Installing spark plugs may not be as simple as you thought

Nov. 1, 2016
Let’s see what some of the experts in the spark plug segment have to say about the proper service of this common component.

In the decade I’ve been with Motor Age, I’ve written dozens of how-to articles on a variety of topics. Many of them have been on common service procedures, like the one I’m going to talk to you about today. And in the process of researching these articles, I often find that I’ve been doing it wrong (in some way) and I’ll bet that, after reading this article, you’ll find out that you have to. Ready?

(Image courtesy of Robert Bosch LLC) Today’s high compression, turbocharged, direct injection engines are placing even higher demands on the spark plug.

First, some fun facts about spark plugs

Many claim to be the first to patent the idea of a “spark plug,” with patent records going back as far as the late 1800s. Names you recognize today were among them: Robert Bosch and Albert Champion, to name a few. There was even a patent submitted by Kenelm Lee Guinness, of the Guinness brewing family. Who knew beer and spark plugs had anything in common?

Albert, it seems, had a rough time in the early years, losing the Champion Ignition Company he founded in Flint, Mich. to investor drama. Shortly after, though, he was named president of the AC Spark Plug Company, formed with backing from Buick. This company’s products were used in a variety of applications including the aircraft used in Lindbergh’s and Earhart’s trans-Atlantic flights and even more cool, to fire off the second- and third-stage rocket engines that powered Neil Armstrong’s mission to the moon.

But it seems that most historians agree that the credit of designing what would become the modern spark plug goes to a Bosch employee, Gottlob Honold. Assigned the task by Robert Bosch in 1901, Honold engineered a magneto ignition system based on what was then known as “electric arc” ignition. This design is credited with making the modern spark ignition engine a possibility.

(Image courtesy of Robert Bosch LLC) By means of two coils on the armature, Bosch’s Gottlob Honold generated a high-voltage current. This was conducted to a spark plug via a simple cable connection.

How does it work?

The terminal (top) end of the spark plug is connected to a high voltage supply source, the ignition coil. While the coil is incorporated in a variety of ways consistent with different ignition system designs, it still serves the same purpose Gottlob’s magneto did. As current flows from the coil, a voltage potential builds between the center (positive) electrode and the side (ground) electrode. Now, of course, the two electrodes do not touch and that means it’s an open circuit, right?

Not entirely. There is something there – air. Or more precisely, the air/fuel mixture that was injected into the cylinder micro-moments before. Initially, this mixture acts as an insulator, but as the voltage potential grows, the gasses begin to change. When the dielectric strength of the gasses is reached, the gasses ionize and become a conductor instead. No more open circuit!

As the current flows across the now-bridged gap, the temperature in the spark channel rises significantly and quickly. This causes the ionized gasses to expand just as quickly, like a small explosion, and results in the “click” you hear when a spark jumps outside of the combustion chamber. Same idea between lightning and the resulting thunder you hear during a summer storm. The heat and pressure force the gasses to react with one another and ideally, near the end of the spark event, a small ball of fire (or kernel) has formed as the gasses burn on their own. This tiny little fireball then expands to consume the surrounding gasses and combustion is realized.

Small part, big job

Based on what I’ve shared so far; you can see that the spark plug plays a critical role in the combustion process. Extending that idea, then, it is easier to understand that even minor discrepancies in the ability of the plug to function can result in inefficiencies of the combustion process.

For example, consider a gap that is too big or too small for the application. Too small, and the resulting spark may be too weak to ignite the air/fuel mixture. The spark will fire each ignition cycle, but still be the root cause of an elusive miss. Too big, and the voltage requirement goes up, perhaps to the point that the plug doesn’t “fire” at all. Excessive gaps, either due to improper initial setting or wear, can also stress ignition coils to the point of failure.

Ever drop a spark plug during a routine service? What did you do with it after you dropped it? The correct answer is “threw it away and got another one!” The drop could cause cracks or internal separations that will lead to premature, if not immediate, failure.

(Image courtesy of Federal-Mogul Motorparts) Federal-Mogul’s Advanced Corona Ignition System uses a high-energy high frequency electrical field to produce repeatable controlled ionization creating multiple streams of ions to ignite the fuel mixture throughout the combustion chamber.

And what about something as simple as removing and reinstalling a plug? Let’s find out from a few men who deal with spark plugs on a daily basis.

What the experts say

Early in my writing career, I learned (the hard way) that I should never rely on my preconceived notions on even the simplest of topics. As I mentioned from the start, I have often learned that I was doing something wrong even in the most routine repairs. I strive to be as professional as possible, and share what I learn in the hope that you will be too. With that said, let’s see what some of the experts in the spark plug segment have to say about the proper service of this common component.

Motor Age: What are common mistakes you see being made by technicians when removing or installing plugs?

Tim Stumpff, Product Manager – Spark Plugs, Robert Bosch LLC: Lack of proper cleaning and improper torque are the most common mistakes. Spark plugs that are worn out and require change have typically been in the cylinder head a long time. This means they can be tough to turn out. Many techs will simply thread the new plug in without cleaning the threads (in the head), which can lead to a plug not sealing properly. Threads should always be cleaned to ensure the plug can be torqued properly. A torque wrench should always be used when installing spark plugs, as over-torque can damage the plug and cause misfires and under-torque can cause the plug to loosen as the engine runs.

Michael Kollenberg, Product Manager – North American Ignition, Federal-Mogul Motorparts (Includes the Champion brand of spark plugs): Additionally, we see techs choosing and installing plugs based on a competitive interchange chart rather than relying on an application-specific catalog recommendation and not verifying spark plug gap prior to installation.

(Image courtesy of Robert Bosch LLC) Heavy ash deposits like this can be caused by the generous over use of fuel additives.
(Image courtesy of Robert Bosch LLC) The presence of an overly rich condition can cause dry deposits like this. If it looks wet and black, lean more toward some form of oil fouling caused by something as simple as an overfilled crankcase.

Jason Norwood, Technical Services Supervisor, NGK Spark Plugs (USA) Inc.: I’d just stress what Michael said earlier about proper torque. Ninety percent of our damage claims (plugs returned as “defective”) are the result of over torque.

Motor Age: The debate over using some form of “never seize” on the threads of the plug continues. What are your recommendations on the use of an anti-seize compound?

Kollenberg:  We discourage the use of anti-seize or other lubricants on the threads. First, because of naturally occurring properties of the base metals used in our shells, Champion plugs already have a certain level of lubricity built into them. In addition, the shells receive a zinc plating to provide an additional lubricating layer.

Second, even when applied with caution, anti-seize or thread lubricant often  contaminates the plug’s firing end. Torquing a plug to the vehicle manufacturer’s spec ensures that it fully seats, properly seals and assists in transferring heat to the cylinder head.

When anti-seize or similar compounds are used, you commonly end up with an over-torque condition, even when a torque wrench has been used. This is due to the difference between “wet” and “dry” torque values. Over-torquing can cause separation of the internal seat area (where the ceramic core contacts the inside of the shell). This will impair the plug’s ability to transfer heat and can lead to damaging pre-ignition or detonation condition.

Excessive gap can cause misfires and stress the ignition coil to the point of failure.
Look close. See where this plug boot has a hole burnt through? The energy is looking for an easier way home.

And Kollenberg’s fellow commentators agree. While there are a few spark plugs out there that may still recommend the use of an anti-seize compound, your best bet is to clean the area around the plug prior to removal and then clean the cylinder head threads prior to installation of the new plug. You will find some info on how to tighten a plug without a torque wrench, but as professional technicians, we know that even those procedures are less than accurate. To make sure you do the job right, use a torque wrench to install and leave the air ratchet on your tool cart. On to the next question.

Plug selection

Motor Age: Is there any truth to the idea that a technician should install the same brand of plug as the car originally came equipped with?

Norwood: In the OEM world, when we are making Dual Fine Element (DFE), Projected Square Platinum Electrode (PSPE) and other designs, it is usually for direct injection or lean burn systems, and normal spark plugs will not operate as efficiently or ignite the mixture as well.

Stumpff: The common perception is that the OE brand must be replaced with the same brand. However, this may not always be the best choice. In some instances, an OE brand may have an aftermarket version to fit the vehicle; and while it may be a less expensive choice it may not be quite the same fit/form/function of that same brand’s OE plug. In other cases, some aftermarket brands may not perform application testing. With today’s complex engines, just because a plug fits or looks like the OE, it does not mean it can withstand the stresses of high compression or turbocharging. Bosch spark plugs are tested for the application and designed to meet or exceed the OE design so a technician can install them with confidence in every application.

Kollenberg: No, but it is important to install the same fundamental spark plug technology – iridium for iridium, double platinum for double platinum, etc. – if you want to achieve the same anticipated service life as the original plug. We offer Champion plugs that are ideally suited for virtually any make or model. As long as the plug you are installing is cataloged for that specific engine application, there is no reason to be concerned about its performance versus the original brand.

Use a flexible fuel hose to get the plug started in the head to avoid accidentally cross threading the plug.

And it should be added that these guys are the same guys that make the OE original equipment. I admit, I was one of those techs that preferred OE for engine management components and I always installed the same brand plug as the car came with. But having had the opportunity to see behind the scenes at many of these big suppliers, I think the key to choosing a plug is to make sure you use a name you know and trust and choose the plug based on application and not based on an interchange chart.

In the shop

Well I learned a lot! I’ll never look at a spark plug change the same way again and I hope you won’t either. Let’s recap some points mentioned (and not yet mentioned) to keep in mind the next time you need to swap plugs on a customer’s car.

  1.  Use a spark plug wire puller to carefully remove the secondary lead from the spark plug. Inspect the boot for any visual sign of damage or arc through and service as needed.
  2. Use compressed air to clean the area around the base of the plug of any accumulated debris. Don’t want anything to drop down into the cylinder, do we?
  3. Inspect the removed plug for signs of damage and to make sure it all came out! While not as common as it once was, it’s still not a bad idea to “read” the plug for clues to any drivability issues you may be troubleshooting.
  4. Tab a small amount of grease (and I mean small!) into the fluted area of a thread chaser and carefully clean the threads in the head. Be sure to position the piston well below TDC to avoid accidental contact and watch out for any open valves! The grease will help catch small metal particles and debris and help you keep the foreign objects out of the engine.
  5. Clean the same thread chaser and run it through one more time to make sure there is no residual grease that may affect the final torque on the plug.
  6. Remove the new plug from its packaging and verify the plug gap is set to the vehicle manufacturer’s specification. Use caution on plugs with precious metal tips to avoid damage. Avoid using the wheel gauge often given out by your friendly local tool guy, and instead us a round-wire gauge.
  7. Carefully start the plug in the head by hand. I use a small section of fuel line to hold the plug as I spin it in. This avoids accidentally cross threading the plug – a problem you don’t want to have if you can avoid it.
  8. With the plug lightly seated, use a torque wrench to finish the install.
  9. Take a small dab of dielectric grease and apply it to the secondary lead boot. This grease will help act as an insulator, seal the plug contact from outside contaminants (water, oil) and make it easier to remove the boot the next time around.

I’ve watched techs rip out the old plugs with ½” impact guns and reinstall them with cordless ratchets, all in an effort to beat the flat rate. I get it – time is money in the shop – but the damage you may be causing in the process will sooner or later come back to bite you. Does it take that much longer to do it right?

I didn’t think so.

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