How to Create a Gap in a Spark Plug

A spark plug needs to have the right gap for the engine to operate smoothly. The gap size affects the spark temperature, directly influencing the combustion of fuel and air in the engine. Widening the gap can produce a larger spark, which is useful for some custom engines aiming for maximum performance. You can learn how to adjust the gap by measuring and making the appropriate adjustments. See step 1 for more detailed information.

Understand the gap specifications for your vehicle's type. Whether you’re buying new spark plugs, checking a newly purchased spark plug before installation, or inspecting the function of the current set, you’ll need to know the proper gap between the two electrodes at the tip of the plug.

• Different vehicles have different gap sizes, typically ranging from 0.07 to 0.15 cm. You can also check your user manual or visit an auto parts store for assistance.
• A tuned engine may require a smaller gap to handle the current flow through the engine. The general rule of thumb: the larger the current, the narrower the gap.

Choose the right gap measuring tool. There are various tools available to measure a spark plug's gap, some of which are better suited for modern spark plugs containing delicate precious metals. Most gap measuring tools have a flat edge, commonly used to gently adjust the electrode gap at the tip of the plug.

• The coin-style gap gauge is typically the most affordable option, where the 'coin' edge is inserted into the gap until it stops. The point where it halts indicates the thickness of the gap. This tool is useful for old spark plugs when you want to check performance, but it can inadvertently widen the gap if used improperly.
• The coin-style wire gauge functions similarly to the regular coin gauge but features wire segments with varying lengths along the edge of the coin.
• The feeler gauge is a highly effective and versatile tool. Similar to a pocket knife, it has various thickness strips, some with wire at the end, others without, and is placed between the electrodes to check the gap. Multiple strips can be used to measure larger gaps, making them especially useful for adjustments.

Clean the spark plug. If you’ve removed the plug from its box, ensure it remains intact, but if you're inspecting the spark plug in your vehicle, it's best to clean it with a clean cloth. The plug may have a slightly white soot buildup on the contact points, so it’s essential to ensure they are clean for accurate results.

• You can use some 90% isopropyl alcohol on the contact points if they're heavily soiled. However, excessive soot or black crust on the contacts may indicate the spark plug has exceeded its useful life.

Measure the gap by passing the tool through the electrodes. Place a feeler gauge or wire tool between the electrodes of the spark plug or pass a coin through the electrodes to check the gap measurement.

Determine if the gap needs adjustment. If the tool passes through the gap without touching the electrodes using the proper measuring method, the gap is too wide. If you can’t insert the tool into the gap, it’s too narrow and needs widening slightly. If the tool fits snugly, you can safely install the spark plug.

• Most modern spark plugs, including iridium plugs, don’t require gap measurement before installation. However, if you own a tuned engine, you may be interested in testing or installing plugs at different gap settings. Adjust them accordingly.

Use a tool to adjust the lower electrode. When holding the spark plug with the electrode facing down, gently bend the lower electrode towards the other electrode to reduce the gap or away from it to slightly widen the gap.

• Never bend more than 1 mm. Apply very little pressure, as the electrode is durable but not that durable.
• If you have trouble using the tool to adjust the gap, bend the tool slightly on a flat surface, such as a table, to apply small pressure on the electrode to adjust it.

Measure the distance again and adjust as needed. Be careful not to touch the center electrode and damage the core. If the electrode breaks or becomes too short, you will need to discard it and purchase a new one.

Always be gentle. Breaking an electrode is time-consuming and easy to do. Only apply slight pressure when bending the electrode, as even a small amount of bending is sufficient.