What Is the Recommended Spark Plug Gap?

The recommended spark plug gap is usually between 0.028 and 0.060 inches (0.7–1.5 mm), but the correct value depends on your specific engine and should always be confirmed in the owner’s manual or service documentation. Modern vehicles often require a precise gap tailored to their ignition system design, fuel type, and operating conditions.

Why Spark Plug Gap Matters

The spark plug gap is the distance between the center electrode and the ground (side) electrode. This small space determines how the spark forms and how effectively the air–fuel mixture ignites inside the combustion chamber. Too small or too large a gap can reduce performance, increase fuel consumption, and even damage ignition components over time.

At its core, the goal of setting the correct spark plug gap is to balance reliable ignition with efficient combustion. A well-set gap helps the engine start smoothly, run efficiently, and produce the power it was designed to deliver, while also reducing emissions.

Typical Recommended Spark Plug Gap Ranges

While every engine has its own specification, there are common ranges used across different types of vehicles and ignition systems. The values below are general guidelines and should not replace the exact specification from the manufacturer.

• Modern gasoline passenger cars (with coil-on-plug or DIS): typically 0.040–0.045 in (1.0–1.15 mm)
• Older distributor-based ignition systems: often 0.028–0.035 in (0.7–0.9 mm)
• Turbocharged or supercharged gasoline engines: commonly slightly tighter gaps, around 0.028–0.035 in (0.7–0.9 mm) to reduce misfires under boost
• High-performance or tuned engines: gap is often customized, but generally in the range of 0.028–0.040 in (0.7–1.0 mm) depending on boost, fuel, and ignition strength
• Small engines (lawn equipment, generators, some motorcycles): usually around 0.024–0.035 in (0.6–0.9 mm)
• Some late-model direct-injection gasoline engines: may specify 0.028–0.039 in (0.7–1.0 mm), with extended-life plugs pre-gapped at the factory

These ranges illustrate how engine design and ignition strength influence the gap. However, manufacturers fine-tune the specification for each engine, so always follow the exact figure specified for your vehicle or equipment.

How to Find the Correct Spark Plug Gap for Your Vehicle

The precise spark plug gap for your engine is published in several places, and using the official specification is the safest way to avoid drivability problems or damage.

Common Sources of Official Gap Specifications

Drivers and technicians have multiple reliable sources to confirm the correct gap. Each source provides the value that matches the engine’s design, emissions requirements, and ignition system.

• Owner’s manual: The most accessible reference for factory-spec spark plug gap and plug type.
• Factory service manual (FSM): Provides detailed specs, including variations by engine code, model year, or market.
• Under-hood emissions or tune-up labels: Many vehicles (especially in North America) list plug gap on a decal near the radiator support or underside of the hood.
• OEM and spark plug manufacturer catalogs: Websites and printed catalogs from NGK, Denso, Bosch, Champion, and others typically list plug type and gap.
• Reputable repair databases: Professional systems like OEM portals or major aftermarket data services mirror factory specs.
• Dealers or certified repair shops: Can confirm the latest spec, especially if there have been technical service bulletins (TSBs) revising earlier recommendations.

Using these sources ensures that the gap you set (or confirm) is not just approximate, but correct for the specific engine code, model year, and regional calibration of your vehicle.

Why Recommended Gaps Vary Between Engines

Spark plug gap is not one-size-fits-all because engines and ignition systems differ dramatically in how they generate and manage spark energy. Engineers select a gap that balances ignition reliability, combustion efficiency, emissions, and component life.

Key Factors That Influence Spark Plug Gap

Several technical and environmental factors drive the recommended gap value. Understanding them helps explain why two similar-looking engines might need different gaps.

• Ignition system strength: Modern coil-on-plug systems support larger gaps thanks to higher voltage and better control, while older systems need smaller gaps to avoid misfires.
• Cylinder pressure: Higher compression ratios and forced induction increase pressure, making it harder for the spark to jump a wide gap; this often calls for slightly tighter gaps.
• Fuel and mixture characteristics: Direct injection, turbocharging, or alternative fuels (like E85) can change how easily the mixture ignites, affecting optimal gap settings.
• Emissions control strategies: Engines designed to minimize emissions may run leaner mixtures or advanced timing, which can benefit from specific gap settings for consistent ignition.
• Plug design and materials: Fine-wire iridium or platinum plugs often come pre-gapped and are more sensitive to bending damage; their geometry supports reliable sparks at relatively larger gaps.
• Operating environment: Engines expected to work in extreme cold, high altitude, or heavy-load environments may have gap specs tuned for those conditions.

Together, these factors show why copying a generic “rule of thumb” gap can be risky. The recommended value reflects extensive testing for a specific engine and usage profile.

Effects of Incorrect Spark Plug Gap

Running an engine with plugs gapped too wide or too narrow can cause noticeable and sometimes costly symptoms. Many drivability issues blamed on “bad fuel” or “worn plugs” actually stem from incorrect gap.

Gap Too Wide

When the spark plug gap is larger than recommended, the ignition system must work harder to fire the plug, especially under load and high cylinder pressure.

• Misfires under load or at high RPM: The spark may fail to jump the gap consistently, leading to stutters or hesitation.
• Hard starting: Cold starts and damp conditions can aggravate weak sparks across a large gap.
• Poor fuel economy: Unburned fuel from incomplete combustion wastes energy and increases consumption.
• Rough idle: Irregular firing can cause shaking, unstable RPM, and vibrations at idle.
• Increased strain on coils and ignition components: Higher voltage demand can overheat or prematurely wear coils, wires, and ignition modules.
• Higher emissions: Misfires and partial burns increase hydrocarbon (HC) emissions and may damage catalytic converters over time.

These problems often appear gradually as plugs wear and the effective gap grows larger, which is why periodical inspection is recommended even for long-life spark plugs.

Gap Too Narrow

A gap smaller than recommended allows the spark to jump easily, but the spark kernel can be smaller and less effective at igniting the mixture.

• Reduced power: Incomplete or slower combustion can slightly diminish engine output, especially under heavy throttle.
• Potentially higher fuel consumption: Less efficient burn may require more throttle input to achieve the same performance.
• Increased carbon buildup: Poor combustion can leave more deposits on plugs, valves, and pistons.
• Higher likelihood of fouling: In engines that already run rich or short-trip frequently, a small gap can contribute to fouled plugs.
• Suboptimal emissions: While less dramatic than misfires, weak combustion can still push emissions above design levels.

Although a slightly tight gap often seems “safer” than too wide, it’s still a deviation from the intended design and can undermine the engine’s efficiency and emissions performance.

How to Check and Set Spark Plug Gap

Even when spark plugs are sold as “pre-gapped,” manufacturing tolerances, shipping, and handling can alter the gap slightly. Verifying the gap before installation is a standard step in professional practice, particularly on engines sensitive to gap variation.

Tools Used to Measure and Adjust Gap

Several simple tools exist to measure and set spark plug gap accurately. Choosing the right tool helps avoid damaging delicate electrodes, especially on iridium and platinum plugs.

• Feeler gauge (flat blades): Individual metal blades marked with thickness; accurate and commonly used in shops.
• Wire-style gap gauge: A selection of round wires of known diameters; less likely to distort the gap when checking.
• Coin or disc-style gap gauge: A tapered disc with markings; useful but easier to misuse and can bend side electrodes if forced.
• Dedicated gapping tool with adjustment slot: Allows you to gently bend the ground electrode to widen or reduce the gap.
• Non-contact digital gauges (specialized): Used mainly in high-precision or racing environments.

Using a quality gauge and gentle technique prevents damage to the plug and ensures the gap matches the exact specification for the engine.

Step-by-Step: Checking and Adjusting Gap

The process of setting spark plug gap is straightforward but should be done carefully, particularly on fine-wire plugs that can be damaged by rough handling.

1. Confirm the correct specification: Look up the precise gap for your engine in the manual or reliable database.
2. Inspect the new plug: Ensure the insulator is intact and threads are clean; reject any plug with visible damage.
3. Measure the existing gap: Insert the correct size feeler or wire gauge between the center and ground electrodes; it should slide with a slight drag.
4. Adjust if the gap is too small: Use a gapping tool to gently bend the ground electrode outward, checking frequently until the gauge fits correctly.
5. Adjust if the gap is too large: Lightly tap or use the tool to nudge the ground electrode inward, rechecking with the gauge after each small adjustment.
6. Avoid touching the center electrode: Do not pry against or bend the fine center electrode; damage can ruin the plug.
7. Recheck final gap: Verify with the correct gauge and, if possible, a second size to ensure accuracy.

This method minimizes the risk of bending electrodes excessively or cracking the insulator, both of which can lead to misfires or premature plug failure once installed.

Special Notes for Iridium and Platinum Spark Plugs

Many modern vehicles use fine-wire iridium or platinum spark plugs, designed for long service life and stable performance. These plugs are more delicate at the tip than older copper designs.

Handling “Pre-Gapped” Long-Life Plugs

With iridium and platinum plugs, the manufacturer usually sets the gap at the factory. However, they still require careful verification before installation.

• Check, don’t assume: Even pre-gapped plugs can be out of spec due to shipping or packaging impacts.
• Use wire gauges: Wire-style gauges are less likely to distort or catch on the fine center electrode.
• Adjust only if allowed: Some plug makers warn against adjusting gap beyond minor corrections; always follow the plug manufacturer’s instructions.
• Make tiny adjustments: If an adjustment is needed, bend the ground electrode only, in very small increments.
• Avoid disc tools that pry aggressively: These can chip fine tips or crack insulators.

Respecting the design limits of iridium and platinum plugs helps maintain their long-life advantage and consistent ignition characteristics across tens of thousands of miles or kilometers.

When to Recheck Spark Plug Gap

Spark plug gaps can widen over time as electrodes wear, especially on high-mileage engines or in severe-service conditions. Periodic inspection can prevent performance loss and misfires.

Key Intervals and Warning Signs

Certain mileage milestones and symptoms serve as practical triggers to inspect spark plug condition and gap.

• Manufacturer’s maintenance interval: Many modern vehicles specify plug inspection or replacement around 60,000–120,000 miles (96,000–193,000 km), depending on plug type.
• Noticeable drop in fuel economy: A steady decline without other obvious causes can point to plug wear and increased gap.
• Rough idle or intermittent misfires: Particularly when the engine is warm and under light load.
• Hard cold starts: Especially in colder climates or after the vehicle sits for long periods.
• Engine codes or check engine light: Misfire codes (such as P0300–P030x on OBD-II systems) can be related to worn plugs and widened gaps.

Addressing spark plug condition at or before these points helps preserve engine efficiency, prevent catalytic converter damage, and maintain reliable operation over the long term.

Summary

The recommended spark plug gap typically falls between 0.028 and 0.060 inches (0.7–1.5 mm), but the correct value is specific to each engine and should always be taken from the owner’s manual, service manual, or an official specification source. Modern ignition systems and engine designs rely on a carefully chosen gap to deliver reliable starts, smooth running, strong performance, good fuel economy, and low emissions. Verifying and, when appropriate, gently adjusting spark plug gap before installation—and periodically checking it over the life of the plugs—remains a critical step in proper engine maintenance.

Does gapping spark plugs increase horsepower?

Yes, gapping spark plugs can increase horsepower, but this is mainly applicable to specific performance modifications like “side-gapping” to improve flame propagation in racing engines. For a typical street car, properly gapping spark plugs is crucial for optimal performance and efficiency, preventing power loss caused by a gap that is too wide or too narrow, rather than increasing it.
 
How gapping affects performance

• Side-gapping: In high-performance or racing applications, modifying the spark plug by side-gapping it can increase horsepower. By unshrouding the spark, the flame can spread faster and more efficiently throughout the combustion chamber, leading to more complete combustion. 
• Proper gapping: For most engines, the goal of gapping is not to increase horsepower, but to ensure the engine runs as it should. 
  • Gap too narrow: A gap that is too small can result in incomplete ignition of the fuel/air mixture, leading to a loss of power. 
  • Gap too wide: A gap that is too wide can prevent the spark from jumping reliably, especially under heavy loads, causing misfires and power loss. 

When to gap spark plugs

• For performance: If you are building a high-performance or racing engine, consulting with a professional or engine tuner to determine the correct gap for your specific setup is recommended. Side-gapping can be an option in these cases. 
• For regular maintenance: If your engine is not running optimally, especially at idle, check your spark plugs. They may need to be gapped or replaced to ensure they are firing correctly for your engine’s needs. 

Important considerations

• Engine-specific needs: The ideal spark plug gap varies depending on the engine, and is affected by factors like boost, compression, and fuel type. 
• Risk of damage: Gapping a spark plug is a delicate process. Incorrectly gapping or side-gapping a plug can cause damage and lead to performance issues. 
• Maintenance is key: Regularly checking and properly gapping your spark plugs is an essential part of routine maintenance that can prevent performance issues. 

Is it better to have more or less spark plug gap?

A wider spark plug gap does a better job of igniting the combustion, but it requires higher voltage to fire. This puts a greater strain on all the high voltage parts, coil, wires, distributor, etc. Too wide a gap causes the ignition parts to fail. An engine runs best with the widest gap the ignition can reliably fire.

How do you know if spark plugs are gapped correctly?

You can know if spark plugs are gapped by using a feeler gauge or a dedicated spark plug gap tool to measure the distance between the center and ground electrodes and compare it to the manufacturer’s recommended specification. The correct gap is crucial for engine performance, so always check it, even on new spark plugs. 
How to check the gap
This video demonstrates how to check the spark plug gap with a feeler gauge: 57sSteve’s Small Engine SaloonYouTube · May 27, 2022

1. Find the correct gap specification: Check the owner’s manual, the vehicle’s under-hood sticker, or the manufacturer’s website (like NGK.com) for the correct gap for your specific engine. 
2. Use a feeler gauge or gap tool: 
   • Feeler gauge: Insert a wire feeler gauge into the gap. The correct size will feel snug with a slight resistance, often described as a “magnetic drag”. You may need to combine two gauges to achieve the correct thickness. 
   • Tapered gap tool: Slide the narrowest end of the tool into the gap. The measurement where you first feel resistance is the current gap size. Do not force the tool. 
3. Compare the measurement:
   • If the tool slides through easily, the gap is too large. 
   • If the tool won’t fit, the gap is too small. 
   • If the tool fits with the correct, slight resistance, the gap is correct. 

What to do if the gap is wrong

• To close the gap: Gently tap the ground electrode on a hard surface or use a gap tool’s slot to gently bend the electrode inward. 
• To open the gap: Use angled needle-nose pliers or the appropriate slot on a gap tool to gently bend the ground electrode outward. 
• Be careful: Avoid touching the center electrode or the precious metal coating on platinum or iridium plugs when making adjustments. 

This video shows how to adjust the spark plug gap using a feeler gauge: 53sRevive My RideYouTube · Oct 7, 2021

What should I set my spark plug gap to?

Finding the Proper Gap Setting
The gap setting is different for each vehicle but most are somewhere between 0.028″ and . 060″. Consult the owner’s manual or Champion ® catalog to find the recommended setting for the vehicle you’re working on.