2020 GMC Sierra 2500HD Spark Plug Type & Gap

Quick reference

Why the right plug matters

The spark plug is a small but critical component in your engine's ignition system. It ignites the air-fuel mixture in the combustion chamber, and using the correct spark plug for your 2020 GMC Sierra 2500HD is paramount for optimal performance, fuel efficiency, and to prevent potential engine damage. There are several key factors to consider:

• Heat Range: Spark plugs are designed with a specific "heat range," which refers to their ability to dissipate heat. A plug that's too "hot" (meaning it dissipates heat slowly) can lead to pre-ignition or knocking, while a plug that's too "cold" (dissipates heat too quickly) can lead to fouling. The correct heat range ensures the plug operates at an optimal temperature, preventing both scenarios.

• Electrode Metal: The material of the center electrode significantly impacts the plug's lifespan and performance:

  • Iridium: This is the most durable and conductive material, offering the longest lifespan and best performance. OEM plugs for modern vehicles like your Sierra 2500HD are typically iridium due to their longevity and consistent spark delivery. They are ideal for high-performance engines and extended service intervals.
  • Platinum: Platinum plugs offer good durability and performance, often outlasting copper plugs. They are a good balance between cost and longevity.
  • Copper: While copper is an excellent conductor, it's softer and wears down quicker. Copper plugs are usually found in older vehicles or those with shorter service intervals.

Using the recommended iridium spark plugs ensures your 6.6L L8T engine runs efficiently and reliably for its intended service life.

How to gap a spark plug

Setting the correct spark plug gap is crucial for a strong, consistent spark. Even pre-gapped plugs should be checked, as shipping and handling can sometimes alter the gap. For your 2020 GMC Sierra 2500HD with the 6.6L L8T engine, the recommended gap is 0.040 inches (1.02 mm).

To gap a spark plug:

1. Use a wire gauge tool: A wire-type spark plug gapping tool is preferred over a flat-blade type, as it provides a more accurate measurement and is less likely to damage the fine electrode of an iridium plug. Always refer to the manufacturer's specifications for the exact gap.
2. Measure the gap: Carefully slide the correct wire gauge through the gap between the center and ground electrodes. It should slide through with a slight drag.
3. Adjust if necessary: If the gap is too small, use the gapping tool's adjuster to gently bend the ground electrode outwards. If the gap is too wide, gently tap the ground electrode on a clean, solid surface to close the gap. Never pry on the center electrode, especially with iridium plugs, as this can easily damage the fine tip.
4. Re-measure: Always re-measure after any adjustment to ensure accuracy.

Replacement procedure tips

Replacing spark plugs on your 2020 GMC Sierra 2500HD 6.6L L8T V8 is a relatively straightforward process for the average DIYer, but some considerations will make the job smoother:

• Cool Engine: Always perform this job on a cool engine to avoid burning yourself and to prevent the possibility of damaging the cylinder head threads, which can happen if you remove plugs from a hot aluminum head.
• Coil-on-Plug System: Your Sierra 2500HD uses a coil-on-plug (COP) ignition system. This means each spark plug has its own individual ignition coil directly mounted on top of it. To access the spark plugs, you'll need to:
  1. Disconnect the electrical connector from the ignition coil.
  2. Remove the bolt securing the ignition coil to the valve cover.
  3. Gently pull the ignition coil straight up and off the spark plug. Be careful not to damage the rubber boot.
• Anti-Seize Debate: There's ongoing debate regarding the use of anti-seize on spark plug threads. Modern spark plugs often have a special coating on the threads that acts as an anti-seize. Applying additional anti-seize can alter the torque readings, potentially leading to over-tightening and damage. ACDelco, the OEM supplier, generally recommends against using anti-seize. If you do choose to use it, apply a very small amount and reduce the torque specification by approximately 10-20% to compensate. However, for most applications, it's best to follow the manufacturer's recommendation and install them dry.
• Dielectric Grease on Boots: A small amount of dielectric grease applied to the inside of the spark plug boots (the rubber part of the coil pack that slides over the spark plug) can aid in future removal and provides an extra layer of insulation against moisture and corrosion. Apply a thin film to the inside of the boot before reinstallation.
• Torque Wrench: Always use a torque wrench to tighten spark plugs to the specified 11 lb-ft (15 Nm). Over-tightening can strip cylinder head threads or damage the spark plug, while under-tightening can lead to poor electrical contact and combustion gas leakage.

Known issues

The 6.6L L8T engine in the 2020 GMC Sierra 2500HD is a relatively new and robust engine. As such, there are no widespread, commonly known spark plug-specific issues like the cracked porcelain on Ford 3-valve engines or broken plug shells seen in some other applications. Regular maintenance with the correct OEM ACDelco parts or equivalent NGK Iridium plugs, combined with proper gapping and torque, should ensure trouble-free operation.

Need plugs replaced? Find a shop on Repairius.