2021 Kenworth T680 Spark Plug Type, Gap & Torque Specs

As an ASE-certified master technician, I often get questions from fleet managers and owner-operators regarding the ignition systems on their heavy-duty trucks. If you are looking for spark plug specifications for a 2021 Kenworth T680, we need to clear the air immediately: the vast majority of these Class 8 trucks are powered by PACCAR MX-11, MX-13, or Cummins X15 diesel engines. Diesel engines use compression ignition and do not have spark plugs.

However, if your fleet ordered the 2021 Kenworth T680 with the Cummins Westport ISX12N Natural Gas (CNG/LNG) engine, your truck does rely on a spark-ignited system. Natural gas engines require highly robust ignition systems to ignite the compressed gas, and maintaining these spark plugs is a critical part of your truck's preventative maintenance schedule.

Here is the essential spark plug data for the 2021 Kenworth T680 equipped with the CNG powertrain.

Quick reference

Why the right plug matters

For natural gas heavy-duty engines, spark plugs endure an incredibly harsh environment. CNG has a higher auto-ignition temperature than gasoline, requiring higher firing voltages from the ignition coils to jump the spark plug gap.

Because of these extreme combustion chamber temperatures and voltage demands, natural gas engines require specialized industrial iridium or double-iridium spark plugs. Iridium is significantly harder and more heat-resistant than platinum or copper. Using the exact OEM heat range is non-negotiable in the ISX12N; a plug that is too hot will cause pre-ignition (which can destroy a heavy-duty piston), while a plug that is too cold will foul out, causing misfires and unburned gas in the exhaust, potentially damaging the catalyst.

How to gap a spark plug

Even if your OEM Cummins/NGK spark plugs come pre-gapped, you should always verify the gap before installation—especially given the ultra-tight 0.015-inch gap required for the ISX12N engine.

1. Select the right tool: Use a wire-style gap gauge or specialized feeler gauge. Avoid cheap coin-style gauges, as they can easily snap the brittle iridium center electrode.
2. Measure: Slide the 0.015-inch wire between the center and ground electrode. It should pass through with a slight "drag."
3. Adjust carefully: If the gap needs widening, use the bending tool on your gauge to lightly bend the ground strap away from the center electrode. Never pry against the center iridium tip—it will crack the ceramic insulator or snap the electrode outright. To close the gap, tap the ground strap gently on a soft wooden workbench.

Replacement procedure tips

Replacing spark plugs on a heavy-duty natural gas engine is a bit different from your standard passenger car. The ISX12N uses a coil-on-plug (COP) setup with long coil boots extending deep into the valve cover.

• Clean the area: Before removing the ignition coils, use compressed air to blow out any dirt, debris, or oil around the coil boots. You do not want debris falling into the engine cylinders when the plugs are removed.
• The anti-seize debate: Cummins generally advises installing plugs dry, as the metal shell of modern NGK-supplied plugs features a specialized zinc or nickel plating that prevents seizing in the aluminum cylinder head. Adding liquid anti-seize alters the friction coefficient, which can cause you to over-torque the plug and damage the cylinder head threads.
• Dielectric grease: Apply a small bead of dielectric grease to the inside of the ignition coil boot. This prevents the rubber from baking onto the ribbed ceramic of the spark plug, making your next 1,500-hour service infinitely easier, while also preventing voltage tracking/arcing down the side of the plug.

Known issues

While you won't encounter the infamous Ford 3-valve two-piece spark plug breakage on a Kenworth, natural gas engines have their own quirks.

The most common issue with CNG spark plugs is rapid electrode wear and coil boot degradation. Because CNG requires much higher voltage to strike an arc, the spark plug gap will widen faster than it would in a gasoline engine. If the gap becomes too wide (beyond 0.020 inches), the resistance becomes so high that the ignition coil will overheat and fail, or the voltage will burn through the rubber coil boot and short out against the cylinder head (known as carbon tracking). Sticking strictly to the 50,000-mile/1,500-hour replacement interval is critical to preventing roadside coil failures.

Need plugs replaced? Find a shop on Repairius.