Where is the spark plug located?
In a typical petrol engine, spark plugs sit in the cylinder head, one per cylinder. They thread into the combustion chamber and are usually accessed from the top of the engine, though exact location varies by layout.
How spark plugs are placed by engine layout
Engine designs arrange spark plugs differently to fit the cylinder head and ignition system. The following outlines common configurations and where you’ll typically find a plug on each.
- Inline engines (four-, six-, or eight-cylinder): Spark plugs line up along the top of the cylinder head in a row, often beneath a removable valve cover or under a plastic engine cover. Access is usually from the top of the engine bay.
- V engines (V6, V8, etc.): Each cylinder head contains its own set of plugs. They’re typically located on the side or top of the heads and may require removing components to reach on some designs; coil-on-plug units are common.
- Boxer (horizontally opposed) engines: Plugs sit on the sides of the cylinder banks, accessible from the sides of the engine bay or under valve covers, depending on layout.
- Small engines (lawn mowers, generators): One spark plug per cylinder in the cylinder head, usually accessible from the top behind or under a cover.
In all cases, the spark plug threads into the combustion chamber through the cylinder head, and the ignition coil or distributor connects to the spark plug via a boot or coil pack. Note that diesel engines do not use spark plugs; they rely on glow plugs or compression ignition instead.
How to locate the spark plugs on your vehicle
To find the exact location on your car, you’ll need to look at the ignition arrangement and consult the owner's manual or service manual. The following steps help you identify the plugs on most vehicles.
- Check for ignition coils or coil packs: If your car uses coil-on-plug ignition, each plug will be topped by a coil or coil-on-plug unit sitting directly above or beside it. If you see a single coil per bank, you likely have a coil pack with multiple plugs inside.
- Follow the spark plug wires (if present): On older designs with a distributor or plug wires, the thick rubber leads run from the coil/distributor to the plug boots on the cylinder head.
- Inspect the valve covers: Spark plugs are threaded into the cylinder head beneath the valve cover; you may need to remove the cover or other components to access the plug wells.
- Consult the manual or a diagram: The exact location can vary widely by model and engine code; a diagram will show plug positions relative to the timing belt/chain and intake/exhaust manifolds.
If you’re unsure or the engine is difficult to access, seek professional help or reference your vehicle’s manual to avoid damage.
Summary
Spark plugs are located in the cylinder head of an internal combustion engine, with one plug per cylinder. The exact position depends on engine layout—inline, V, or boxer—and modern cars often use coil-on-plug arrangements that place the coil directly over each plug. Always refer to your vehicle’s manual for the precise location and access method.
