Spark Plugs
The spark is the heart of every petrol engine. Without perfectly functioning spark plugs, nothing works. In this guide, you will learn everything you need to know about these small but vital components: from the different materials and technical specifications to change intervals and the right selection.
What are spark plugs and how do they work?
Spark plugs are electrical components that reliably ignite the petrol-air mixture in the combustion chamber of a petrol engine. They consist of an insulator base, a centre electrode and a ground electrode. High voltage of up to 30,000 volts jumps across the electrode gap – typically between 0.7 and 1.3 mm – and creates a spark that ignites the mixture. The combustion drives the piston and provides propulsion.
Over time, the electrodes wear down, the gap widens and the spark quality decreases. Therefore, regular replacement is crucial for engine performance, smooth idling and low consumption.
Construction of a modern spark plug
A spark plug consists of several precisely coordinated components: the aluminium oxide insulator, which withstands high voltage up to 42 kV, the metallic housing with thread (common are M14, M12, M10), the centre electrode (often with a copper core for better heat dissipation) and the ground electrode. Depending on the design, precious metal tips made of platinum or iridium may be welded on to minimise burn-off and drastically increase service life.
Spark generation in a petrol engine
The ignition coil builds up the necessary high voltage, which is passed on to the spark plug via the distributor or directly (in modern engines with individual coils per cylinder – coil-on-plug). The spark jumps exactly when the piston is shortly before reaching top dead centre, triggered by the engine control unit. Deviations in the electrode gap or worn insulation lead to misfires, which cause the mixture to burn incompletely.
The most important spark plug types compared
Choosing the right spark plug material is a key purchasing decision. It influences not only the service life but also ignition reliability and behaviour at high temperatures. There are essentially three main groups: copper plugs, platinum plugs and iridium plugs. In the automotive sector, double platinum and laser iridium variants have also become established.
The following table summarises the key characteristics in a practical way:
| Material | Electrode condition | Service life (km) | Thermal conductivity |
|---|---|---|---|
| Copper / nickel | Nickel-alloy centre electrode with copper core; solid ground electrode | 20.000–30.000 | Very good (high heat dissipation) |
| Platinum | Fine platinum tip on centre electrode, often round platinum pads | 60.000–100.000 | High |
| Iridium | Laser-welded iridium tip (Ø 0.4–0.6 mm) with copper core, sometimes double iridium | 100.000–160.000 | Excellent |
The price range depends on the manufacturer (NGK, Bosch, Denso, Beru, Champion) and vehicle type. Double iridium plugs with two precious metal electrodes achieve over 180,000 km, but then cost 15–30 € each.
Copper spark plugs (standard)
Copper plugs are the classic. They have a centre electrode made of a nickel alloy with a solid copper core for optimal heat dissipation. The electrode gap is usually 1.0–1.3 mm. They burn evenly and are very inexpensive. However, their service life of 20,000–30,000 km is the shortest of all types, as the nickel electrode wears more quickly. For older vehicles without a high-performance ignition system or with short maintenance intervals, they are a solid choice.
Platinum spark plugs
Welded-on platinum tips – often only on the centre electrode – greatly slow down wear. Platinum has a high melting point (1,768 °C) and also resists aggressive combustion residues. The service life is 60,000 to 100,000 km. Typical electrode gaps are 0.8–1.1 mm. Platinum plugs are particularly suitable for modern engines with high compression and are often original equipment for extended maintenance intervals.
Iridium spark plugs
Iridium is harder than platinum and only melts at 2,446 °C. The tip can therefore be extremely thin (Ø 0.4–0.6 mm), which reduces the ignition voltage and ensures a powerful, stable spark – even at extreme combustion temperatures. The service life is 100,000 to 160,000 km. The typical electrode gap is only 0.7–0.8 mm. They are the first choice for turbocharged and direct-injection engines and provide consistent engine running even after many kilometres. As with modern TPMS sensors, material quality is the decisive factor for reliability here.
Other special variants
In addition to the basic types, there are special forms such as laser platinum with a reinforced platinum coating and double iridium plugs, which have iridium tips on both electrodes. These achieve mileages of over 180,000 km and are often used in high-performance engines or as a long-life upgrade. Surface discharge plugs or ruthenium plugs (e.g. NGK Ruthenium HX) are also available on the market for special requirements.
Heat range and electrode gap: the decisive technical parameters
In addition to the material, two technical parameters largely determine whether a spark plug works optimally in the engine: the heat range and the electrode gap.
The heat range describes the ability to dissipate heat from the insulator nose. A too hot plug (poor heat dissipation) can glow and cause uncontrolled pre-ignition. A too cold plug (excessive heat dissipation) cools too quickly, and soot and deposits form. Manufacturer scales differ: with NGK, a low value (e.g. 5) means a hot plug, a high value (e.g. 9) a cold one. Bosch, on the other hand, increases the numbers with increasing coldness, i.e. a Bosch 08 is colder than a 05. In case of doubt, the specification approved by the vehicle manufacturer must always be observed.
The electrode gap is usually set at the factory between 0.7 and 1.3 mm. Modern direct-injection and turbo engines require narrower gaps of around 0.7–0.8 mm to keep the ignition voltage low and ensure reliable ignition. Older naturally aspirated engines use gaps of 1.0–1.3 mm. Incorrectly set gaps or gaps that have become too large through wear lead to misfires and rough running.
Recognising the symptoms of faulty spark plugs
Faulty or heavily worn spark plugs make themselves known through a series of typical symptoms. Anyone who recognises these early can avoid consequential damage to the catalytic converter and lambda sensor.
- Poor engine starting: The engine cranks for longer and starts sluggishly, especially in cold weather.
- Rough engine running / juddering: The engine shakes at idle, and the vehicle judders at low engine speeds.
- Loss of power: Acceleration feels sluggish, and top speed is no longer reached.
- Increased fuel consumption: Incomplete combustion can push consumption up by as much as 15%.
- Misfires: Brief combustion misfires make themselves felt as strong vibrations or juddering under acceleration.
- Illumination of the engine management light: The control unit registers combustion misfires and stores a fault code in the OBD memory.
- Black or oily deposits on the plug: An indication of an overly rich mixture or excessive oil consumption through the piston rings.
A simple visual check of the old spark plugs often already provides precise indications of the engine's condition and the combustion mixture. A light brown discolouration is normal, whereas heavy sooting or carbon fouling is alarming.
When do spark plugs need changing? Change intervals for every type
Change intervals are not a fixed quantity; they depend on the spark plug type, the vehicle model and the driving profile. Many manufacturers specify rigid inspection intervals, but these can often be exceeded by choosing a longer-lasting technology.
At a glance:
- Copper/nickel plugs: every 20,000–30,000 km or 2 years
- Platinum plugs: every 60,000–90,000 km, some manufacturers approve up to 100,000 km
- Iridium plugs: every 100,000–160,000 km, and up to 180,000 km for long-life variants
Modern vehicles with turbochargers and direct injection are often fitted with iridium or double platinum plugs at the factory and thus achieve service intervals of frequently 90,000 to 120,000 km. BMW, Mercedes-Benz or Audi, for example, specify a spark plug change every 90,000–120,000 km for many models – easily achievable with iridium technology. Under high engine load, short-distance driving or with tuned engines, the intervals should be set more conservatively, for example 60,000 km for platinum plugs.
Changing spark plugs yourself – step-by-step instructions
With the right tools and a little manual skill, changing the spark plugs is easily done yourself on most inline and V engines. However, always observe the specifications in your vehicle's workshop manual.
- Allow the engine to cool: Never work on a hot cylinder head – risk of burns and of damaging the threads.
- Remove the spark plug connectors or ignition coils: On coil-on-plug systems, carefully unlock the connectors and pull them off.
- Clean the area around the spark plug: Blow dirt and dust out of the plug recess with compressed air so that nothing falls into the combustion chamber.
- Loosen the old spark plugs: Carefully unscrew them with a suitable spark plug socket (often 16 mm or 14 mm) and an extension.
- Check the new plugs: Check the electrode gap (especially with copper plugs). Modern precious metal plugs are pre-set and should not be bent.
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Screw in the new plugs by hand: Fit them without cross-threading and first screw them in by hand. Then tighten them to the specified tightening torque – depending on the thread:
M14 thread: 25–30 Nm
M12 thread: 15–20 Nm
M10 thread: 10–15 Nm
A torque wrench prevents over- or under-tightening and protects the aluminium thread in the cylinder head from damage. - Refit the ignition coils/connectors: A firm fit and the correct order are essential.
If you are unsure, leave the change to a specialist workshop.
FAQ – Frequently asked questions about spark plugs
Which spark plugs are the best?
That depends on the engine. For modern direct-injection and turbo engines, we recommend iridium spark plugs, as they offer the highest melting point and the most stable ignition performance. Platinum plugs are the economical middle ground, while copper plugs are sufficient for older vehicles. What matters is always adhering exactly to the heat rating and electrode gap approved by the vehicle manufacturer.
How do I recognise worn spark plugs?
Typical signs are a worn electrode gap (>1.3 mm for copper), heavy burn marks, cracks in the insulator or discolouration. Even a simple removal shows the condition – a light brown colour is normal, black soot deposits or oily residues indicate problems. The engine runs unevenly and is difficult to start.
Can I change spark plugs myself?
Yes, this is possible on most vehicles. You need a suitable spark plug socket, an extension, a torque wrench and a little sensitivity. Observe the exact tightening torques and let the engine cool down completely beforehand. On engines that are difficult to access (e.g. transversely mounted V6), a workshop is often the better choice.
How much does a spark plug change cost at a workshop?
The costs vary depending on the vehicle and workshop. For a 4-cylinder engine with standard accessibility, you are usually between 80 and 150 €. For 6- or 8-cylinders and hard-to-reach cores, it can be up to 250–400 €, including labour time and high-quality spark plugs. Changing them yourself with components from Electronicx saves significantly compared with workshop prices.
Which heat range is the right one?
The heat range must correspond exactly to the vehicle specification. A plug that is too hot can cause engine damage through pre-ignition, one that is too cold soots up and leads to misfires. The values differ by manufacturer: with NGK, for example, 5 stands for hot, 9 for cold. Bosch increases the number with increasing coldness. Please refer to the type code of the original plug or the vehicle key in our online shop filters.
How long do iridium spark plugs last?
Iridium spark plugs achieve a service life of 100,000 to 160,000 km. Double iridium variants can even last over 180,000 km. The electrode gap remains very stable over the entire service life, which ensures constant engine running and low emissions.
What happens if I use spark plugs that are too cold or too hot?
A plug that is too hot can reach the surface temperature at which the mixture ignites uncontrollably (pre-ignition). This can lead to severe engine damage through piston seizure or cylinder head gasket defects. A plug that is too cold becomes contaminated, the insulator soots up and creep currents and misfires occur. The correct heat range must therefore be observed without fail.
Where can I buy spark plugs online?
We stock a wide range from all major manufacturers (NGK, Bosch, Denso, Beru) and deliver throughout Germany free of charge by DHL. For individual advice, you can reach us on +49 7135 7194106 or [email protected].