2020 Kenworth T680 Spark Plug Type & Gap Guide

When discussing spark plugs for a heavy-duty Class 8 commercial truck like the 2020 Kenworth T680, we have to start with a massive clarification. The vast majority of T680s are equipped with diesel engines—specifically the PACCAR MX-13, MX-11, or the Cummins X15. Diesel engines do not have spark plugs. They rely on extreme compression to ignite the fuel.

However, the 2020 Kenworth T680 is also offered with the Cummins ISX12N (11.9L) Natural Gas (CNG/LNG) engine. Unlike diesels, natural gas engines use a spark-ignited combustion cycle and absolutely require spark plugs. Maintaining these plugs is one of the most critical aspects of keeping a CNG rig on the road, as natural gas combustion is incredibly hard on ignition components.

Here is the exact spark plug data you need for the natural gas-powered 2020 Kenworth T680.

Quick reference

Why the right plug matters

Using the correct spark plug in a heavy-duty natural gas engine is not a suggestion; it is a strict requirement. Natural gas burns much hotter and drier than liquid fuels like gasoline. Because there is no liquid fuel to provide a latent cooling effect in the combustion chamber, CNG spark plugs operate in a brutal thermal environment.

To survive these conditions, OEM and high-tier aftermarket plugs for the ISX12N feature heavily reinforced iridium electrodes. Copper and platinum plugs will melt or erode in a matter of days under heavy hauling loads. Furthermore, maintaining the exact heat range dictated by Cummins ensures the plug tip stays hot enough to burn off combustion deposits, but cool enough to prevent pre-ignition and catastrophic piston damage.

How to gap a spark plug

Even if you are installing pre-gapped OEM Denso or Cummins iridium plugs, you must verify the gap before installation. Shipping drops can easily close a gap, leading to heavy-load misfires.

1. Use the right tool: Do not use cheap coin-style gap tools on heavy-duty iridium plugs. The central electrode is fragile and can easily snap. Use a wire-type feeler gauge.
2. Measure gently: Slide the 0.015" (0.38 mm) wire between the center and ground electrode. It should pass through with a slight "drag."
3. Adjusting the gap: If the gap is too tight, use the bending notch on the gapping tool to grab only the ground strap. Apply a light bend to open it up. Never pry against the center iridium tip.

Replacement procedure tips

Replacing plugs on the Cummins ISX12N in your T680 involves a specialized coil-on-plug/coil-near-plug setup connected via ignition extensions (boots).

• Clean the tubes: Before removing the old plugs, use compressed air to blow out the deep spark plug tubes in the cylinder head. Road grime and dust easily settle here and will fall straight into your cylinders if not cleared.
• The anti-seize debate: Cummins strictly dictates torque values without anti-seize. The metal plating on premium Denso/Cummins plugs acts as its own release agent. Applying anti-seize alters the friction coefficient, causing you to over-torque the plug, which stretches the threads and can distort the plug's internal ceramic seal.
• Dielectric grease: Always apply a small dab of dielectric grease to the inside lip of the ignition coil boot. Natural gas engines require extremely high voltage to jump the spark gap. Without dielectric grease, high-voltage electricity will arc down the side of the ceramic (carbon tracking) instead of firing at the tip.

Known issues

Short Lifespans and Carbon Tracking: The most common issue with natural gas commercial engines like the ISX12N is ignition system wear. A CNG engine eats spark plugs. While a passenger car might get 100,000 miles out of an iridium plug, a CNG T680 will demand new plugs every 1,000 engine hours (roughly 30,000 to 40,000 miles). Ignoring this interval leads to severe misfires, reduced fuel economy, and expensive damage to the ignition coils. Additionally, if the ignition coil extension boots are not replaced regularly, they will develop microscopic pinholes, leading to voltage leaks and misfires under heavy boost.

Need plugs replaced? Find a shop on Repairius.