How To Set Spark Plug Gap

As time progresses, the little details of mechanics are often lost. Unless your parent or grandparent taught you how to set spark plug gap, you probably didn’t even know that was a thing. Yes, spark plugs are manufactured with a gap, but that spark plug gap is rarely what it is supposed to be on your particular vehicle. There are several factors that alter the spark plug gap range for the the plugs, and the difference can mean running, running well, and running great. Your fuel economy will suffer to if not adjusted correctly. That’s why learning how to set a spark plug gap is so important.

Every time your ignition system sends a spark down the plug wire to the plug, it reaches the center electrode and has to jump across the spark plug gap to the ground electrode. The space between the two electrodes is call the spark plug gap. There are base standards for the gap, specifically .025” for a stock street engine, and .028” for a performance engine. These are just the starting points, and every engine manufacturer has a specific setting or range for the plugs for their ignition system. These standards are for rotary distributor engines with a single coil. Most modern vehicles run distributorless coil-on-plug systems that run gaps as high as .06”. If you run a tight spark plug gap, there will not be as much spark area to flame to grow, leaving you with unburned fuel. If the gap on a spark plug is too big, then the spark may struggle to jump the gap at all. Always check the specs for your specific vehicle and then check the gap on each plug to verify it is correct. Unless you are a serious engine tuner, stick to the spark plug gap specifications for your engine or else you could wind up with an engine that doesn’t run right.

Spark Plug Type Matters

The type of spark plug makes a difference on gapping. Multiple electrode plugs like E3 are vehicle specific and gapped at the factory, so there is no way to really figure out what is the spark plug gap on this kind of plug or change it. Iridium electrode plugs can be damaged with the typical keyring style tools, so you have to use a proper spark plug gapping tool.

If you don’t have one of these special spark plugs then you won’t have any problems setting spark plug gap.

Checking Spark Plug Gap

It is a good idea to check the gap on your plugs. This can be done with a keyring tool, commonly sold at the counter of your favorite NAPA Auto Parts Store, or with gapping tool such as SER 166 for just a few dollars. This is one of those tools that everyone should have one in their toolbox.

To check the spark plug gap, insert the keyring tool, wire tool, or feeler gauges into the gap between the two electrodes. Do this carefully so that you don’t damage them. You are looking for the snug fit, not tight, not loose. You should feel slight drag as the tool passes through the spark plug gap, but it should not feel forced.

How To Gap A Spark Plug

If the spark plug gap is too small or too big, you need to adjust the ground electrode to match the gap. Luckily setting spark plug gap is easy. How you do this depends on the type of plug you have. If you are using a standard spark plug with a large center electrode, the keyring tool works just fine.

The keyring tool has a chamfered hole at the top, where you might put it on a keyring. Place the ground electrode inside the hole and then lightly lift up on the opposite side of the tool to open the gap. If you need to close the gap, a small pair of needle nose pliers works well and does not risk damaging the electrodes. Iridium and platinum plugs have a very thin electrode, and a keyring tool will damage them.

Instead, the tool we are using will work on any gappable plug without damaging it. Use the slotted keyway on the tool to open or close the spark plug gap. This does not put any stress on the center electrode. Then re-check the gap. You can pry on the bottom of the tool to close a gap that is too large. 

All spark plugs should be gap checked, this includes your vehicle’s engine, motorcycles, lawnmowers, jet skis, any engine that has a spark plug. The proper spark plug gap ensures the best running conditions for your engine. Now that you know how to gap spark plugs you can keep your own engines running in peak condition. Just make sure to change your spark plugs at the interval specified in your vehicle owner’s manual for maximum engine efficiency.

Check out all the electrical & ignition system products available on NAPAOnline or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on gapping spark plugs including what is the gap on spark plugs for your vehicle, chat with a knowledgeable expert at your local NAPA Auto Parts store.