Spark Plugs

For a gasoline powered internal combustion engine to run it must have the proper mixture of air and fuel, compression, and a well-timed spark. Fuel mixes with air in a carburetor, or in the intake manifold or port after the fuel injector on electronic fuel injected engines, and is then drawn into the cylinder through the intake valve when the piston descends on the intake stroke. The piston compresses the air/fuel mixture into a small, dense volume when it rises in the cylinder on the compression stroke. High voltage produced by the ignition coil is sent to the spark plug, which creates a spark igniting the air/fuel mixture, generating combustion pressure that forces the piston down on the power stroke. On a 4-cycle engine the spark plugs fire 300-400 times per minute at idle and 1000-1200 times per minute at cruising speed, and they do this for many thousands of miles. With that kind of firing frequency, it’s no surprise that even the best plugs wear out eventually, and when they do you can count on our replacement spark plugs to restore performance.

A typical spark plug consists of a copper core center electrode and metal terminal inside a ceramic insulator that is encased in a steel shell. The terminal connects the spark plug to the ignition system. The center electrode has a nickel alloy, platinum, iridium or other precious metal firing tip. Resistor spark plugs include a resistor with the center electrode to prevent electrical interference that can adversely affect radio reception and vehicle computer controls. The steel shell is threaded for secure fitment in the cylinder head and to provide a good ground and effective heat transfer from the plug to the cylinder head. The side electrode, or ground strap electrode, is made of high nickel steel that’s welded to the steel shell. Double platinum spark plugs have a platinum disc welded to the side electrode.

The ignition coil turns the 12 volts from the battery into 15,000-40,000 volts. The coil is directly connected to the spark plug on COP (Coil On Plug) ignition systems, connected via spark plug wire on DIS (Distributorless Ignition Systems), and via spark plug wire and distributor on breaker point and distributor-type electronic ignition systems. When the breaker points, ignition module, or ECU (Engine Control Unit) triggers the coil, the high voltage transmitted through the spark plug arcs across the gap from the center electrode to the side electrode and ignites the air/fuel mixture. The pressure created by combustion of the air/fuel mixture pushes the piston downward, providing the force that turns the crankshaft. While all spark plugs function the same way and have similar construction, they are tailored to their applications and ignition systems with different electrodes, thread sizes, seat type, heat ranges, and reach.

There are four common spark plug thread diameter x thread pitch combinations: M10 x 1.0, M12 x 1.25, M14 x 1.25, and M18 x 1.5. Spark plugs can have a flat seat that uses a gasket to seal the plug to the combustion chamber or a tapered seat where the spark plug shell forms the seal. A spark plug’s heat range is its ability to transfer heat from the combustion chamber to the cylinder head. On a “hot” spark plug, much of the ceramic insulator will be exposed to the combustion chamber, so it retains heat and burns off deposits. In contrast, very little of the ceramic insulator is exposed on a “cold” plug, enabling it to transfer heat better and run cooler. A plug that runs too hot for the application can cause pre-ignition and knock, and one that runs too cold can result in carbon buildup and misfire. Cold plugs are generally used on modified high-performance and racing engines. Spark plug reach is the distance the plug projects into the combustion chamber. The plug must project far enough for good combustion without getting too hot or hitting the piston and should not be so shallow that it’s shrouded, resulting in poor combustion.

Copper core spark plugs with nickel alloy firing tips, commonly called copper/nickel plugs, are usually specified for breaker point and some electronic distributor ignition systems because the softer nickel alloy erodes faster and won’t last as long as platinum or iridium when used on modern DIS and COP ignition systems. However, because they run cooler, copper/nickel plugs are used on some late-model performance engines. The firing tips on platinum plugs are much harder than copper/nickel plugs; they will retain their sharp edges and gap dimensions much longer than copper/nickel plugs, so you can count on many more miles of good performance. Platinum plugs also operate at a higher temperature, which resists carbon buildup. Some platinum plugs have fine-wire tips, which need less voltage to fire and increase firing efficiency.

Double platinum plugs were designed for DIS ignition systems. In a DIS system each coil supplies voltage to the spark plugs in two cylinders on every revolution of the crankshaft, with one spark plug firing in the cylinder on the compression stroke and the other in the opposite cylinder in the firing order that is on the exhaust stroke. Because the spark in the cylinder on the exhaust stroke has no effect on engine operation, this is commonly called a “waste spark” ignition system. A double platinum plug, with a platinum disc welded to the side electrode, is needed in this system because, while the spark plug in the cylinder on the compression stroke fires in the conventional manner, from the center to the side electrode, the spark in the cylinder on the exhaust stroke jumps from the side electrode to the center electrode. If spark plugs without platinum side electrodes are used in a DIS, the side electrode will wear prematurely.

Iridium is even harder than platinum, and iridium plugs last even longer than platinum, and now there are ruthenium-tipped plugs that are harder still. The hardness of precious metal tipped plugs is emphasized by vehicle makers and spark plug manufacturers because hardness provides durability. Harder electrodes will maintain their sharp edges and the plug gap longer, for good spark and consistent performance over many thousands of miles and extended change intervals. Along with precious metal and fine wire firing tips, some spark plug manufacturers form U-shaped or V-shaped channels in the side electrode or shorten or alter the shape of the side electrode to reduce the quenching effect that can be caused by conventional electrodes and cause incomplete combustion.

Every time a spark plug fires, a small amount of electrode material is lost, eventually resulting in rounded electrodes and wider gaps, which require more voltage to fire, and could cause misfire. This occurs on all spark plugs regardless of the type of electrode, it just takes materials like platinum and iridium longer to wear. In addition to normal wear, spark plugs are also subject to engine conditions like a rich air/fuel mixture, worn piston rings or valve seals that can cause fuel or oil fouling or excessive deposits, and detonation or pre-ignition damage caused by incorrect ignition timing, use of fuel with too low an octane, lean air/fuel mixture, defective EGR (Exhaust Gas Recirculation) system, or excessive carbon deposits in the combustion chamber. Symptoms of worn, fouled, or damaged plugs include misfire, rough idle, hard starting, poor acceleration and reduced fuel mileage. Spark plugs should be replaced when recommended by the vehicle manufacturer or when there are symptoms of poor performance. Upon removal, plugs should be examined for evidence of the engine conditions described above.

The engine design dictates spark plug features like diameter, thread pitch and seat, but other features like heat range can be changed. However, unless the engine has been significantly altered and you are an experienced tuner, when replacing spark plugs you should stay with the type of spark plug recommended by the vehicle manufacturer. We make it easy for you to select the plugs that meet or exceed your engine’s requirements. Just plug in your year, make, model, and engine size in the fields at the top of this page and you’ll instantly be shown a selection of spark plugs from the top brands, and each one will deliver the performance, fuel economy, and durability you’re looking for at an affordable price. Besides spark plugs, we offer spark plug tubes and seals for engines that require them, as well as tune-up kits for select vehicles that include plugs, distributor cap and rotor, plug wires, filters, and more.