Swapping out your spark plugs
DIYers must consider the type, thread length and heat range
If you ever want to start a heated group discussion in a group of auto gearhead DIYers, simply ask, “What’s the best spark plug to use?” You’ll get as many opinions and reasons as if you asked some locals in a Scottish pub, “What’s the best single-malt scotch?”
While swapping out spark plugs on most V6 engines in a front-drive vehicle (or some V8s in a rear driver) can be a challenge for even a seasoned wrench-veteran, it’s a relatively easy task on most four-cylinder engines and involves few special tools.
Spark plug types are pretty much divided into three main groups: copper-core, iridium and platinum. They all perform the same function: deliver a clean, clear, and bright electrical arc to ignite the air-fuel mix in the engine’s cylinders. So why bother upgrading?
The problem with plain old copper-core plugs is that the electrode tip wears over time. When this happens, the all-important gap between the tip of the plug and the electrode increases to the point where the spark weakens and/or becomes unreliable. This is why these types of plugs are usually recommended for replacement by most carmakers between 30,000 and 50,000 km. Leave them in any longer and you risk, among other things, a no-start condition on a cold, wet winter morning.
The benefits of iridium or platinum spark plugs over copper-core is that the electrode tips are coated with these stronger metals, giving them a much longer lifespan. Most auto manufacturers that install platinum plugs at the factory will recommend their replacement at 100,000 km or more.
One thing to note here is that with all the improvement in plug technology they still use steel threads, which can cause a world of headaches when screwing them into aluminum cylinder heads. Leave this type of setup together too long and you can easily run into seized and/ or broken plugs when trying to do a tune-up.
This is why some carmakers recommend inspecting spark plugs at 50,000 km intervals, even though they’re platinum types that don’t require replacement until 100,000 km. The inspection process will ensure that the threads don’t become permanently seized.
Within the main grouping of spark plugs there’s another category of heat range. Some plugs will fire with a hotter spark than others and many DIYers believe hotter is better. Most drivers won’t see any improvement in performance or fuel mileage by running a hotter plug. The same can be said for certain specialized plugs, such as v-grooves.
If you’re swapping out one spark plug manufacturer for another, make sure of your cross reference. Every parts store will have a sparkplug book that lists all the acceptable choices. Even with this assurance, always visually compare the new plug to the one that’s coming out. Make certain the plugs have the exact same thread length because with many engines there’s no room to extend a longer plug into the combustion chamber. Put too long a plug in and you risk damaging the piston as well as the new plug.
Most spark plugs come pregapped, meaning the gap between the two electrodes is correct — but never trust it. A spark plug gapping tool is one of the cheapest things you can buy for your tool box and they are very easy to use.
Also check the small threaded adapter cap on the end of the plug opposite the electrode. Most vehicles require it and a quick twist with a set of pliers is all it takes to make sure it’s tight.
If you’re going to step up to platinum plugs, make sure to get double-platinum tips. This means there’s platinum on both the plug’s tip and its side electrode to ensure long life. Single-tip platinum plugs are slightly cheaper but will wear out faster.