Know-How Notes: How To Set Spark Plug Gap

Know-How Notes: 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 gap spark plugs, you probably didn’t even know that was a thing. Yes, spark plugs are manufactured with a gap, but that gap is rarely what it is supposed to be on your particular vehicle. There are several factors that alter the 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.

A standard spark plug with a large electrode can be gapped with any gapping tool, but the plugs with thin electrodes require the tool we are using in this article. .

A standard spark plug with a large electrode can be gapped with any gapping tool, but the plugs with thin electrodes require the tool we are using in this article. .

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 gap to the ground electrode. The space between the two electrodes is call the 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 gap, there will not be as much spark area to flame to grow, leaving you with unburned fuel. If the gap 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.

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. Iridium electrode plugs can be damaged with the typical keyring style tools, so you have to use a proper gapping tool.

This is an E3 brand spark plug, it is factory gapped, it cannot be altered.

This is an E3 brand spark plug, it is factory gapped, it cannot be altered.

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 tool costs a few bucks and it is great for checking and adjusting gaps for older engines. Newer engines used really large gaps, and you may need a different tool.

This tool costs a few bucks and it is great for checking and adjusting gaps for older engines. Newer engines used really large gaps, and you may need a different tool.

To check the 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.

Checking the gap is easy. Simply slide the wire between the electrode and ground strap.

Checking the gap is easy. Simply slide the wire between the electrode and ground strap.

 

The wire should just touch both parts, it should not be tight or have a gap.

The wire should just touch both parts, it should not be tight or have a gap.

Setting Spark Plug Gap

If the gap is too small or too big, you need to adjust the ground electrode to match the gap. 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.

This feeler gauge also works for measuring the gap.

This feeler gauge also works for measuring the gap.

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 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. 

The hook on the tool is for gapping the plug. Simply pry up gently until you have reached the specified gap.

The hook on the tool is for gapping the plug. Simply pry up gently until you have reached the specified gap.

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 gap ensures the best running conditions for your engine. 

Check out all the electrical & ignition system products available on NAPA Online or trust one of our 16,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on gapping spark plugs, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

about author

Jefferson Bryant

A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.

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