How to Properly Gap a Spark Plug

Properly setting the gap on the electrode at the base of a spark plug is an important skill that all do-it-yourself mechanics should have. Many of the spark plugs sold today come pre-gapped, but should a plug be dropped or hit something, the gap could change. If the gap is wrong, the intensity and quality of the spark will suffer, causing poor fuel/air charge ignition in the cylinders and decreasing the performance of the engine overall.

Things You'll Need

• Feeler gauge

Instructions

1. Open the box and remove the spark plug. Obtain the proper spark gap for your car either from the owner’s manual or off the emissions sticker on the radiator support under the hood.

2. Select the proper blade thicknesses from a feeler gauge to make the desired gap size. If the gap is .45, you may need to combine a .40 and .05 blade or some other similar combination. If you have never used a feeler gauge, it is nothing more than a tool with many thin blades in it that are marked to indicate the thickness of a blade.

3. Slide the selected blades into the gap between the electrode and the ground on the base of the spark plug. The blade should be snug, but you should not have to force it through the gap. If it will not go in, open the gap slightly with a small, thin screwdriver. If it slides right through and does touch the ground or electrode, close the gap up by lightly tapping the base of the plug on a firm surface.

4. Set the plug aside once the gap is set and continue on to the next one. Check every plug before you put them in the car. This will prevent you from having to remove and regap them later.

References

• "Jeep Cherokee Factory Service Manual"; DiamlerChrysler Corp; 1998
• "GMC 1500 Factory Service Manual"; General Motors Corporation; 1995