Spark plugs last a long time but they don’t last forever. Most spark plugs have a factory service interval of 100,000 miles, though some may be as much as 120,000 miles. Long-life platinum and iridium spark plugs will typically last up to 100,000 miles or longer provided the engine isn’t using oil or doesn’t spend a lot of time idling.
Replacing spark plugs at 100,000 miles can be troublesome on some applications. When a spark plug has been in the cylinder head for that many miles, carbon and corrosion can make it difficult to remove. There is a risk of damaging the threads in aluminum cylinder heads. Many spark plugs have nickel-plated threads on the plug shell to prevent corrosion between the steel threads on the plug and the aluminum threads in the cylinder head. Some spark plugs have a black oxide coating on the shell to inhibit corrosion, but it’s not as effective as a nickel coating. Plain uncoated spark plugs offer no corrosion resistance whatsoever when installed in aluminum heads. Dissimilar metals can react over time causing the spark plug to stick in the head.
The use of anti-seize compound on spark plug threads to make future replacement easier is not recommended for two reasons: anti-seize acts like a lubricant, which increases the risk of over-tightening the spark plug and damaging the threads in the cylinder head, and it can contaminate the electrode causing the plug to foul and misfire. Spark plugs should be installed dry and tightened to specifications.
On some applications, like 2004 to 2008 Ford trucks with 5.4L V8 and 6.8L V10 engines and 2005 to 2007 Ford Mustangs with 4.6L engines, the original equipment spark plugs are a two-piece welded design with a very long shroud around the electrode. The plugs tend to stick in the heads, and often break off leaving the tip of the electrode stuck in the head. If the broken tip can’t be extracted, expensive head repairs are required. Some aftermarket replacement spark plugs for these applications are a one-piece design to resist breakage. Replacing the OE plugs before they reach 40,000 miles can also reduce the risk of breakage.
Replacement spark plugs do not have to be the same brand as the original equipment spark plugs. Any brand of spark plug will work in any engine as long as the spark plug fits correctly and has the right heat range for the application.
Heat range refers to the operating temperature range of the spark plug. A spark plug must run hot enough to burn off fouling deposits but not so hot that it causes preignition or detonation. Most spark plugs use a copper core and electrode design that gives them a broad heat range and makes them suitable for a broad range of engine applications.
Replacement spark plugs should usually be the same type (platinum or iridium) as the original if they are expected to last 100,000 miles. Standard spark plugs are only good for about 40,000 miles.
On some applications, a double-platinum or double-iridium spark plug may be recommended. This type of spark plug uses platinum or iridium on the center and ground electrodes, not just the center electrode to improve wear resistance and extend the service life of the spark plug. Such plugs are often specified for engines that have a “waste spark” ignition system that fires twice the rate of a standard DIS or COP ignition system, or boosted engines that require a higher firing voltage.