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How to Gap a Spark Plug

How to Gap a Spark Plug

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After purchasing a new spark plug for your car engine, it may be indicated that it’s pre-gapped. However, you may need to make some adjustments to the gap for more suitability to your vehicle. This happens because the spark plug may have been dropped or mishandled during packaging or shipping. Before learning how to gap a spark plug and follow the steps closely, you should always read the manufacturer’s instruction manual. This will tell the ideal gap for your car’s spark plug. 

How to Gap a Spark Plug

What Is A Spark Plug?

The first spark plug that worked effectively was invented by Oliver Lodge in 1903. Spark plugs are insulated plugs screwed into the head of an internal combustion engine cylinder. It’s in place to provide the spark that ignites fuel and air mixture in the combustion chamber. They are also relevant to transfer heat away from the combustion chamber. If you own a four-cylinder car, then you will need four spark plugs. On the other hand, when you own a six-cylinder you will need six spark plugs, and so on. Nevertheless, if a HEMI engine is in place, then you’ll need two plugs for one cylinder.

Two Basic Varieties of Spark Plugs

In relation to temperatures, there are two basic types of spark plugs. That is; cold plugs and hot plugs.


  • Cold plugs


These plugs work effectively on high-horsepower and high compression engines. Their insulation is little; they can transfer more heat away from the combustion chamber to the outside of your engine. The plug needs to be cold enough for a particular application to get enough heat out of the piston chamber. Failure of this could lead to knocking, pre-ignition and permanent damage to the engine. It’s always important to consult your manufacturer or a professional auto mechanic to tell you clearly which spark plug heat range to use. 


  • Hot plugs


These are found in most standard engines. They have more insulation to keep the temperature of your plug high enough to completely burn the carbon deposits. This provides more time between the spark plug changes.

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What Is A Spark Plug’s Gap?

A spark plug’s gap is an air gap that’s found between the ground electrode and center electrode. This gap needs to be precisely set for the effective functioning of your car. It’s the most important part of the spark plug.

Why Does the Gap of a Spark Plug Matter?

A large gap of a spark plug provides a big and fat spark for igniting the mixture. Nevertheless, this large gap requires the ignition components to work harder for the spark to jump it. The gap enlarges as the spark plug is put to use. This is due to wearing away the constant arcing at the metal of both electrodes causing the gap to open up over time. Having this in mind, it’s important to regularly measure and adjust the plug gap to its ideal measurement for better engine performance.

Previously, the center electrodes were made of copper, and there was a specific way of gapping the plugs. Nevertheless, in recent times, precious metals are being used to construct spark plugs. The metals include iridium, platinum, and more. They’re generally applied to coat the electrodes. Manufacturers considered them because they can withstand extreme temperatures, they’re good conductors, and not corrosive. You will, however, need to spend more on these spark plugs.  With the features in place, the wire diameter of the center electrode became finer since narrower wires were used to cut the cost. 

Most of the modern spark plug electrodes are made of a very narrow wire, or have a fine coating of precious metals on them. The gap is prone to opening up over time, and gapping needs to be done. They may be indicated as pre-gapped, but you should always double-check the spark plugs to ensure that the gap is set correctly depending on the type of car. You won’t have to do gapping frequently, as it was in the past, but you will need to do it at some point.

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Steps for Gapping A Spark Plug

We will divide this section into two; preparation for gapping, and adjusting the gap.

Preparation for gapping


  • Be aware of the specifications provided for your vehicle’s gap


This information can be found on your manufacturer’s manual. You should check keenly the recommended gap for the engine’s best performance to determine whether that particular gap is in place or needs some adjustments.

  • Most vehicles have a spark plug gap of 0.028 to 0.6 inches. You can consult a professional auto mechanic to look at it if you can’t find the measurements on the manual easily. 
  • Smaller gap settings are necessary for modified engines to account for the amount of power being put through the engine. You should always have in mind that a smaller gap produces more power.


  • Select a suitable spark plug gapper


Not every gapper will work for your car’s engine. You should choose one that will provide accurate measurements for a perfect gapping. Some tools are suitable for the modern spark plugs that utilize delicate precious metals, while others are appropriate for the copper spark plugs. Most of them will have a flat edge necessary for bending the plug’s bottom electrode when slightly adjusting the gap.

Spark plug gappers available include the following;


  • Coin style gap tool


This is among the cheapest options in the market. It runs the edge of the coin through the gap until it stops. Markings of this edge are similar to that of a ruler, making it easy to show its thickness at a certain point. It’s an effective tool for measuring the gap of older spark plugs. They can also adjust the gap of these plugs.


  • A wire coin gap tool


The operation of this tool is similar to that of a coin style gap. A wire coin gap tool, however, features different wire measures of varying lengths around the coin’s edge.


  • A blade-style feeler gauge


This is an effective and versatile tool. Its construction is similar to that of a pocket knife. It features various blades with different widths. Some of them have wire on the end, while others don’t. The blades are placed on the gap to check its measurement. Multiple blades can be used to accurately measure larger gaps. Interestingly, they can also be used to adjust gaps size when necessary. 


  • Spark plug gauge


The gauge comes in two main types;


This plug gauge has a look similar to a silver dollar. It has numbers marked all around the edge of its face, and the outer edge is thin with a gradual change in thickness all around the coin. They’re among the cheapest gauges for gapping and can be easily bent with less trouble, you don’t need the assistance of other tools. 

Nevertheless, due to the gradual slope of their bending and measuring edge, they’re difficult to read, hence fail to provide very accurate measurements.

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Wire loop-style

This spark plug gauge looks like a coin and has a variety of wire loops with different thickness on their outside edge. They also have a hook or a driver to bend the electrode on a plug when adjusting the gap. They are more accurate than the ramp-style and are easy to use without the need for other tools for bending. 

However, the limited loops available may not accurately measure an engine with an odd gap size due to a lack of a loop to efficiently fit it. 


  • Clean the spark plug


You may not need to clean a new spark plug, but for a spark plug that you have been using, some cleaning needs to be done for the most accurate reading. There is some whitish soot that builds-up on the plug’s contact points, thus you’ll need to clean it off using a clean cloth. 

  • You can do some quick-drying with 90% alcohol on the contact points. This will clear the dirt off to make your work easy as you take the measurements. If the dirt is quite a lot or there are blackening on the contact points, it’s a sign that the spark plug is past its prime.


  • Take the measurements


With your selected tool, you can now measure the gap by passing it through the electrodes. Place the appropriate wire or blade on your gap tool between your plug’s electrode tips, or pass the coin through the electrodes to get the most accurate measurement.

Making Adjustments on The Gap


  • Determine the gap needs to be adjusted


Your spark plug’s gap is too wide when the spark plug gapper passes through the gap without touching electrodes at the required measure. On the other hand, if the gapper can fit between the electrodes, then the gap is too small and needs to be adjusted to the appropriate measure. Gap adjustments are not necessary when the gap tool fits closely to the required measurement.


  • Make the adjustments


Use your selected tool to make adjustments on the bottom electrode. Hold the plug with the electrodes facing the floor. Gently bend the bottom electrode inside towards the other electrode when the gap needs to be narrowed. On the contrary, when you need to widen the gap, then you need to bend it out.

  • Don’t bend the electrode more than 0.2 fractions of an inch as it won’t take much pressure. Additionally, be very careful in the process since any fracturing on the spark plugs could cause misfires.
  • In case it becomes troublesome to use the tool; you could bend it in using a flat surface to get very gentle pressure on the electrode for proper adjustments.


  • Re-measure the gap


Take the measurements using the procedure provided previously. If the appropriate measurement is still not in place, you’ll need to make some adjustments once more.


  • Be gentle in the whole process


You need to be  careful as you handle the spark plug and the electrode since when snapped off, your efforts will be in vain. You’ll just need to apply a small amount of pressure when bending the electrode. Damage on the plug will force you to buy a new one.

What Should the Gap Be On My Spark Plugs?

There are many different spark plug gap configurations. This ensures that you get the best performance from your engine. The spark plugs have to be carefully matched with the appropriate application and intended use. 

To get a satisfying performance from a spark plug, there are factors that should correspond with the plug’s gap style, size, and shape.


  • Pressure in the combustion chamber when discharging the spark


Engines with high compression and forced induction have higher pressure in the combustion chamber when discharging a spark. Higher cylinder pressure needs a higher voltage for the spark to occur. Such engines require spark plug gap smaller than .032″.


  • Concentration and type of fuel


When using fuels with high alcohol content, a lower A/F ratio is required and the spark plug gap should be smaller. 


  • The ignition system’s potential


The spark voltage which is usually the electrical pressure or voltage potential corresponds with the distance that a spark can cross at a given environment and air pressure. Running a large gap in high cylinder pressure applications will need a high voltage to have the spark jump the plug’s gap. Consequently, you should consider the ignition potential of your car to determine a suitable gap for your engine’s best performance.


  • Engine load and intended use


The combustion chamber’s pressure is high when the engine is under load. This is because more air is aspirated, thus it becomes harder for the spark to occur. Higher pressure calls for a smaller plug gap while lower pressure requires a larger gap.

What Will Happen When A Plug Gap Is Too Big?

As mentioned above, having the right spark plug gap provides and maintains top engine performance. This means that you’re likely to be faced with some challenges when the gap is narrower.

A wide spark plug gap increases the duration in which the spark travels to ignite the air and fuel mixture. This increased duration, weakens it taking away the sufficient charge necessary for ignition. You will realize that the spark plug gap is wider when there is a possible no-start condition, cylinder misfiring, engine hesitation and rough idle. When the plugs are inspected, they are found to appear black, wet and fouled due to the unburned fuel. The spark plug gap may widen due to normal electrode aging and wear, hence once fixed the engine will work as expected.

What Will Happen If Plug Gap Is Narrower?

Having a narrower gap than expected leads to a decline in the amount of room needed for the air-fuel mixture, the group strap, and hot tip. Subsequently, the spark travels for a small distance which makes it not to remain hot enough with sufficient charge necessary to ignite the mixture of air and fuel. You would learn that the gap is narrower when you get an engine hesitation, continuous cylinder misses and hard starting when there are narrow gaps on all the plugs. 

After inspecting a spark plug, the one that doesn’t fire due to a narrow gap will appear wet or black. The wetness or black appearance is an indication of unburned fuel. 

What will happen if the spark plug gap is wrong?

Below are some of the common symptoms that you would experience with your car when the gap is wrongly placed.

  • An incorrectly placed gap could cause uneven firing of spark plugs leading to a delay in engine combustion. This can cause an engine to idle or miss unpredictably.
  • The engine’s performance is likely to decline as a result of weakened ignition spark, delayed arcing of the ignition spark which alters the engine’s combustion.
  • You could also experience engine hesitation since the spark loses its strength as it crosses the gap.
  • Wrongly gapped spark plugs would result in engine knocking. This is due to the continuous incomplete and delayed engine combustion when accelerating.

It’s now very clear that gapping a spark plug is very crucial for the effective performance and maintenance of your car. It’s always wise to consult your manufacturer or a professional auto mechanic to offer assistance in case of any troubles when gapping the spark plug.  This will ensure that you don’t cause any damage to the electrodes or the plug itself when in the process of measuring or adjusting the gap. 


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