Spark Plug Buying Guide
Last updated: February 2026 · 6 min read
Spark plugs are cheap, easy to replace on most cars, and make a noticeable difference to how your engine runs. Despite this, they're one of the most overlooked service items. This guide explains the different types, when to change them, and why spending a few extra pounds on better plugs pays for itself.
How Spark Plugs Work
A spark plug creates the electrical spark that ignites the air-fuel mixture in each cylinder of a petrol engine. It does this thousands of times per minute, in conditions of extreme heat (up to 2,500°C at the electrode tip) and pressure. Over time, the electrode erodes and the gap between the electrodes widens, making the spark weaker and less consistent. This gradually reduces engine efficiency, power, and fuel economy.
The Three Types of Spark Plug
Copper / Nickel (Standard)
The traditional type, with a nickel alloy electrode and copper core. These are the cheapest but wear the fastest because nickel is a relatively soft metal. They actually provide excellent conductivity when new, which is why some high-performance and older engines specify copper plugs. But for everyday cars, they need replacing more frequently — every 20,000 to 30,000 miles.
Typical UK price: £2–£5 each
Platinum
A platinum disc welded to the centre electrode resists erosion much better than nickel, roughly doubling the lifespan to 40,000–60,000 miles. Single-platinum plugs have platinum on the centre electrode only, while double-platinum plugs have it on both the centre and ground electrode (required for waste-spark ignition systems used by some manufacturers). Most modern cars from around 2000 onwards specify platinum or better.
Typical UK price: £5–£10 each
Iridium
Iridium is harder and more durable than platinum, allowing a finer centre electrode (typically 0.4–0.6mm versus 0.8mm for platinum). The finer electrode requires less voltage to fire and produces a more focused spark, improving combustion efficiency. Iridium plugs last 60,000–100,000 miles and maintain more consistent performance throughout their life. Most new cars now come with iridium plugs as standard.
Typical UK price: £8–£15 each
Getting the Right Plug for Your Engine
Spark plugs are not interchangeable between engines. The key specifications are thread diameter (usually 12mm or 14mm), thread length (reach), hex size (for the socket), heat range, and electrode gap. The easiest way to get the right plug is to look up your car's registration or use the manufacturer's part number cross-reference. NGK and Denso both have online lookup tools on their websites.
The heat range is particularly important. It determines how quickly the plug dissipates heat from the combustion chamber. A plug that's too cold will foul up with carbon deposits; one that's too hot can cause pre-ignition and engine damage. Always match the heat range specified by the engine manufacturer unless you've made significant modifications.
Brand Guide
NGK is the world's largest spark plug manufacturer and the OEM supplier to the majority of car makers. Their standard BKR/BPR range covers copper plugs, the G-Power range is platinum, and the Iridium IX and Laser Iridium ranges are their premium offerings. If in doubt, NGK is always a safe choice — they almost certainly made the plugs your car came with.
Denso is the OEM supplier for Toyota, Honda, and most other Japanese manufacturers, and also supplies to several European makers. Their Iridium TT plugs feature a patented twin-tip design that's claimed to improve ignitability. Particularly good for Japanese cars but covers most makes.
Bosch is the OEM supplier for many German manufacturers including VW, BMW, and Mercedes. Their standard range is solid, and the Double Iridium plugs are their premium offering. Bosch plugs tend to be slightly more expensive than NGK/Denso equivalents but are excellent for German cars.
Champion is a well-known brand with a long history, now owned by Federal-Mogul. Their standard range is adequate and affordable, but for premium applications NGK, Denso, or Bosch are generally preferred by mechanics.
DIY Replacement Tips
Spark plugs are one of the most accessible DIY jobs. On most four-cylinder engines, you just need a spark plug socket (16mm or 21mm depending on the plug), a ratchet with an extension bar, and a torque wrench. Remove plugs when the engine is cold, check the gap matches the specification (most modern plugs come pre-gapped), and tighten to the manufacturer's torque setting — over-tightening can damage the cylinder head threads. Apply a thin coating of anti-seize compound to the threads to prevent them seizing in the head over time. On some modern engines with coil-on-plug ignition, you'll also need to unclip the ignition coils first.
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