisture out of the spark plug boot and insulates the terminal to spark plug connection[15]. Likewise, the same benefits are afforded to the wire’s connection at the magneto socket. Also, dielectric grease eases the installation and removal of the terminal connections or spark plug boots.

Other notes:
I am usually not a fan of ‘seat-of-the-pants’ testing because we are oftentimes inclined and hopeful to feel the impact of our latest modification regardless whether it is actually authenticated or not… but as soon as I pulled on the starter strap for the first time I DEFINITELY noticed a difference. I used the machine for about two hours and it ran and sounded different. Much of this can, rightfully, be simply attributed to the replacing of the magneto. However, the difficulty I experienced in attempting to crank/run this machine AFTER exchanging the magneto AND replacing the condenser was COMPLETELY eliminated AFTER replacing the spark plug wire with a new solid core wire.

Now, after running this newly invigorated machine for a about three hours, I pulled the spark plug just to inspect this item. Previously, the plug was always covered in soot – now, the plug is CLEAN. Something definitely was awry previously with my ‘spark delivery system’. Further, although winter temperatures have not hit us yet, I did perform a cold-start with the ambient air temperature of 41F and the machine fired right up on the first pull. I am going to adjust the carburetor also since the efficiency of this machine’s combustion process has improved greatly.

I purchased a set of ‘do it yourself’ copper solid core spark plug wires. The kit was for a four-cylinder engine so I could replace the spark plug wire on all four of my Gravelys. The kit was relatively inexpensive at $9.58 though single ‘plug & play’ single assembled wires for Gravelys are available at some of the Gravely supply shops and Ebay. Toddnails posted some really good information on another thread ( about making your own plug wires.

As delving into the topic of solid core spark plug wires, I repeatedly came across references to Packard 440 wire. According to what I read, this is a solid core material is ideal for fabricating spark plug wires… if you can find it. This was OEM for many ignition systems prior to the suppression-type wires.

The weak point of the spark plug wires assembly oftentimes is the connections. It is imperative that a proper connection is made between the magneto/spark plug terminals and solid core wire itself. Because of the spine numbing vibrations from our Gravelys, initially I surmised that a soldered connection would logically serve better than a crimped connection (i.e., a soldered connection would be stronger and more resilient to such vibrations) but, based on my research, this may not always be the case. Issues can arise from the knowledge and experience level of the person doing the soldering and, likewise, the tools, skills, and techniques used in performing the crimping. Some industries no longer use soldered connections at all preferring to use only crimped connections[20, 21]. After investigating this I opted to crimp mine but I strongly encourage others to do their own homework in this specific area as the jury is still out on this topic.

Ensure that a good electrical connection exists between the magneto’s cap and the spark plug wire terminal. Oxidation on these surfaces may have developed in the 32+ years since these ‘L’ and ‘C’ model machines last left the factory. Also, the spark plug wire’s terminal should fit snuggly in the magneto cap and likewise of the other end of the spark plug wire’s terminal attaching to the spark plug’s terminal.

Unless you have owned your Gravely since new, you have no idea what the previous owner(s) have done to the machine. This also includes changes like substituting a resistor-type spark plug wire in place of the required solid core spark plug wire… which is a huge no-no! Regardless, the spark plug wire should be considered as a ‘wear item’ and at least be inspected and/or replaced at some sort of schedule[19]. I postulate that the schedule is based on the age, condition, and run hours of the machine though I could not find any actual recommended schedule. For me, I will be inspecting and measuring resistance on the spark plug wire on an annual basis for all my machines – it is a quick and easy check and can yield some great benefits (as I have witnessed)!

In conclusion:
Spark plug wires have a service life. Visually inspect these wires for damage. Use a proper device to check the resistance (Ohms) of the plug wire. If you deem it still serviceable, clean and lubricate the connectors as required. If there is any doubt whatsoever, replace this spark plug wire with a new solid core wire, proper connector terminals, and caps and boots.

Just as any other research, oftentimes you come across credible information that is in direct opposition to other credible information that you have run across. This journey was no different as I, too, saw several areas where something was stated as being entirely one way while another source disputed this fact. Your opinions may vary differently to the what I have written above and my desire is NOT to argue any of these points… in fact, I will not argue any points because there is no advantage in doing that activity. Instead, I implore you to simply check your spark plug wire as having a properly functioning spark plug wire can make a huge difference in the actual performance of our Gravely magneto-equipped machines.

For any sticklers for details, I realize that my approach to documenting my sources does not adhere to the accepted methods of APA and MLA styles. I opted for readability and accessibility over the more formal methods. The sources I used are listed in the next thread (who knew, there is a limit of 18,000 characters?).

27 Posts

Excellent job of putting the facts together about an often overlooked and/or misunderstood part of a healthy ignition system. No matter whether it's a magneto or battery ignition. A particular part that I will remember to consider now is that a poor condition plug wire could be the cause of a plug that consistently carbon fouls even after carburetor adjustments and jetting, adjusting points and timing, plug heat range changes, etc. Something to certainly put on your check list when trying to start an old engine that has been sitting for a long period of time especially if it has been left out in the weather. Well done.


Premium Member
3,529 Posts
Great info Fireant/Daryl
A true cornucopia of information. I must admitt I have to revisit it because I couldnt read it all ( eye issues, will nod-off and drop cig on laptop as I have before lol).
But part of what you described almost stopped a van I have from running recently and the mechanic said it was "jumping spark" AKA some kind of arking.
Great job as usual.
Will keep up w/it . And get solid core plug wire.
1 - 5 of 5 Posts