2025 Chevrolet Silverado 2500HD Spark Plug Spec Guide

As an ASE-certified master technician, I regularly field questions about heavy-duty truck maintenance. If you drive a 2025 Chevrolet Silverado 2500HD equipped with the 6.6L V8 L8T gas engine, you already know it’s a workhorse built for towing and hauling. But keeping that massive 6.6L V8 firing smoothly under heavy loads requires the ignition system to be in peak condition.

(Note: If you drive the 6.6L Duramax diesel, your engine uses glow plugs, not spark plugs.)

Here is your comprehensive guide to the correct spark plug types, gaps, and torque specs for your 2025 Silverado 2500HD gas V8.

Quick reference

Why the right plug matters

When you are pulling a 14,000-pound trailer up a grade, cylinder pressures in that 6.6L L8T are immense. Using the correct spark plug is non-negotiable.

Electrode Metal: General Motors specifies Iridium spark plugs (like the OEM ACDelco 41-162) for a reason. Iridium is significantly harder and more durable than platinum or traditional copper. Copper plugs offer great conductivity but will burn up rapidly in a modern high-compression HD engine. Iridium resists spark erosion, maintaining a precise gap over the 97,500-mile service interval.

Heat Range: The heat range dictates how fast the plug transfers heat from the firing tip to the cylinder head. A plug that is too hot will cause pre-ignition and engine knock (which can destroy a loaded-down truck engine), while a plug that is too cold will foul out with carbon. Always stick to the factory heat range unless you have extensively modified the engine with forced induction.

How to gap a spark plug

Modern iridium plugs generally come pre-gapped, but a good technician always verifies the gap before installation. Dropped boxes in shipping happen more often than you'd think.

1. Use the right tool: Do not use the old-school coin-style gapping tools on iridium plugs. The iridium center electrode is exceptionally thin and brittle; a coin tool will snap it right off. Use a wire-style feeler gauge instead.
2. Checking the gap: Gently slide the 0.040 in (1.01 mm) wire gauge between the center and ground electrode. It should pass through with a slight "drag."
3. Adjusting (if necessary): If the gap is too tight, use a dedicated gapping tool to gently bend the ground strap away from the center electrode. Never pry against the fragile center electrode itself.

Replacement procedure tips

The 6.6L L8T utilizes an individual coil-near-plug ignition system, meaning each cylinder has its own ignition coil connected to the spark plug via a short boot/wire.

• Clear the debris: Before removing the old plugs, use compressed air to blow out the spark plug wells. Heavy-duty trucks see a lot of job sites, dirt, and gravel. You don't want sand falling directly into your cylinders.
• The anti-seize debate: GM specifically advises against using anti-seize on the threads of their modern spark plugs. The ACDelco plugs feature a special zinc/nickel coating that prevents seizing. Adding anti-seize acts as a lubricant, which often leads to over-torquing and stretching the threads in the aluminum cylinder head.
• Dielectric grease: Always apply a small dab of dielectric grease to the inside of the spark plug boot before snapping it onto the new plug. This prevents moisture intrusion, prevents voltage leaks, and ensures the boot won't bake onto the ceramic insulator over the next 100,000 miles.
• Torque it right: Thread the plugs in by hand first to prevent cross-threading. Then, use a torque wrench to tighten them to exactly 11 lb-ft (15 Nm).

Known issues

Unlike the notorious older Ford 3-valve engines that suffered from 2-piece spark plug breakages, or older GM engines that had extreme oil consumption fouling out plugs, the 6.6L L8T is remarkably robust.

However, GM's modern ignition coils are highly sensitive to resistance changes. The Engine Control Module (ECM) uses the spark plug to essentially "measure" the combustion event through ionization sensing. If you use cheap copper plugs or the wrong gap, it alters the electrical resistance. The ECM may interpret this as a misfire, leading to a flashing check engine light, reduced engine power, and poor transmission shifting. Stick to the ACDelco Iridium or high-tier NGK/Denso equivalents to keep the computer happy.

Need plugs replaced? Find a shop on Repairius.