2025 GMC Sierra 2500HD Spark Plug Type & Gap

Quick reference

Why the right plug matters

Choosing the correct spark plug for your 2025 GMC Sierra 2500HD is crucial for optimal engine performance, fuel efficiency, and emissions. There are several key factors to consider:

• Heat Range: Spark plugs are designed with a specific "heat range" that dictates how well they dissipate heat from the combustion chamber. A plug that's too "hot" (dissipates heat slowly) can lead to pre-ignition and engine damage, while a plug that's too "cold" (dissipates heat too quickly) can foul with carbon deposits. Your Sierra's engine is engineered for a precise heat range, and using anything outside of that can compromise performance and longevity.

• Electrode Metal: The material of the center and ground electrodes significantly impacts a spark plug's lifespan and performance characteristics:

  • Iridium: These are the most common type for modern vehicles due to their extremely fine electrodes and exceptional durability. Iridium plugs offer superior ignitability and can last for extended intervals, often up to 100,000 miles.
  • Platinum: Platinum plugs are also long-lasting, typically offering service intervals of 60,000-100,000 miles. They provide good performance and are a step up from traditional copper plugs.
  • Copper: While still used in some older or specialty applications, copper plugs have the shortest lifespan (around 20,000-30,000 miles) due to their softer electrode material. They are excellent conductors but wear quickly.

The 2025 GMC Sierra 2500HD's 6.6L L8T V8 gasoline engine is designed for long-life iridium spark plugs from the factory, which contribute to its relatively long service interval.

How to gap a spark plug

While most modern iridium spark plugs come pre-gapped from the factory, it's always a good practice to verify the gap before installation. This is especially true for aftermarket plugs, as they might need minor adjustments to match your engine's exact specifications. A spark plug gap tool (either a coin-style or wire gauge tool) is essential for this task.

To gap a new plug:

1. Use a wire gauge tool: Slide the correct wire gauge (e.g., 0.040 inches for your Sierra) between the center and ground electrodes.
2. Adjust if necessary: If the gauge doesn't slide through with slight resistance, carefully bend the ground electrode. If the gap is too small, use the gapping tool to gently pry the electrode open. If it's too wide, carefully tap the ground electrode on a clean, hard surface to close the gap. Never bend or exert force on the center electrode, especially with iridium plugs, as this can easily damage the delicate tip.
3. Re-check: Always re-check the gap after any adjustment to ensure it falls within the specified range.

Replacement procedure tips

Replacing spark plugs on your 2025 GMC Sierra 2500HD's 6.6L L8T engine requires careful attention to detail:

1. Engine Cooling: Always ensure the engine is completely cool before attempting to remove spark plugs. Removing plugs from a hot aluminum cylinder head can lead to stripped threads.

2. Coil-on-Plug Removal: The 6.6L L8T uses a coil-on-plug (COP) ignition system. Each spark plug has its own ignition coil mounted directly above it. Disconnect the electrical connector from each coil and then unbolt the coil (typically one or two small bolts) before gently pulling it straight up and off the spark plug. Be careful not to damage the coil or the associated wiring.

3. Anti-Seize Debate: There's an ongoing debate about using anti-seize on spark plug threads. For spark plugs with nickel-plated shells, manufacturers like NGK and Denso generally advise against using anti-seize, as it can alter torque readings and potentially lead to over-tightening and plug breakage or cylinder head damage. The nickel plating provides adequate anti-seize properties. If you opt to use anti-seize (and only a very thin coating), reduce the specified torque by roughly 10-20% to compensate for the lubricity.

4. Dielectric Grease on Boots: A small amount of dielectric grease applied inside the spark plug boot (where it contacts the ceramic insulator of the plug) can help prevent moisture intrusion and make future removal easier. Be cautious not to get grease on the electrode or the inside of the boot where the spring contact sits.

5. Torque is Critical: Using a torque wrench is paramount during installation. Under-tightened plugs can vibrate loose, cause misfires, and transfer heat inefficiently. Over-tightened plugs can damage the cylinder head threads or even crack the spark plug insulator. Stick strictly to the manufacturer's recommended torque specification.

Known issues

Fortunately, the 6.6L L8T V8 engine in the 2025 GMC Sierra 2500HD is not widely known for specific spark plug-related issues like the Ford 3-valve engine's propensity for broken two-piece plugs or certain GM engines with ionization-sensing coils. However, always be vigilant for common signs of spark plug wear or failure, such as:

• Misfires: A rough idle, hesitation, or a "check engine" light with a misfire code often points to a spark plug issue.
• Reduced Fuel Economy: Worn-out plugs can lead to inefficient combustion.
• Loss of Power: An engine that struggles to accelerate can be a symptom of failing spark plugs.

Always use genuine ACDelco or recommended aftermarket plugs to ensure compatibility and performance.

Need plugs replaced? Find a shop on Repairius.