What the heck is spark plug indexing?

Indexing involves installing and torquing the spark plug so the ground electrode is facing away from the fuel injector or intake valve

High ignite-ability is really important in late model engines with direct injection lean burn fuel systems. To ignite the lean mixture, carmakers require spark plug indexing where you screw in the plug and tighten the plug with a torque wrench. When it reaches the specified torque, it will be properly aligned so the flame kernel can spread most efficiently.

Why is spark plug indexing important

On some late model engines equipped with gasoline direct injection, it’s important for the open end of the spark plug’s ground electrode to not be directly facing either the fuel injector or the intake valve.

Once fully torqued, the open section of the spark plug, where the center electrode can be seen, will be within the center of a 45° range of the injector and intake valve. This final positioning allows for “high ignite-ability.”

How to install a spark plug in an index-designed engine

At the present time (Spring 2023), the following manufacturers require spark plug indexing in some of their engines:

• Automotive: Mercedes Benz, Land Rover, Jaguar, Nissan/Infiniti, and Freightliner
• Non-Auto: Ski-Doo Snowmobiles w/E Tech Engines

The cylinder head spark plug threads and the recommended spark plugs have their threads oriented so the spark plug is properly indexed when installed to specification using a torque wrench.

1) Start threading the spark plug by hand using the proper spark plug socket
2)Then tighten to spec using a calibrated torque wrench.

Do not use an air or battery powered ratchet to install spark plugs, especially in an index-designed engine.

Remember, when properly installed the open end of the spark plug should be facing the GDI fuel injector and the intake valve. The only way to accomplish this orientation is to use a torque wrench.

Also, never install a non-indexed spark plug in these applications.

©, 2023 Rick Muscoplat