Second Chance Garage
All About Car Restoration
Second Chance Garage
All About Car Restoration


Putting Spark Back in those Antique Spark Plugs with Thermal Heating

By Allen K. Williams

Allen K. Williams is a lifelong car enthusiast of some twenty five years. He has restored a number of antique vehicles during this time and is nearing completion of his latest project: a 1948 Buick Special, Model 46S.

The author's current project: 1958 Buick Special, Model46S.

The author's current project: 1958 Buick Special, Model46S.

Life can be a real headache for the antique car owner when your car's performance suffers for lack of the proper plug. Ever had to throw away a set of vintage spark plugs because they were no longer servicable? Too fouled or dirty to re-use? No replacements? This isn't a big deal if you have access to plenty of spares, have found a good modern replacement or are talented enough to build your own spark plug like Robert T Wells did for Autolite.

Virtually any spark plug with the correct thread size and an adjustable gap will make do, but only the correct plug lets your motor deliver the power as it was designed to do. The search for a serviceable replacement entails a lot of research because modern engines use High Energy Ignition and sophisticated combustion controls that require a wider gap, usually between 0.040 and 0.045 thousanths. Adapting this type of spark plug to older cars tends to wear them more quickly because of the exaggerated bend angles needed to get the correct settings. I had tried adapting a number of the newer types without success, absolutely nothing worked as well as the original spark plugs. Every new plug I tried soon fouled to a point where it was unusable.

Deposits built up on the new plugs quickly because standard modern heat ranges are usually too cold for older engines and I only operate my antique vehicle over short distances and too infrequently. And,if your car happens to burn oil as mine did, then the problem is accentuated, and cleaning becomes more difficult.

Chemical cleaning isn't effective in the case of heavy plug fouling because the carbon based deposits aren't soluble in any of the typical cleaners, such as mineral spirits, toluene, and other common organic solvents. Abrasive cleaning works on anything contacted by the particulates but usually doesn't reach everything.

The spark plug's electrode may look clean after sand blasting but when reinstalled in the motor, it may not work any better than before the cleaning. I found that there was generally a small layer of residue nestled up inside the electrode cavity, out of reach of most cleaning methods, that shorted out the High Voltage.

I confess to having felt a little queasy more than once as I discarded plugs that looked O.K. after blasting but either misfired immediately on reinstallation or after a few miles of driving. I discovered that electrode appearance as the sole means of determining service adequacy wasn't bery reliable. There had to be a way to clean and recycle dirty plugs, and know they were reusable.

One fall afternoon I was burning a number of tree limbs that had fallen in the back yard through the summer of 1993 from a series of rain storms down in southern Texas. It took me all afternoon that particular day to burn the debris I had gathered up and near dusk I had finally gathered the last pile of branches near the fire. I placed another few chunks of wood on the fire in the center of the pit in front of me, sitting down to watch it burn. I was resting from all the wood cutting activities of the day, all that was left was a small pile of wood next to me. I reached over occasionally and added more wood to the fire, spreading and breaking up embers, to help the burning.

It was getting dark as I placed the last of the wood on the hot coals. As I sat there, I remembered that the trashman would be by in the morning and I didn't want to forget to put out a batch of oil soaked plugs that I had stored in an old bucket.

The red glow of the ash was no longer visible on the surface now as I leaned over again to stir the coals. I wanted to see how much real fire remained before deciding whether or not to wet it down and retire for the night. As, I broke a section of ash, I saw a vast number of tiny red particles throbbing with heat just below the cooler surface. I sat there staring at it for a moment more and then an idea hit me like a ton of bricks! Since, I was going to throw the old plugs out anyway, why not see if this fine hot ash could do something to remove those stubborn deposits. Heat treating in a furnace might be a good idea but as I got up and started for the bucket, I immediately thought of several problems.

The first thing I remembered was that solids don't burn only the gases they emit do. So, a reasonable contact surface with the heat was needed for the solids to gasify and ignite. Engine deposits have a high combustion temperature, so simply heating plugs in an open flame doesn't remove contaminants. Something was needed to get the heat up inside the densely packed areas around the insulator cavity.
I needed something that could get up inside the electrode cavity to attack the deposit causing it to burn like a lighted cigarette. The plugs would require extended exposure but this is exactly what the hot particles in my fire promised if the temperature could be kept from getting too hot or the coals dying out too soon.


My thermal treatment idea also needed to work without damaging or cracking the porcelain insulators. I thought If I could place the plugs in the ash deep enough to get up inside the cavity but without burying the insulators, it might work. Now, if the fire I had built could last long enough, maybe I could test this idea.

I rushed over to the old bucket and brought it back to where I had been sitting. I retrieved several of the plugs and soon was gingerly placing them in the hot ash taking care not to bury them much beyond thread depth. The coals were really hot and I had to use a pair of pliars to place the plugs where I wanted them around the outside of the glowing embers. I gave them a little twist as I set them in the ash to help work it up inside.

It was a quiet cool night with no wind, so I decided to leave them in the hot embers over night as the fire slowly extinguished itself. The open air should help the plugs cool through their exposed tips.

As a precaution, I soaked a wide circular area surrounding the coals to reduce the probability of the wind stirring up the ash and igniting the surrounding grass, and then I retired for the night.

The next day I returned to inspect the plugs, bringing a small wire brush and magnetic retriever with me. At first I didn't see anything in the ash because they had all apparently fallen over and been covered up. Everything was cool now, so I fished around with the magnet for one of the plugs along the edge where I thought I had placed it the night before. I wasn't expecting anything at this point after discovering that they had all fallen over and been buried.

I figured all the porcelain insulators were probably shot. And, even if the plug was clean, what good was it with a damaged insulator? Pulling one loose from the retriever that I just fished out, I began to brush away the clinging white residue, seeing immediately that part of the porcelain near the identifying number of the plug had a severe discoloration. However, it didn't appear to be cracked on a close inspection. A few more brush strokes around the electrode and the tip was visible. It looked like a spark plug that had just come out of a new package, I was pleasantly surprised!

Insulator discoloration

Insulator discoloration

I continued retrieving and cleaning more plugs, observing that many of the lower insulators were now near white in appearance while others had a light brown tinge. Now, I was excited! These plugs had been covered with hard combustion deposits and some had been severely oil soaked as well. To fully appreciate the magnitude of what the hot ash had accomplished, it must be emphasized that I had over two dozen badly fouled plugs in that bucket, including a set that was so badly fouled that the engine I removed them from barely ran. But now, the worst of the plugs looked as if they had received only light to moderate use, in an engine in good condition. I finished cleaning the last of the plugs and began checking the spark gaps. Everything looked O.K.

I continued checking until I had identified eight likely candidates to reinstall. All, the electrodes looked fine but would any of the plugs work? I found myself wishing that I had something more than just appearance, to judge how effective the cleaning was before deciding to remove the last good set of antique plugs I owned and chance these.

Ideally, the method would be something measurable and repeatable, and of course, reliable. But right now, I had nothing else but the motor to test them in, so I installed the cleaned plugs and started the engine. To my amazement, the engine ran as well as the set of plugs I had just removed with less than a hundred miles of service. Now, I was curious about the other plugs. Would they work,too?

Rejuvenated Spark Plug

Rejuvenated Spark Plug

I began to replace one or two at a time, leaving the rest in place, to see if I could detect any missing. A few had failed to clean well from their appearance, still showing some darkish grey deposits, and began to miss fire shortly after the engine started. Several other plugs with discolored porcelain failed to fire at all, although the electrodes were visibly clean. These plugs had been damaged by excessive temperature and had to be discarded. The plugs with lighter to no discolorations on the porcelain and clean electrodes tended to fire O.K.

I decided to start driving the car with only my thermally reconditioned plugs, saving my remaining good set for an unexpected emergency. As the miles accumulated, I saw no difference in service life or performance compared to new plugs. The reconditioned plugs began to fail at about the same mileage as before (less than 200) because of new deposits and oil. I was anxious to treat these plugs a second time to see if my results were more than a fluke.

After a second treatment in hot ash from a smaller fire, the plug insulators didn't come out quite as white as before but I didn't have any glazed or discolored porcelain, either. Again, the plugs with the best appearances worked as well as they did after the first treatment. But ther were a few exceptions. O.K., maybe I had a method here but I was still puzzled over why some of the treated plugs with like insulator appearance didn't work while others did.

I was able to recondition most of the plugs in subsequent thermal treating operations anyway, occasionally discarding a few plugs here and there with insulator discoloration that had failed to fire. Approximately, 5 percent of the plugs that I treated were lost early on because of too high a temperature but 95% had been rejuvenated, helping the car retain its originality.

I reduced the available heat from my make shift furnace so it wasn't getting as hot as in the earlier tests and this tended to produce more plugs that were harder to judge by appearance. Some of the ones that looked as good as ones in the first treatment would miss fire right away and had to be removed. I simply replaced them out of a pile of good plugs that I had previously tested in the engine and recycled the offenders for additional thermal cleaning.

My heat treating operation was time consuming (and I was running out of branches!)but it did allow me to continue using my two sets of original spark plugs, alternating one set while I thermally treated the other. I continued heat treating the plugs over the next several years with pretty good success. By now, I was back to cleaning everything at the same time, still using visual inspection as a guide for reuse. I refined the fire building technique somewhat and wasn't using nearly as much wood or time as in the earlier trials.

I had finally gotten around to having my engine rebuilt and the oil fouling was gone. I thought this would be the end of my need for the furnace. However, there remained some occasional deposit buildup from a worn out set of metering rod jets in the carbuerator that I hadn't been able to replace, so there was just enough light carbon deposits to cause the plug to misfire. But, now I didn't need to treat the plugs nearly as often since the buildup happened more slowly. Amazingly, I was still using the same set of spark plugs from four years earlier due to the benefit of the thermal treatment. It dawned on me that these new combustion deposits should contain enough carbon that resistance might be a good way to determine whether or not the plug was serviceable.

Internal resistance has long been the way to suppress RF spikes produced during ignition that interferes with the radio and other accessories. Hot rodders use the manufacturted resistance to maximize performance by balancing the internal resistance of each plug for maximum power transfer. But, resistance inside the insulator cavity allows high voltage to leak off through an alternate path to ground, causing the plug to misfire. So it's logical that this resistance might be a good indication of how clean the plugs were getting after thermal treatment.

To determine the external resistance of a plug, measure between the bottom center electrode and ground, not from the top of the plug. Measurement from the top to ground, displays the plugs internal resistance designed for radio shielding, and will falsely indicate a dirty plug. Any ohm meter with at least 30,000 ohms per volt is satisfactory.

I began by marking the plugs as either good or questionable based on actual engine operation. Then, using an ohm meter, I randomly selected plugs and checked the DC resistance. I have acquired an additional set of NOS plugs which I never use, keeping them for historical, emergency, and as a resistance reference. A new plug shows infinite resistance between ground and the lower electrode as expected. Plugs that were fouled showed low resistance readings that were highly repeatable. Generally, the more severe the fouling, the lower the resistance because there were more paths to ground. The dirty lugs always indicated a low resistance, 10 megaohms or less while plugs which had not been treated sufficiently in the furnace measured 1 megaohm or less. Serviceable, but not 100% clean plugs, show less than new but considerably more resistance than fouled ones. More on this later.

Portable furnace.

Portable furnace.

I use an old 20 inch diameter portable barbeque pit with a cover to function as a furnace. It should have a minimum depth of five inches to hold the ash bed. A good depth of ash provides the necessary heat retention for extended operation. It takes about three to five hours to build the ash layers and you won't need several cords of wood! I like to use plain old pine because it burns fast to generate ash. It can be any sort of scrap wood for the fireplace. The finer the particles of the ash layer, the more effective the cleaning is on accumulated deposits.

Wood has a flame temperature between 900F and 2500F, so some preparation in fire construction is needed to stay on the low side. You want a combustion temperature between 1000F and 1550F, hot enough to completely burn off the oil or carbon material without damaging the porcelain insulators. This is attainable using a majority of soft wood which burns and extinguishes relatively fast.

Add just enough hard wood so the ash accumulates more slowly and takes longer to cool. (I've had to do several heat treatments on the same plugs using soft pine alone because the ash burns out very quickly.) A little experimentation will produce the right wood mix to get the proper temperature. Its easy for the average hobbyist to do with some practice. A severely fouled plug does not have to be restored to the same pristine 'out of box' condition to be reusable.

It takes about eight hours in a furnace to satisfactorily clean spark plugs so I set things up to burn overnight. (Don't use the family pit for the furnace, buy another one, wives don't appreciate automotive cleaning operations on equipment used to prepare food. You might also get some unpleasant looks from guests who will not fully appreciate the economy of doing both!) Don't use charcoal to develop the ash; its combustion produces much hotter flame temperatures than wood and quickly exceeds the maximum rated 1560F thermal rating for porcelain insulators.

Once the fire is going good, I start piling the wood on and begin building the ash layers. Lightly stoke the ash between layers to break up the larger lumps as you add more wood, careful to avoid extingishing the fire when stoking. Keep adding wood until the ash level is about an inch or two high, then add a little hard wood (oak, ash, etc).

Keep building and stoking the ash until you have a three to four inch thick bed of fine hot particles letting it burn down to where you see only a soft red glow just under the surface. Now stir to get the hot particles near dust consistancy. Continue adding small amounts of wood so the fire doesn't die. The finer the particles the better the penetration into the plug recesses to burn out deposits. Keep the hot ash concentrated near the center of the pit allowing a small area along the walls to cool.

Place your hand near the glowing particles in the center (but not in them). The temperature is about right when you can't keep your fingers at spark plug length for more than a few seconds without feeling real discomfort. Don't stoke or stir the ash from this point on and don't add any more wood. For those who might have high temperature measurement capability, such as a flame pyrometer, you want the temperature around 1500F for the most efficient cleaning.

When you think the ash consistancy is about right, put on a heat insulating glove and grab a fouled plug using a pair of tongs. You might want to begin by treating plugs you planned on throwing out until you get the hang of building the proper fire. Now, gently twist the spark plug down into the edge of the hot ash to where the leading edge of the porcelain is just about to be buried but pointing away from the center of the heat, see Figure 4. The cooler outer edge gives some additional protection against porcelain damage in case the plug topples over later in the process. The twisting helps work the fine hot ash particles up into the nooks and crannies of the insulator cavity. Once the proper temperature and ash consistency is attained, the carbon burns like a cigarette with the ash traveling upwards towards the taper.

Spark plugs in the furnace.

Spark plugs in the furnace.

If you feel comfortable enough, place the rest of your dirty plugs in the hot ash moving around the center leaving enough clearance for each one so that if a plug should topple it doesn't knock over the others. Now, cover the pit with the lid making sure the top vent is open so the escaping hot gases will draw combustion air from underneath. This keeps the radiant heat trapped under the cover allowing the furnace to cool slowly. I locate the furnace in a secure place where wind or curious animals can't topple it and start a fire, while I'm in bed, and then retire for the evening. A word of caution, don't use your garage or another confined air space occupied by people or animals as the secure location. These furnaces produce a lot of carbon monoxide, a deadly gas that is best left outside. Don't worry about the environment, carbon monoxide oxidizes in free air to carbon dioxide used by the plant world.

If you got the furnace conditions right, you'll notice a marked difference in the appearance of the plug after the heat treatment. The porcelain insulator will be light grey or brown to a near white condition, depending on how well the temperature held during the process. Temperatures near the maximum allowable will return the electrode insulators to a near pristine white condition. Higher temperatures will cause glazing on the porcelain closest to the immersion point. Lower temperatures produce a mixture of medium to dark discolorations on the insulator and are an indication that the proper thermal cleaning conditions were not met.

Checking the cleaned spark plugs

Checking the cleaned spark plugs

Now, place an ohm meter on the plug between gap ground and the lower electrode to measure the resistance as shown.
Improperly cleaned plugs will display low external resistance and indicate that ash consistancy was too coarse or the furnace cooled too quickly. If the plug has been throughly cleaned, the resistance reading will be the same as for a new plug (open circuit) but this rarely happens in a single treatment of a severely fouled plug. If you get readings below 10 megaohms for a 10000 volt, secondary ignition system, the plug will have to be treated again. This value is about the transition point for low voltage ignition systems, defining the probability that a given plug will spark succesfully. I have gotten good results on re-installation of every plug that measured above 10 megaohms. Systems that have higher secondary voltages will have correspondingly higher transitional resistance readings that define acceptable service and must be determined experimentally for your vehicle.


I have one set of AC-48 and another of AC-47 spark plugs that I have used alternately over the last eight years, cleaning whichever one is in use about once a year. I have seen no deterioration in the spark gaps or mechanical damage to the insulators after the annual furnace cleanings. I continue to run on the same set of spark plugs until I detect a degrading of performance and then I swap sets until I have time to heat treat the dirty plugs.

Sometimes, one or more of the treated plugs fails to meet resistance criteria and I simply return them for another treatment later. There is no limit to how often the plugs can be heat treated as long as the furnace temperature remains below 1560F and you have the time and patience.

Now, you don't have to throw those rare plugs away, you can bring them back to life with this simple heat treating method right in your own back yard!

data-matched-content-ui-type="image_card_stacked" data-matched-content-rows-num="3" data-matched-content-columns-num="1" data-ad-format="autorelaxed">

Terms of Use | Who We Are | Contact Us | Sitemap | Privacy Policy

© 2018 Second Chance Garage, LLC. All Rights Reserved. Reproducing any material on this website without permission is prohibited.