2024 GMC Sierra 2500HD Spark Plug Specs: Type, Gap, Torque & Guide

As an ASE-certified master technician, I know that heavy-duty trucks demand precision maintenance to keep performing at their peak. If you're maintaining a 2024 GMC Sierra 2500HD, it's crucial to understand your spark plug requirements. Note that the 2024 Sierra 2500HD is offered with two engines: the 6.6L Duramax Turbo-Diesel V8 and the 6.6L L8T Gas V8. Since diesel engines utilize glow plugs rather than spark plugs, this guide focuses entirely on the 6.6L Gas V8.

Using the correct spark plug type, achieving the precise gap, and applying the proper torque will ensure your L8T V8 continues to tow and haul reliably.

Quick reference

Note: Always verify exact part numbers with your VIN, as GM occasionally supersedes ACDelco part numbers.

Why the right plug matters

Modern engines like the 6.6L L8T V8 rely on exact spark timing and a very specific heat range to prevent pre-ignition, misfires, and catalytic converter damage.

Heat Range: The heat range dictates how fast the spark plug pulls heat away from its tip and transfers it to the cylinder head. If you use a plug that is too hot, it can cause detonation (engine knock). Too cold, and the plug will foul with carbon deposits.

Electrode Metal: The 6.6L V8 requires Iridium spark plugs. Iridium is significantly harder and more durable than platinum or standard copper. Because Iridium handles the intense cylinder pressures of modern direct-injected engines so well, it allows for a fine-wire center electrode. This concentrates the electrical charge, resulting in a cleaner, more efficient burn and an extended service interval of nearly 100,000 miles. Never downgrade to copper or single-platinum plugs in this engine.

How to gap a spark plug

Even though most premium iridium spark plugs come pre-gapped from the factory, they can be bumped or dropped during shipping. It is best practice to verify the gap before installation.

1. Use the right tool: Do not use a cheap coin-style gapper on iridium plugs, as it can easily snap the fragile fine-wire center electrode. Instead, use a wire-style feeler gauge.
2. Measure: Gently pass the 0.040" (1.0 mm) wire between the center and ground electrodes. There should be a slight drag.
3. Adjust carefully: If the gap is too tight, use a dedicated spark plug gapping tool to gently bend the ground strap outward. Never lever against the center iridium tip. If it is too loose, lightly tap the ground strap on a soft wooden workbench to close it slightly.

Replacement procedure tips

Servicing the plugs on the 2024 Sierra 2500HD is relatively straightforward thanks to its coil-on-plug design, but a few professional practices should be observed:

• Coil-on-plug removal: The L8T uses individual ignition coils mounted on the valve covers with short plug wires bridging the gap. Twist the plug wire boots gently before pulling to break the heat seal.
• The anti-seize debate: GM and ACDelco strongly advise against using anti-seize compound on the spark plug threads. Modern ACDelco and NGK plugs feature a special nickel-plated coating that acts as a release agent. Adding anti-seize alters the thread friction, which frequently leads to over-torquing and catastrophic cylinder head damage.
• Dielectric grease: Apply a pea-sized amount of dielectric silicone grease to the inside of the spark plug wire boots. This prevents moisture intrusion, stops high-voltage tracking (misfires), and ensures the boots won't weld themselves to the ceramic insulator over the next 100,000 miles.
• Torque accurately: Thread the plugs in by hand to prevent cross-threading, then use a calibrated torque wrench to tighten them to 11 lb-ft (15 Nm).

Known issues

While the heavy-duty 6.6L L8T is highly durable and doesn't suffer from the notorious broken two-piece spark plug shell issues seen in older Ford 3-valve engines, there are a couple of things to watch for.

Because the Gen V V8 architecture uses Direct Injection, the ignition coils have built-in ionization sensing used for knock detection and combustion monitoring. Using cheap aftermarket plugs or incorrect resistance plug wires can interfere with this system, leading to phantom misfire codes and rough idling. Always stick to OEM ACDelco or high-tier equivalents like NGK. Additionally, access to the rear cylinders (cylinders 7 and 8) near the firewall can be tight; use a combination of a swivel socket and various extensions to reach them without cracking the porcelain insulators.

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