Spark plug gap problems

A good question about spark plug gap from a site visitor. He was replacing spark plugs on his Toyota vehicle and noticed that the plugs he was removing had a much larger gap than the plugs he was installing.

Picture of actual spark plug gap

spark plug gap

The difference in gap was so drastic that he thought may be something was wrong, or he had received the wrong replacement spark plugs. The Gentleman decided to buy the correct automotive special tool and gap them as stated in the service manual.

His e-mail question was why the distance was so large on the old plugs that he removed and so small on the new ones. On his vehicle the spark-plugs last 100,000 miles. In that time the center electrode or the tip of the spark plug wears and a large gap is created. After 10 years 100,000 miles this can be noticeable even with the naked eye. Also not all new plugs are gapped correctly at the factory or can change during shipping.

How to Gap spark plugs

Picture of gap measuring tool

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Gap measuring tool

Both new and use spark-plug should have their air gaps not only checked but set the precise manufacturers specifications. On some vehicles the spark plug gap is included on a tag that is often applied to the radiator shroud or sometimes on the inside of the hood. This is also where the size of the engine and the model year are also indicated.

Some manufacturers especially in the case of Toyota provide the spark plug gap in the owner’s manual. On other vehicles you may have to purchase an auto repair manual to achieve the specifications. Mechanics both driveway and professional should use a spark plug adjusting tool.

Picture of gap adjustment tool

gap adjuster tool

This is an inexpensive tool specifically designed to measure the gap between the electrode and the ground strap and also to make adjustments to that air space. These tools are especially designed to perform these adjustments without damaging the spark plug.

I have seen mechanics use screwdrivers and tapping hammers to make these adjustments. Although you can get lucky with this procedure is recommended to spend a few dollars to get the proper tool. This could save you time in the long run.

Always check the air gap of a new spark-plug before installing it. Never assume the gap is correct just because the spark plug is new. Although it might be tempting you should never try to reduce the plugs air gap by tapping it on the ground or hard surface. Try to make small adjustments and check it often. The ground strap is made of a soft material and can be adjusted easier than people think but you do not want to go back and forth more than needed.

Non-adjustable sparkplugs

Some engines are equipped with sparkplugs that have more than one ground strap or electrode. The gap between the center electrodes in each ground strap should be checked. If the gap between the center electrode and one of the straps is less than the others the spark will occur at the smallest gap.

This is also true of the V-shaped ground strap plugs that have become popular aftermarket replacements in recent years. There are also ring fire type plugs that jump from the center to the edge. This is sometimes known as a surface gap and the electrode cannot be adjusted with conventional tools and therefore most manufacturers recommend that the gap be left alone.

Replacing spark plugs has always been a favorite of do-it-yourself driveway mechanics. On some vehicles it is just as easy to replace spark plugs as it was years ago. On other vehicles it has become extremely difficult and should only be done by a professional technician. A good way to determine the difference is to review the procedure in the service manual and decide whether it’s within your skill level. For more articles this next link will take you back to the blog about auto repair.