How To Gap Spark Plugs? | Spark Plug Gap

What is Spark Plug Gap?

The spark plug gap is where spark plug spark discharge is designed to take place. In a conventional spark plug, this is the area between the center and the ground electrode.

Since the spark always follows the path of least resistance, the spark gap is generally the closest point between the center electrode of the spark plug and the ground electrode of the spark plug, which is sometimes formed by the spark plug cup itself.

The only case the spark will travel a longer path to the ground is when the longer path is more conductive (offers less resistance). This can be caused, for example, by the loss of the insulating properties of ceramics due to conductive carbon that accumulates from the combustion process (spark plug fouling).

Setting a large spark plug gap requires a higher voltage (electrical pressure) so that the spark splits the large gap. This means that a higher voltage builds up in the ignition system (ignition coil, distributor, ignition cable) before the spark discharge.

This is generally desirable in high power ignition coil applications and later naturally aspirated, lower compression engines where cylinder pressure is relatively low and spark discharge can easily occur.

However running a large gap in a high cylinder pressure application (which makes it very hard for a spark to occur), will demand such a high voltage (electrical pressure) for a spark to jump the spark plug gap.

That the spark will find an easier way to the ground, possibly where the ignition wire is close to the ground (engine block, etc.) or inside of distributor cap, inside of the ignition coil, on the outside of the spark plug (between the plug and the boot), etc. 

In either case, the spark will not occur between the spark plug electrodes in the combustion chamber and the engine will misfire.

Importance of the Spark Plug Gap

When replacing spark plugs, ensuring that the plugs have the proper gap is critical to engine performance. The gap is the distance between the center and side electrodes, set so arcing occurs at the proper voltage that ignites the fuel and generates the combustion that makes the engine run.

Do Spark Plugs Always Have to Be Gapped?

Not always. In the past, it was necessary to gap spark plugs, but today spark plugs are usually pre-gapped. It is advisable to double-check that the gap is correctly set to the vehicle’s recommended setting when installing spark plugs.

Finding the Proper Gap Setting

Spark plugs in automobiles generally have a gap between 0.6 and 1.8 mm (0.024 and 0.071 in). The gap may require adjustment from the out-of-the-box gap. A spark plug gap gauge is a disc with a sloping edge, or with round wires of precise diameters, and is used to measure the gap.

The gap setting is different for each vehicle but most are somewhere between 0.028″ and .060″. Consult the owner’s manual to find the recommended setting for the vehicle you’re working on.

How Do I Gap a Spark Plug?

Step 1: Locate a gap gauge or feeler gauge.

Spark plug gap is where spark plug spark discharge is designed to take place. In a conventional spark plug, this is the area between the center and the ground electrode.

Step 2: Clean the spark plug.

If it is new, this shouldn’t be an issue; however, if you are checking a spark plug that is currently in the vehicle, it might be dirty on the contact points. Never use abrasive materials to clean a spark plug.

Step 3: Measure the existing gap of the spark plug.

Using your gap gauge or feeler gauge, run the tool through the electrodes to determine the measurement. Take note of the measurement and compare it to the recommended setting listed in the owner’s manual.

Step 4: Make the necessary adjustments.

Choose the right size for your gap or feeler gauge. If you can’t get the correct feeler gauge through the gap, the gap will need to be increased. If the tool passes through the gap without touching the electrodes, the gap is too large and needs to be narrowed.

Use your spark plug gap tool to adjust the gap. Gently bend the ground electrode in to narrow the gap, or bend it out to widen it.

Be very careful when making adjustments. Do not bend the electrode more than a few inches. It’s durable, but not designed to take a lot of pressure. Also, be careful not to hit the center electrode.

If you break the bottom electrode or damage the center electrode by prying it excessively, you will need a new spark plug. Precious metal iridium or platinum center electrodes can be very delicate when it comes to prying them. It is important to use the correct spark plug splitting tool.

Step 5: Check the gap again.

Repeat the adjustment process until the tool fits closely between the electrodes.

Consequences of Incorrect Spark Plug Gap

If the spark plug gap is incorrectly set, it can lead to engine issues. The customer may experience loss of power, misfires, spark plug fouling, increased plug wear, or poor gas mileage. Too small of a gap may give too weak of a spark to complete the combustion process within the engine; too wide of a gap can lead to the spark plug not firing correctly, causing misfires at high speeds.


Do spark plugs need to be gapped?

Not always. In the past, it was necessary to gap spark plugs, but today spark plugs are usually pre-gapped. It is advisable to double-check that the gap is correctly set to the vehicle’s recommended setting when installing spark plugs.

How do you gap a spark plug without a tool?

All you need to do is slide the spark plug electrode opening into the gap opener and gently move it over to the smallest side of the gapper. Once it hits the 0.02” area, press the electrode down until it’s flush on each end of the gapper.

How do I know if my spark plug gap is wrong?

A list of some of the most common symptoms of incorrectly gapped spark plugs is laid out below.

  • Rough Engine Idle. An engine that has a rough, irregular engine idle is often due to spark plugs that are incorrectly gapped.
  • Engine Hesitation.
  • Engine Missing.
  • Poor Engine Performance.
  • Engine Knocking.

What happens if the gap is too small on a spark plug?

One of the key factors that make a spark plug work well, or not, is the size of the gap between the electrodes. If the gap is too small, the spark will likely be too weak and cause the engine to run poorly or with poor efficiency.

How tight are you supposed to tighten spark plugs?

Tighten the spark plug finger-tight until the gasket reaches the cylinder head, then tighten about ½ – ⅔ turn more with a spark plug wrench. (Taper seat: About 1/16 turn more.)

What can I use as a spark plug gapper?

Can you use a feeler gauge to gap spark plugs?

You can use either a feeler gauge or a spark plug gauge to gap your spark plugs.

Does NGK spark plugs come pre-gapped?

While most NGK spark plugs are pre-gapped, there are occasions when the gap requires adjustment. Care must be taken to avoid bending or breaking off the fine-wire electrodes. NGK recommends a round wire-style or pin gauge gap tool to measure the gap.

Why does my car shake after I changed the spark plugs?

Either the spark plugs have been connected up in the right firing order, the leads aren’t pushed onto the spark plug terminals properly or they’ve been damaged or one or more of the new spark plugs are faulty or they’ve been damaged while fitting them!

How far off can a spark plug gap be?

Excessive changing of the spark plug gap setting will result in weakening of the spark plug ground electrode and can lead to breakage. Also, the spark plug gap should never exceed. 055″ unless pre-set by the manufacturer. The spark plug gap is where spark plug spark discharge is designed to take place.

Is a bigger spark plug gap better?

The larger the gap, the more voltage is needed to jump the gap. Most experienced tuners know that increasing the gap size increases the spark area exposed to the air-fuel mixture, which maximizes burn efficiency.

What happens if you don’t torque spark plugs?

if not tightened properly the spark plug will work out of the hole & most likely damage the threads on their way out. If a spark plug is cross threaded it will damage the plug & threads in the head. Sometimes a reamer thread tool can repair the head.

Are feeler gauges accurate?

010 feeler gauge has a thickness tolerance of ±. 00035. If one of the. 010” feeler gages is measured with a quality micrometer it could actually measure.

How does spark plug gap affect timing?

It won’t affect the timing, but it will affect the ability of the spark to ignite the fuel. Modern engines use a very lean fuel mixture and require a wide plug gap to ensure there are sufficient fuel molecules between the electrodes to start combustion.

Can you buy pre-gapped spark plugs?

Most spark plug manufacturers will pre-gap their spark plugs for their most popular application but considering that same spark plug might fit over a hundred different vehicles or engines, checking that it matches your vehicle’s specifications is a good idea.

Does spark plugs increase horsepower?

In short, yes, in some situations spark plugs can increase horsepower.

Why is there oil on my spark plug?

You may find oil on a spark plug because: Too much oil was mixed with the gasoline. The piston rings (the component that seals the piston and cylinder) are failing. If these rings are broken or failing in some way, they can let oil slip past the piston and get on the spark plug.