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
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.
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.
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.