2021 Chevrolet Silverado 2500HD Spark Plug Specs & Replacement Guide

As an ASE-certified master technician, I’ve serviced countless heavy-duty trucks, and the 2021 Chevrolet Silverado 2500HD is a frequent visitor to the bay. Whether you use it for hauling heavy equipment or long-distance towing, keeping the ignition system in top shape is critical for maintaining power and fuel economy. Below, I’ll walk you through everything you need to know about replacing the spark plugs on your 2021 Silverado 2500HD equipped with the 6.6L L8T gas V8.

(Note: If your 2500HD has the 6.6L Duramax diesel engine, it utilizes glow plugs, not spark plugs, which have an entirely different service procedure.)

Quick reference

Why the right plug matters

Modern heavy-duty truck engines are extremely sensitive to ignition component variances. The 6.6L L8T V8 demands an exact heat range to prevent pre-ignition when under heavy towing loads.

The OEM ACDelco plugs are made with iridium center electrodes. Iridium is significantly harder and more durable than platinum or traditional copper. This extreme durability is what allows General Motors to specify an interval near 100,000 miles. If you substitute a cheaper copper plug, the center electrode will degrade quickly under the high cylinder pressures of a heavy-duty engine, leading to misfires, reduced fuel economy, and potential catalytic converter damage within just 20,000 miles. Stick to the OEM ACDelco Iridium or premium equivalents from NGK or Denso.

How to gap a spark plug

Even though iridium spark plugs usually come pre-gapped from the factory, an ASE tech never installs a plug without verifying the gap. Shipping and handling can easily bump an electrode, closing the gap.

1. Use the right tool: Do not use a cheap coin-style gauge on iridium plugs; sliding the thick metal edge through the gap can easily snap the fragile iridium tip. Use a wire-style feeler gauge.
2. Measure: Gently pass the 0.040" wire through the gap. It should slide through with a slight "drag."
3. Adjust: If the gap is too tight, use the bending tool on the gauge to slightly pull the ground strap outward. Never pry against the center electrode.

Replacement procedure tips

The 6.6L gas V8 utilizes a Coil-on-Plug (COP) system, specifically a coil-near-plug design with short spark plug wires connecting the ignition coils to the plugs.

• Wire removal: The spark plug wires have metal heat shields around the boots to protect them from the exhaust manifolds. Twist the boot slightly before pulling to break the seal. Do not pull by the wire itself, or you will tear the conductive core.
• The Anti-Seize debate: GM specifically recommends against using anti-seize compound on the spark plug threads. The ACDelco plugs have a special zinc-nickel coating that prevents them from seizing in the aluminum cylinder heads. Adding anti-seize acts as a lubricant, which often leads to over-torquing the plugs and stripping the head threads. Install them dry.
• Dielectric grease: Apply a small dab of dielectric grease to the inside of the spark plug wire boot before reinstallation. This prevents moisture intrusion and ensures you'll be able to remove the boot easily next time.

Known issues

While you won't face the infamous 2-piece plug breakage seen on older Ford 3-valve engines, the modern GM V8s have their own quirks.

The ignition coils on these engines are highly sensitive. GM often utilizes the ignition coils for ionization sensing to detect misfires and knock. If you install sub-par aftermarket coils or low-quality spark plugs with the incorrect resistance, the engine control module (ECM) may register false knock or misfires, pulling ignition timing and drastically reducing engine power. Furthermore, the metal heat shields on the plug wires can sometimes rub against the exhaust manifold over time; inspect the wires closely for burns or chafing while you have them off.

Need plugs replaced? Find a shop on Repairius.