2025 International LT Spark Plug Type & Gap Specs

As an ASE-certified master technician, I often get questions from fleet managers and owner-operators about ignition maintenance on heavy-duty commercial trucks. When discussing the 2025 International LT, there is an enormous elephant in the room: standard Class 8 trucks run on diesel engines (like the International S13 or Cummins X15), which are compression-ignition engines and do not have spark plugs.

However, with the massive industry push toward alternative fuels, the 2025 International LT can be spec’d with the Cummins X15N, a 15-liter Compressed Natural Gas (CNG) engine. Because natural gas has a much higher auto-ignition temperature than diesel, the X15N is a spark-ignited engine requiring specialized spark plugs and coil-on-plug ignition systems.

Here is your comprehensive guide to servicing the spark plugs on the CNG-powered 2025 International LT.

Quick reference

(Note: Never substitute standard automotive spark plugs in a heavy-duty CNG application. Always use the specified industrial Cummins/NGK plugs).

Why the right plug matters

Spark plugs in a heavy-duty natural gas engine operate in one of the harshest environments imaginable. CNG burns significantly hotter and slower than gasoline, and the cylinder pressures in a 15-liter block are immense.

Because of this, you cannot use standard copper or platinum plugs. The X15N requires heavy-duty iridium electrodes. Iridium has an extremely high melting point (over 4,000°F) and resists the aggressive electrical erosion caused by the high-voltage demands of igniting compressed natural gas. Using an incorrect heat range or standard electrode metal will result in pre-ignition, severe engine misfires, melted electrodes, and potential catastrophic piston damage.

How to gap a spark plug

Heavy-duty CNG engines require incredibly tight gaps—often right around 0.015 inches (0.38 mm)—to prevent the spark from blowing out under high cylinder pressures.

1. Use the right tool: Do not use a cheap coin-style gapper on these expensive iridium plugs. Use a dedicated wire gauge tool.
2. Measure carefully: Gently slide the 0.015-inch wire between the center and ground electrode. It should pass through with a slight "drag."
3. Adjusting the gap: If the gap needs adjustment, use a specialized bending tool to manipulate the ground strap. Never pry against the fragile iridium center electrode. Iridium is incredibly hard but brittle; prying on it will snap the tip, ruining a very expensive spark plug.

Replacement procedure tips

Servicing the X15N CNG engine requires attention to detail. This engine uses a coil-on-plug design with long, rigid extensions reaching deep into the cylinder head.

• Clean the plug well: Before removing the old plugs, use compressed air to blow out the deep spark plug wells. Debris falling into a 15-liter engine cylinder will cause catastrophic scoring.
• The anti-seize debate: Cummins generally advises installing spark plugs dry to ensure proper torque and heat transfer. If you must use anti-seize due to extreme fleet environments, use a microscopic amount of nickel anti-seize on the upper threads only, and reduce your torque by 10-15% to prevent over-tightening.
• Dielectric grease: Always apply a small bead of dielectric grease to the inside of the coil-on-plug boots. The heat in the X15N valve cover area is immense; without dielectric grease, the rubber boots will vulcanize to the porcelain, ripping apart during the next service interval.

Known issues

While heavy-duty CNG engines are generally reliable, their ignition systems are highly sensitive.

• Broken plug shells and cracked porcelain: You may have heard horror stories about the notorious Ford 3-valve 2-piece plug breakage or GM 5.3 ionization-sensing coil failures. Fortunately, the Cummins X15N uses a standard one-piece plug design, so the Ford 3-valve extraction nightmares don't apply here. However, because the plug wells are very deep, technicians must use a high-quality magnetic spark plug socket and a locking extension. Allowing the socket to wobble or applying side-load during installation will crack the porcelain insulator, leading to immediate high-load misfires.
• Short lifespan: Unlike modern passenger cars that can go 100,000 miles on a set of plugs, CNG heavy-duty engines chew through plugs. Expect to replace them every 40,000 miles or 1,000 operating hours. Do not push the interval, or coil failure will quickly follow.

Need plugs replaced? Find a shop on Repairius.