What is Spark Plug Gap?
The spark plug gap is where spark plug spark discharge is designed to take place. In a conventional spark plug, this is the area between the center and the ground electrode.
Since the spark always follows the path of least resistance, the spark gap is generally the closest point between the center electrode of the spark plug and the ground electrode of the spark plug, which is sometimes formed by the spark plug cup itself.
The only case the spark will travel a longer path to the ground is when the longer path is more conductive (offers less resistance). This can be caused, for example, by the loss of the insulating properties of ceramics due to conductive carbon that accumulates from the combustion process (spark plug fouling).
Setting a large spark plug gap requires a higher voltage (electrical pressure) so that the spark splits the large gap. This means that a higher voltage builds up in the ignition system (ignition coil, distributor, ignition cable) before the spark discharge.
This is generally desirable in high power ignition coil applications and later naturally aspirated, lower compression engines where cylinder pressure is relatively low and spark discharge can easily occur.
However running a large gap in a high cylinder pressure application (which makes it very hard for a spark to occur), will demand such a high voltage (electrical pressure) for a spark to jump the spark plug gap.
That the spark will find an easier way to the ground, possibly where the ignition wire is close to the ground (engine block, etc.) or inside of distributor cap, inside of the ignition coil, on the outside of the spark plug (between the plug and the boot), etc.
In either case, the spark will not occur between the spark plug electrodes in the combustion chamber and the engine will misfire.
Importance of the Spark Plug Gap
When replacing spark plugs, ensuring that the plugs have the proper gap is critical to engine performance. The gap is the distance between the center and side electrodes, set so arcing occurs at the proper voltage that ignites the fuel and generates the combustion that makes the engine run.
Do Spark Plugs Always Have to Be Gapped?
Not always. In the past, it was necessary to gap spark plugs, but today spark plugs are usually pre-gapped. It is advisable to double-check that the gap is correctly set to the vehicle’s recommended setting when installing spark plugs.
Finding the Proper Gap Setting
Spark plugs in automobiles generally have a gap between 0.6 and 1.8 mm (0.024 and 0.071 in). The gap may require adjustment from the out-of-the-box gap. A spark plug gap gauge is a disc with a sloping edge, or with round wires of precise diameters, and is used to measure the gap.
The gap setting is different for each vehicle but most are somewhere between 0.028″ and .060″. Consult the owner’s manual to find the recommended setting for the vehicle you’re working on.
How Do I Gap a Spark Plug?
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 the necessary adjustments.
Choose the right size for your gap or feeler gauge. If you can’t get the correct feeler gauge through the gap, the gap will need to be increased. If the tool passes through the gap without touching the electrodes, the gap is too large and needs to 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. Do not bend the electrode more than a few inches. It’s durable, but not designed to take a lot of pressure. Also, be careful not to hit the center electrode.
If you break the bottom electrode or damage the center electrode by prying it excessively, you will 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 correct spark plug splitting tool.
Step 5: Check the gap again.
Repeat the adjustment process until the tool fits closely between the electrodes.
Consequences of Incorrect Spark Plug Gap
If the spark plug gap is incorrectly set, it can lead to engine issues. The customer may experience loss of power, misfires, spark plug fouling, increased plug wear, or poor gas mileage. Too small of a gap may give too weak of a spark to complete the combustion process within the engine; too wide of a gap can lead to the spark plug not firing correctly, causing misfires at high speeds.