Spark plug gapping is overlooked so often, yet it’s a vital step in almost every tune up. There are many different cars and trucks on the road – so to make things easier on dealerships, manufacturers, and auto parts stores, each spark plug is designed to fit multiple applications. Because of this, however, when you pull that spark plug out of its box, the electrode may not be properly gapped for your specific car or truck. So let’s take a look at how to measure and adjust the gap on a spark plug.

By “gap” we are talking about the space between the electrode and the spark plug’s ground (see fig.1). Lucky for us, engineers have found a way to imbue gapping tools with measuring technology.

Step one will be picking your tool of choice. Gapping tools come in a number of styles (fig.2), so choose the one that suits your needs best. I like to use the coin style – there are just less swinging and sliding pieces to mess with.

Now, you have to know what your plug’s gap should be. Sometimes this is listed in an owner’s manual, and it is always listed in a repair manual. If you’re unsure, call up your local auto parts store. That information is always available to your parts professionals. Remember to always confirm your gap when you purchase new plugs. Sometimes the gap will differ based on the brand of plug you purchase!

Now you’re armed with everything you need: plugs, gapping tool and knowledge of your proper spark plug gap.

Measure the current gap of the plugs.

In measuring the gap of your spark plugs, you want the gapping tool to be snuggly in the gap. (fig.3) You shouldn’t force the gapping tool into the gap. You want an accurate measurement without widening the gap. That measurement will be your starting point. If you’re lucky, the plug is already gapped to where you need it. If not, then it’s time to adjust the gap a bit.

If the gap is too narrow, you need to carefully angle the gapping tool to press against the ground to bend it away from the electrode (fig.4). The gapping tool should be lifted away from the electrode – so as to not damage it – and never hard enough to break off the ground. After you adjust the gap, measure it again.

If the gap is too wide, the ground needs to be bent down closer to the electrode. You’ll want a hard surface with a little bit of give – a wooden workbench works great for this. Place the spark plug on its electrode end and press down just hard enough to bend the ground inward. Remember, we’re working with fractions of an inch, so a little pressure is all you need. After you adjust the gap, measure it again.

Once your spark plugs are at the appropriate gap, they’re ready to be installed!

Remember!

Not all spark plugs are meant to be gapped. Iridium spark plugs and multi-ground spark plugs should not be gapped. In addition to these, there are specialty plugs and pre-gapped plugs. So always check with your parts professional before gapping your new plugs!