2015 Kenworth T680 Spark Plug Type, Gap & Torque Specs

As an ASE-certified master technician, I often get questions about spark plugs on heavy-duty Class 8 trucks like the 2015 Kenworth T680. It’s important to clear the air immediately: the vast majority of T680s are equipped with the PACCAR MX-13 or Cummins ISX15 diesel engines, which do not have spark plugs because they rely on compression ignition.

However, if your T680 is running on CNG or LNG (Compressed/Liquefied Natural Gas), it is equipped with the Cummins Westport ISX12 G spark-ignited natural gas engine. Unlike diesel, this engine relies heavily on a robust coil-on-plug ignition system. Here are the exact spark plug specifications for the natural gas T680.

Quick reference

Why the right plug matters

When dealing with natural gas engines like the ISX12 G, the right spark plug isn't just a recommendation—it's critical for the engine to run. Natural gas burns significantly hotter and is harder to ignite than atomized gasoline. Because of this, Cummins utilizes specialized, heavy-duty iridium-tipped spark plugs.

Iridium provides exceptional resistance to the intense thermal environment and high cylinder pressures of a dedicated CNG/LNG engine. Using a cheaper platinum or standard copper plug will result in rapid electrode erosion, severe misfires, and potential catalytic converter damage. Stick strictly to the OEM Cummins-specified heat range.

How to gap a spark plug

Cummins natural gas spark plugs typically come pre-gapped, but a master tech always verifies before installation. The ISX12 G requires a very tight gap (around 0.015 inches).

1. Use a wire-style gauge or feeler gauge: Avoid the cheap coin-style gauges, as they can easily snap the brittle iridium center electrode.
2. Light bend: If adjustment is necessary, use a dedicated gapping tool to bend the ground strap slightly. Never pry against the center electrode or the ceramic insulator.
3. Double-check: Ensure the gauge slides through with a slight drag.

Replacement procedure tips

Servicing the ISX12 G in your T680 requires precision. These engines use a coil-on-plug setup with long boot extensions reaching down into the deep cylinder head wells.

• Coil-on-plug removal: Carefully twist the coil boots before pulling up to avoid tearing the silicone. If a boot tears and stays on the plug, you'll need long-reach needle-nose pliers to fish it out.
• The anti-seize debate: Do not use anti-seize on the threads unless explicitly directed by the updated Cummins service bulletin. The plugs feature a special plating designed to prevent seizing; adding liquid anti-seize can alter the torque friction, leading to over-tightening and stripped cylinder heads.
• Dielectric grease: Always apply a small dab of dielectric grease to the inside of the coil boot to prevent moisture intrusion, stop high-voltage tracking, and make future removal easier.

Known issues

The biggest known issue with spark-ignited heavy-duty engines is accelerated plug wear and boot degradation. Because natural gas requires higher voltage to jump the spark plug gap, the ignition coils work incredibly hard. The heat from the ISX12 G can bake the coil boots over time, leading to micro-cracks and spark tracking (where the spark grounds out on the cylinder head instead of jumping the plug gap). Always replace the boots when you replace the plugs at the 1,000-hour mark.

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