The Spark Plug Gap Guide (What It Is + How To “Gap”)

When you get a new spark plug, do you check the spark plug gap?

This might be old news, but even a difference of a few thousandths of an inch in this tiny gap could spell engine misfire.

So, what is a spark plug gap?
And why is it so important?

We’ll discuss the spark plug gap in this article — its significance, why you should measure the gap size, how to do so, and a few other related FAQs.

Let’s strike a spark and shed some light.

What Is The Spark Plug Gap?

The spark plug gap defines the distance between a spark plug’s center electrode and the ground electrode (also called a ground strap which is a curved piece of steel at the end of the plug).

This gap width is critical so electrical arcing (a “spark”) can occur at the right voltage, igniting the air fuel mixture in an engine’s internal combustion chamber.

Setting the correct gap distance is called “gapping” the spark plug.
But why is the spark gap significant?

Why Is The Spark Plug Gap Important?

The spark plug gap influences the heat range of the spark produced to ignite the air fuel mixture in the combustion chamber. This makes it critical to engine performance.

How so?
A smaller gap ensures the spark plug fires on each cycle. But if the spark gap is too small, there won’t be enough room for the airfuel mixture between the ground electrode and center electrode. So when the spark plug fires, there’s a possibility that nothing ignites.

A wide gap between the center electrode and ground electrode allows for a strong, clean burn. But if the gap is too wide, the spark will be weak and won’t have enough energy to ignite the charge, leading to cylinder misfire. And while the spark plug is a pretty simple piece of equipment, it has to work in very tough conditions.

Some engine environments make it even harder for ignition to happen.
For example:

Engines with higher compression or forced induction have higher pressures in the combustion chamber during spark discharge. More cylinder pressure means denser airfuel mixtures that demand a higher voltage for ignition.

Engines with high flow intake ports (like racing engines) will have lots of charge swirl and movement in the combustion chamber, making ignition harder.

Engines with a high RPM give the distributor little dwell time to build up a solid spark, also making ignition tougher.

In all cases, the spark plug gap must be wide enough for a sufficiently energized spark, but not too wide that the spark skips at higher speeds — which will result in cylinder drag or engine rattling.

Next, let’s see how you go about gapping a spark plug to its recommended gap.

How To “Gap” Spark Plugs (Step-By-Step)

Before you begin gapping, make sure you have the right spark plug for your engine. For this task, you’ll need a gap tool like a feeler gauge or gap gauge.

Note that:

Multiple electrode spark plugs are vehicle-specific and come with a factory gap that isn’t adjustable.
A platinum or iridium plug has a delicate center electrode that can be damaged by the usual keyring-styled tool, so you’ll need a proper gapping tool to make adjustments.

And always — if you’re not familiar with spark plug installation, let a professional mechanic deal with it.

Now, here’s how to do spark plug gapping:

Step 1: Clean The Spark Plug

If you have a new plug, this shouldn’t be an issue.
However, old spark plugs may be dirty around contact points.

You’d want to avoid introducing dirt into the engine’s combustion chamber or have it weaken the ignition spark. In this case, use a wire brush with gentle motions to clean off debris, and never use abrasive materials.

Step 2: Measure Current Spark Plug Gap

Check the owner’s manual for the recommended gap size. Then, run the feeler gauge or gap gauge between the electrodes to determine the current measurement.

If the feeler gauge or gap gauge can’t fit at the needed gap size, the plug gap needs to widen.

If the feeler gauge or gap gauge (with the correct size) passes through the gap without touching any electrode, the gap is too wide and needs to be narrowed.

Step 3: Adjust The Spark Plug Gap

Use the gap tool to adjust the spark plug gap size. Gently bend the ground electrode to widen or narrow the gap.

Here are a few essential things to note:

Never exert any force on the center electrode or its ceramic insulator
Don’t bend the ground strap more than 0.008” in either direction

Step 4: Check The Gap Again

Repeat the adjustment until the gap tool fits closely between the spark plug electrodes.

However, you shouldn’t adjust the ground electrode more than 3 times. It’s durable, but it’s not meant to deal with lots of pressure and might snap off. Any damage to either electrode means you’ll need a new spark plug.

Now, you may wonder why you’d need to measure the spark plug gap, especially on a new plug.

Why Should I Measure The Spark Plug Gap?

Firstly, while manufacturers try to make stock plugs the same, it’s simply unrealistic in a mass-production environment. There will be toleranced differences.

Secondly, there’s no way to account for spark plug handling from its factory release until it gets to you.

And thirdly, the same spark plug can be specified for many different engines, but each engine may require a different gap size.

A new spark plug could have a factory gap for a V6 engine, and you’d likely be able to install all 6 plugs as is. But in a V8 engine, those 8 stock plugs would need gapping.

Let’s go through a couple of FAQs to understand the spark plug gap a bit more.

6 Spark Plug Gap FAQs

Here’s some additional information on the spark plug gap:

1. How Does The Spark Plug Gap Work?

The ignition system delivers a spark (electricity) down the spark plug wire (or through the spark plug boot for cars using coil packs) to the plug.

The electrical spark always follows the path of least resistance. As the plug gap is typically the closest point from the center electrode to the ground electrode, the spark will jump this gap.

When it does so, it ignites the air fuel mixture between the gap, creating combustion (like how a fire starter lights up a gas stove).

2. What Happens If The Spark Plug Gap Is Wrong?

An incorrect gap can lead to spark plug performance issues like engine misfires or increased plug wear.

It won’t be as directly noticeable if the spark only fails intermittently in igniting the air fuel mixture. However, the firing failure will manifest as engine power loss and fuel inefficiency.

3. Why Are There Different Spark Plug Gaps?

For the best spark plug performance, the plug needs to be matched to its intended use.
As different engines offer varied working environments, the style, size, and shape of the spark plug is influenced by factors like:

Fuel type and concentration — fuel with a higher alcohol content requires a lower air fuel ratio and a narrower spark plug gap.

Combustion chamber pressure (at spark discharge) — higher compression engines need a higher voltage to generate a spark.

Engine load and application — engines experience more cylinder pressure in the combustion chamber when under load as there is more air aspirated, making it harder to generate a spark.

4. What Are Typical Spark Plug Gap Settings?

Here are some typical gap settings for cars:

Standard street engine (ignition coil with distributor): 0.025″
Performance engine (ignition coil with distributor): 0.028″
Distributorless ignition system (coil on plug): up to 0.060”

Additionally, engine manufacturers often define a range for the spark plug gap. Here’re the benefits of setting at opposing extremes:

Setting the new plug gap at the minimum recommended gap (instead of the center of the specified range) can ensure a longer interval between plug changes.

Setting the plug gap at the maximum recommended width allows for the largest gap with a strong, reliable ignition. This can be used for high-performance applications at the cost of re-gapping or replacing the spark plugs more frequently.

5. What’s A Spark Plug Feeler Gauge?

The feeler gauge has several metal blades or wires of different thicknesses and often looks like a pocket knife. Each blade is a specific thickness marked in millimeters (metric) or thousandths of an inch (imperial).

6. How Does An Eroded Spark Plug Look?

On an eroded spark plug:

The center electrode (which used to be a cylindrical rod) would be a dark bump
The ground electrode won’t have square edges anymore

The spark plug gap will widen as the plug erodes, making it harder to fire a spark. The ceramic insulation on old spark plugs can also be compromised by carbon combustion deposits and old fuel saturation, resulting in a weaker spark.

Closing Words

All spark plugs gaps should be checked, whether it’s for your car engine or lawnmower. Using the right spark plug with the correct gap ensures the best running conditions for engine performance.

If you need professional help gapping your spark plugs, why not get hold of AutoNation Mobile Service?

AutoNation Mobile Service is a mobile vehicle repair and maintenance solution available seven days a week. We can help you gap your spark plugs, replace them, and look into any other issues that may plague your vehicle.

Contact us, and our expert technicians will drop by to get your car sparking in no time!