Gapping Spark Plugs

Gapping Spark Plugs

For modern fine-wire spark plugs, you should not measure the gap unless instructed by the manufacturer.

Article courtesy Underhood Service.

For a long time, the copper core spark plug ruled the roads. For under a dollar, you could buy a plug that would fit, but you had to gap the plug the correct distance between the electrodes yourself.

This was due to differing ignition system designs with different output voltages. You could even adjust the gap to suit modifications to the vehicle. But the softer electrodes made of copper, chromium or nickel-iron tended to erode every time the spark plug fired. Eventually, the gap would open up or the shape of the center electrode changed to the point where it would cause a misfire. 

In the 1990s, ignition systems changed with the introduction of fine-wire spark plugs. These plugs took advantage of increased ignition system outputs by utilizing small-tip electrodes made of, or plated with, hard precious metals like platinum, palladium or iridium. These plugs could last longer because the metals did not erode as fast as softer metals.

These improvements have changed the spark plug from a 25,000-mile item to a 100,000-mile item. Over the past decade, we have seen spark plug design grow by leaps and bounds. It’s not uncommon to see a specific spark plug designed just for one engine’s combustion chamber design and flame front.

Gapping Methods

For modern fine-wire spark plugs, you should not measure the gap unless instructed by the manufacturer because you could damage the center electrode. But, it is still a good idea to inspect the new spark plugs for damage or defects. The majority of these plugs come pre-gapped from the factory.

On these types of plugs, bending the ground electrode(s) can alter the geometry and thermal performance of the electrodes to the point where it could cause a misfire.

On older applications, gapping may still be required. Gapping specifications can be found in the service information or emissions tag under the hood. Also, gapping specifications can be found in electronic and paper catalogs.

Measuring the gap requires the right tools. A coil-style gap gauge will work for copper core plugs. For fine-wire or precious metal plugs, it is recommended to use a wire-type gauge.

If it is necessary to widen the gap, do so with a tool that only pulls back on the ground electrode without touching the center electrode or the porcelain. To close the gap on a plug, gently tap the plug, electrode first, on a hard surface.

You May Also Like

ECM Damage

Engineers have devised two strategies that can be called the “immune system” for the electrical system.

With even the most basic vehicles having more than 10 modules connected to hundreds of circuits, there is always a possibility that wires can get crossed. Engineers want to prevent damage to wiring, circuit boards and sensors if something goes wrong that causes a short or open. To avoid damage and possible fires, engineers have devised two strategies that can be called the “immune system” for the electrical system.

Fuel Pumps and Cranking

Diagnosing the problem comes down to understanding what causes a loss of fuel pressure.

PCV System DTC

PCV systems control crankcase pressure.

A/C Condenser Clogs

When a compressor fails, the question for most technicians is whether to flush the condenser or replace it.

Checking The Auxiliary Water Pump

A car or truck comes into the shop with a complaint of poor heater performance. The issue may be the auxiliary water pump.

Tech holds up heater core

Other Posts

It’s Got Spark!

Why can’t you trust some spark tests?

Understanding Differences Between Aluminum And Cast Iron

Sheer material differences aside, what are the features and benefits of aluminum and iron engine components?

Diagnosing Misfires

What if there are no codes and a misfire is intermittent? This is where it gets complicated. 

Fuel Pressure Diagnostic Service

Use a scan tool that can look at special direct fuel injection parameters and perform bidirectional tests.