How To Gap a Spark Plug? (Easy 5 Step Guide)

Want to know how to gap a spark plug? You've come to the right place!

In this guide, you'll learn:

  • What you need to know about gapping spark plugs
  • Supplies you'll need for gapping spark plugs
  • The steps to gapping a spark plug

And much more!

How to Gap A Spark Plug

Is it really necessary to gap the spark plug before installing it? In a lot of cases, no.

However, an improperly gapped spark plug can cause plenty of engine problems. 

Luckily for you, it doesn't have to be a difficult or cumbersome process. 

Which is exactly why we created this helpful guide to walk you through the process.

Now, before you do service the spark plugs on an engine, I recommend you read this quick little blurb about why a properly gapped spark plug is important.

What You Need To Know About Gapping Spark Plugs

What is the spark plug gap? The spark plug gap is the gap between the electrode tips that makes the engine run the most effectively.

Gap adjustment is easy to do, and some engines do have a specific gap adjustment that is different than the out of the box.

The typical spark plug gap ranges from 0.6 to 1.8 millimeters.

Sure, most spark plugs come pre-gapped to spec, and you don't have to worry about gapping them for proper function. However, did you know that the spark plug gap widens over time with engine use? 

A spark plug that has the proper gap will have a blue spark, and you can test this visually on small engines by removing the spark plug from the engine head, grounding it out on the engine, leaving the spark plug connecter installed and pulling on the crank of the engine. 

How can you know what the gap specification is for your vehicle?

The spark plug will come out of the factory at a stock gap specification, and your engine may also have a recommended specification that is different than the spark plug that is recommended. 

A spark plug gap that is too small will cause the engine to run poorly and inefficiently because the spark only travels a small distance in between electrodes, and doesn't allow the tip to get hot enough for fuel combustion.

spark plug gap that is too large will cause the engine to run rough or not run at all because the spark will be traveling across a larger distance, or the spark will not occur.

Supplies You’ll Need For Gapping Spark Plugs  

Feeler gauges or a spark plug gap tool aren't the only tools you'll need. Each engine has its distinct requirements for what is needed to access the spark plugs. 

Some small engines like a lawnmower, for example, require simply a crescent wrench whereas newer vehicles will require a few extra items. Here is a complete list of tools you might need:

  • Spark plug socket
  • Ratchet
  • Pliers
  • Basic wrench set
  • Spark plug gap gauge/feeler gauges
  • Flashlight
  • Socket extension/universal joint
  • Magnet pickup tool
Did you know: The typical spark plug needs to head up to 1,475 degrees Fahrenheit!

How To Gap a Spark Plug (5 Step Guide)

  1. Remove the spark plug
  2. Inspect the spark plug for damage/fouling
  3. Gap and clean the spark plug
  4. Reinstall
  5. Analyze the Results

Step 1: Remove the Spark Plug

Remove the spark plug wire boots/ignition coils to gain access to the spark plugs. 

Ignition coils are typically mounted on to the engine using bolts. You'll need the appropriate socket/screwdriver to remove them.

Spark plug wire boots are simply installed straight onto the spark plugs. You can use either your hands or a set of pliers to pull off the wire boots. 

After you have exposed the spark plugs, stick a spark plug socket on and unscrew the spark plugs. This is where a magnetic pickup tool is convenient to stick down into the cylinder head and remove the spark plug easily. 

Note: Some vehicles may have a plastic engine cover that you must pull off first thing.

Step 2: Inspect the Spark Plug for Damage/Fouling

Use your eyes to do a physical inspection of the plugs. A flashlight is always nice to have to identify small cracks in the ceramic insulator. 

What you are looking for on the electrode tips is signs of corrosion, blistering, and excessive oil/carbon buildup. Any of these characteristics mean that you should replace the spark plug if possible.

A used plug that has normal wear will appear gray, tan, or brown on the side electrode. If you see any of these colors, your spark plug is probably still good.

Step 3: Gap and Clean the Plug

Sometimes spark plugs aren't performing simply because they are covered with carbon deposits.

You can clean the side electrode and center electrode tips on the plug with anything from gasoline to carb cleaner. 

Use a brush or piece of sandpaper to create a new metal surface for cleaner combustion on the electrodes.

Measuring the gap is easy with the gapping tool; you simply stick it in between the two electrodes.

If the gap is too small, simply hook the gauge or tool in between the side/ground electrode and center electrode and tug or bend into the right width. 

If it is too large, press, or tap the electrode against a clean surface with the measuring tool installed to create the correct width. 

Slowly adjust the gap, and don't adjust more than .2 mm at a time to avoid misaligning the two electrodes. 

Step 4: Reinstall

Carefully reinstall the spark plug, and tighten it with your hands.

A spark plug is easy to over-torque, and the ceramic insulator will break and misfire if you do, so a good technique if you don't have a torque wrench, is to grasp the ratchet with your fist closest to where the shaft meets the extension/socket, and tighten it firmly from there.

You can purchase quality brand spark plugs like NGK, Bosch, ACDelco, and more on sites like Amazon. It isn't always necessary to replace the spark plugs when servicing.

Clean and gap the spark plugs if they don't appear to have any damage and reinstall them without any problems. 

And remember, the nicer the engine, the more important it is that you torque to spec.

Step 5: Analyze the Results

So, you've gapped each one of your spark plugs, but did it make your engine perform better?

It might have, or you could just be fooling yourself by wasting your time on spark plug removal when the real problem was a bad ignition coil, valve seal, piston ring, or gasket issue. Not to mention the other causes of a misfire in a modern-day vehicle like 02 sensors, electrical problems, and ECU problems. 

But one thing is sure, small carbureted engines need the spark plugs serviced a lot more regularly than vehicles do.

Read More >> How to Clean a Spark Plug

Other Valuable Resources on How to Gap a Spark Plug

Did nothing change after gapping your spark plug

Don't get discouraged. Here are a few other suggestions you can try for improving your engine performance.

Clean the fuel injectors(fuel injected engines)

High mileage engines need some tender love and care given to the injectors that mist fuel into the combustion chamber. 

Cleaning the injectors is also a relatively easy procedure.

Either you can run injector cleaner straight through the gas tank and clean as you drive, or you can remove the injectors, apply a voltage through them, and run injector cleaner through with a connected hose.

The second method mentioned is the most effective.

Adjust Carburetor Jetting (Carbureted Engines)

A spark plug is easily fouled by a bad fuel mixture in the carburetor. For all of you small and old engine repair guys, be aware of the carburetor when servicing and inspecting the spark plugs. 

Service Ignition Coils/Distributor Cap

Fuel-injected engines have ignition coils that sometimes go bad and cause the engine to misfire just like spark plugs. You can easily test an ignition coil by unplugging it and trading it out with one from a different cylinder.

Older engines that have a carburetor require servicing the distributor cap and rotor.

The rotor spins around inside the distributor like the arrow on a clock making contact with metal points inside the distributor cap to send electricity to each spark plug.

These wear down and corrode over time just like a spark plug does.

Wiring Issues

Never eliminate the possibility of a wiring issue when servicing your ignition system.

Spark plug wires that have been exposed to excess engine heat can sometimes be the cause of problems with engine performance. 

It is smart to change out the spark plug wires every 100 thousand miles on carbureted engines to ensure good engine performance. 

You could also check the voltage of the ignition coil wires in the case of an unknown misfire.

Change engine air filter regularly

Remember that your engine not only needs a steady supply of fuel, but it also needs a steady supply of air. Switch out your engine air filter when performing a tune-up to ensure the best fuel economy. 

Oil leaks

Oil leaking from valve covers or even the piston rings can cause spark plugs to foul. Take care of any significant oil leaks beforehand to avoid having to buy a second set of new spark plugs. 

We hope this page helps you when servicing your engine. Enjoy the ride!

Read More >> How to Check a Spark Plug

About Your Mechanic

About Ryan Nichols

Ryan here! My pro mechanic career began as a technician for Mercedes-Benz. After two years of that great experience, I went out on my own in both the automotive and construction fields. I've since pulled my share of salvage yard parts and fixed more vehicles than I can count.

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