Spark Plug Cover

What Is a Spark Plug Cover?

A spark plug cover — also called a spark plug cap or resistor cover — is the protective terminal connector that fits over the top of a spark plug, linking the high-voltage ignition wire or coil to the plug itself. It is a small but critical part of the secondary ignition system. Without a properly seated, undamaged cover, the high-voltage signal traveling from the ignition coil can leak, arc, or fail to reach the electrode with enough energy to reliably ignite the air-fuel mixture inside the combustion chamber.

Beyond simply making an electrical connection, spark plug covers serve several important functions. Many incorporate a built-in resistor that suppresses radio frequency interference (RFI) — the electrical noise generated during each spark event that can disrupt onboard electronics, sensors, and even radio reception. The cover's rubber or plastic boot also seals out moisture, oil, and road debris, protecting the connection from corrosion and short circuits that can cause misfires, rough running, or a no-start condition.

Where Are Spark Plug Covers Used?

While spark plug covers are found on virtually any gasoline-powered engine that uses spark plug wires, they are especially common in the powersports world — motorcycles, ATVs, UTVs, and snowmobiles. These vehicles are frequently exposed to wet conditions, vibration, and tight engine bays where the cover's angle (straight, 90°, or 120° elbow) makes a meaningful difference in fit and wire routing. Automotive applications such as passenger cars and trucks also use them, particularly on older distributor-based systems and on engines where individual plug wires are still routed to each cylinder.

What to Look for When Shopping Spark Plug Covers

• Thread size compatibility: Common sizes are 10mm, 12mm, and 14mm. Verify your spark plug's terminal thread size before purchasing.
• Angle/type: Straight, 90-degree, and 120-degree elbow configurations are available to accommodate different engine layouts and wire routing needs.
• Resistor value: Most covers designed to suppress RFI use a 5kΩ resistor. Check your vehicle's service manual if a specific resistance value is required.
• Material: Rubber covers offer more flexibility and a tighter seal in off-road or wet environments; hard plastic covers are common for standard street and automotive use.
• Wire diameter: Confirm that the cover's allowable wire diameter matches your ignition wire gauge.

Top Brands in Spark Plug Covers

K&S Technologies is one of the most recognized names in powersports electrical components and carries the largest selection of spark plug covers in this category. Their lineup spans straight-type, 90-degree, and 120-degree elbow configurations across 10mm, 12mm, and 14mm thread sizes, in both rubber and plastic, making them a go-to source for universal-fit replacement caps for motorcycles, ATVs, and more.

Parts Unlimited is well known as a leading distributor of powersports aftermarket accessories, and their spark plug caps are a staple in the off-road and street motorcycle market. Their neoprene spark plug caps with spring connectors offer a reliable, cost-effective option for riders who need a quick, quality replacement.

Subaru offers OEM-specification spark plug covers designed for precise fitment on their horizontally-opposed (boxer) engines. For Subaru owners seeking a factory-correct replacement that meets original engineering tolerances, going with a genuine Subaru part ensures the right seal, fit, and electrical integrity for their specific engine.