2020 International LT Spark Plug Type & Gap Guide

As an ASE-certified master technician, I often get questions from fleet managers and owner-operators about ignition maintenance on heavy-duty trucks. When discussing the 2020 International LT, we first have to address the elephant in the room: the vast majority of these trucks are equipped with either the Cummins X15 or the International A26 engines. Both are heavy-duty diesel engines that use compression ignition—meaning they do not have spark plugs.

However, if your truck is spec’d for alternative fuels and runs the Cummins ISX12N Compressed Natural Gas (CNG) engine, spark plugs are a highly critical, frequently serviced maintenance item. Natural gas burns much hotter and requires a very powerful, precisely timed spark to ignite properly. Here is everything you need to know about servicing the ignition system on a CNG-powered 2020 International LT.

Quick reference

Note: Always verify the exact engine serial number (ESN) with your dealer, as CNG ignition components are updated frequently.

Why the right plug matters

Using the correct spark plug is non-negotiable, especially in a commercial CNG application. Natural gas is more difficult to ignite than atomized liquid gasoline, requiring higher firing voltages.

Electrode Metal (Iridium vs. Platinum vs. Copper): For heavy-duty CNG engines like the ISX12N, standard copper or even platinum plugs will erode almost immediately. The extreme combustion chamber temperatures demand premium iridium center and ground electrodes. Iridium is incredibly hard and resists spark erosion and chemical corrosion far better than other metals, which is necessary to maintain the extremely tight gap required for natural gas ignition.

Heat Range: CNG burns extremely hot. An incorrect heat range will either cause the plug to foul (if too cold) or turn the spark plug into a glow plug, causing severe pre-ignition and catastrophic engine damage (if too hot). Always stick strictly to the OEM-specified heat range.

How to gap a spark plug

The gap on a CNG spark plug is exceptionally tight (around 0.015 inches).

1. Use the right gauge tool: Never use a cheap coin-style gap tool on an iridium plug. The fragile iridium tip can easily snap off. Always use precision wire feeler gauges.
2. Measure gently: Slide the wire gauge between the electrodes. It should have a slight drag.
3. The light bend: If the gap needs adjustment, use a specialized gapping tool to lightly bend the ground strap. Never pry against the center electrode. Because the gap is so small, make micro-adjustments and re-measure frequently.

Replacement procedure tips

Replacing plugs on the Cummins ISX12N requires care, as commercial truck environments are harsh.

• Coil-on-Plug Handling: The ISX12N utilizes heavy-duty individual coil-on-plug (COP) ignition coils, connected via long extensions. When removing them, twist gently before pulling to break the seal and avoid tearing the silicone boots.
• The Anti-Seize Debate: Most modern NGK and OEM Cummins plugs feature a specialized trivalent metal plating that acts as its own anti-seize. Adding traditional liquid anti-seize can alter the thread friction, causing you to over-torque the plug and potentially strip the cylinder head. Install them dry unless the specific service manual for your ESN explicitly states otherwise.
• Dielectric Grease: Always apply a small dab of dielectric grease to the inside of the spark plug boot before reinstallation. This prevents moisture intrusion, stops high-voltage tracking (misfires), and ensures the boot won't bake onto the ceramic over the next 45,000 miles.

Known issues

While the 2020 International LT doesn't suffer from the infamous two-piece broken plug issues of early 2000s Ford 3-valve engines, the CNG engine does have its own quirks.

The most common issue is accelerated electrode wear and gap growth. Because CNG operates at such high cylinder pressures and temperatures, a gap that starts at 0.015 inches can quickly erode to 0.020+ inches, leading to a weak spark, hesitation under heavy loads, and triggering misfire codes. Stick strictly to the severe-duty replacement interval (often every 1,000 to 1,500 engine hours or 45,000 miles). Additionally, inspect the coil extensions carefully; the extreme radiant heat from the cylinder head can cause the boots to become brittle and crack, leading to spark jump.

Need plugs replaced? Find a shop on Repairius.