How To Gap Spark Plugs

Most drivers understand that the spark plug gap, the space between the two electrodes on the spark plug tip, is a critical element of how the component operates. However, some drivers don’t understand that the gap has a direct effect on the operation and exertion put forth by the ignition system to actually start the engine. In other words, if the gap is too large, the system must work harder to ignite the air/fuel mixture. Although, despite your best efforts, the gap will gradually increase over time. Typically, a properly gapped spark plug will last roughly 70,000 miles, depending on the material, before it needs to be replaced.

When replacing your spark plugs, your choice of gapping tool matters. Those coin-style gap tools may not always be the best option. While those tools are OK for traditional copper/nickel spark plugs, many plug manufacturers have moved towards precious metals. These precious metals, such as platinum, iridium, and ruthenium, are harder than their copper/nickel counterparts which allows the center electrode design to be smaller, typically under 1 mmm in diameter. These precious metal tips are more resistant to wear and have a higher melting point than the larger copper/nickel tips, but they are more fragile, especially when gapping. Any lateral force applied to the tiny center electrode could cause the tip to break off. Therefore, before you go using a coin gapper on any spark plug, make sure you understand how it works and why a precise gap matters.

Why Gapping Matters

The distance of the gap on a spark plug depends on the voltage and amperage generated by the ignition coil. The gap specification is determined by the vehicle manufacturer during engine design and is commonly found in the vehicle owner’s manual or the emission sticker under the hood. The accuracy of the gap is critical to an engine’s operation and efficiency. However, let’s further explain for those asking, “what happens when plugs aren’t gapped properly?”

Spark plugs that do not have the proper gap lead to premature wear, ignition system stress, and even cylinder misfires. If the gap is too wide the ignition system components are required to work harder to jump across the extended gap leading to spark plug, ignition coil, and control module failures. It’s also possible the spark could be “blown out” in highly turbulent combustion chambers. A gap that is too small will not fully ignite the air/fuel mixture leading to low power and higher tailpipe emissions.

Check Pre-Gapped Plugs

Pre-gapped spark plugs often create a little confusion or over-confidence. While it is true that many spark plug manufacturers will say that their plugs are indeed pre-gapped, that does not necessarily mean that the plugs are gapped for your specific engine or vehicle. Pre-gapped only means that the device is pre-measured to work in a variety of engines. Therefore, always measure the gap to ensure maximum efficiency and longevity of the plug in your specific engine.

There are a few types of gapping tools you can use.

Using a Feeler Gauge or Wire-Type Gap Gauge

Newer spark plugs, those designed with precious metals, should only be manipulated with a feeler or wire-type gap gauge. While a coin-style tool can be used in a pinch, albeit cautiously, it is far too easy to damage the iridium or platinum metals, which decreases the lifespan of the plug. Also, wire and feeler gauge tools provide a more accurate measurement than coin-style devices because they are designed with a linear surface. Additionally, the linear advantage of these tools provides a way to check the squareness and thickness of the gap, which is difficult with the coin tool. Beyond more accurate measuring, these tools also have a better mechanism for expanding the gap than found in coin-style gauges.

How to Use a Coin-Style Gap Tool

Coin-style gap tools are some of the most common and readily available tools on the market. However, not everyone understands how to use these devices properly, and not every spark plug is designed with this type of tool in mind. Therefore, you will want to locate in your car’s manual the type of spark plug you should be using. If the plug uses precious metals, like iridium, platinum, or ruthenium, then you want to avoid using a coin-style gapper, but if your vehicle uses copper/nickel plugs, then this tool is fine. Although you must use it correctly.

1. Insert ramp between electrodes

The coin tool has a little ramp going around its perimeter that is labeled with measurements. You want to place the narrowest end of the slope between the electrodes of the plug and slowly rotate until you feel a slight resistance. Then, stop. Look at the measurement to determine the current gap.

2. If Too Wide

If the measurement that is revealed is too wide for your engine, then you need to decrease the gap. Thankfully, this is not difficult. Tap the gapped side of the plug against a hard surface gently. If you do this too hard, you can damage the porcelain or the insulator.

3. If Too Narrow

This is when most people make a mistake and use the ramp to widen the gap. Don’t do that. The ramp is only used to measure the gap, nothing else. The appropriate way to widen the gap with the coin tool is to use the predrilled hole. While it might appear to be a place for a key ring, the hole is actually an opening for inserting the ground electrode. Inside of the whole is a lip or edge, which is called the gapper. Hook the ground electrode onto the rim and pry the gap open with the tool. However, be gentle so that you don’t damage the integrity of the plug.

4. Remeasure and Adjust

Be sure to check the gap frequently while making these minor adjustments. A little pressure goes a long way, and if you are not careful, you might adjust too much and weaken the ground electrode strap. Tap or pry a little, and then measure. It is better to work incrementally than to try and force it in one maneuver.

Therefore, the answers to the questions, “why do spark plugs need to be gapped?” and, “how do you gap spark plugs?” are simple. Spark plugs require a precise gap to ensure proper ignition of the air/fuel mixture, and gapping them requires appropriate tools for your vehicle’s specific plugs. Therefore, if your spark plugs need replacing, go out and pick up not only the proper plugs but the tools that will make gapping them easier. Also, if you chose to use a coin-style tool regardless of the plug type, then just remember to be careful and avoid damaging any potential precious metals. You can get the parts you need at your local AutoZone Store. If the job is too big for you, seek out one of our Preferred Shops to help you do the job.

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