Step 1
Locate a gap gauge or feeler gauge.
Step 2
Clean the spark plug. If it is new, this shouldn't be an issue; however, if you are checking a spark plug that is currently in the vehicle, it might be dirty on the contact points. Never use abrasive materials to clean a spark plug.
Step 3
Measure the existing gap of the spark plug. Using your gap gauge or feeler gauge, run the tool through the electrodes to determine the measurement. Take note of the measurement and compare it to the recommended setting listed in the owner's manual.
Step 4
Make any needed adjustments. Select the correct measurement on your gap gauge or feeler gauge. If you can’t get the correct feeler gauge through the gap, the gap needs to be widened. If the tool runs through the gap without touching the electrodes, the gap is too big and must be narrowed.
Use your spark plug gap tool to adjust the gap. Gently bend the ground electrode in to narrow the gap or bend it out to widen it.
Be very careful when making adjustments. Don't bend the electrode more than a few fractions of an inch. It is durable but isn't designed to take a lot of pressure. Also, take care not to hit the center electrode. If you snap off the bottom electrode or damage the center electrode from excessively prying on it, you'll need a new spark plug. Precious metal iridium or platinum center electrodes can be very delicate when it comes to prying them. It is important to use the proper spark plug gapping tool.
Step 5
Check the gap again. Repeat the adjustment process until the tool fits closely between the electrodes.
