2021 Ford F-350 Super Duty Spark Plug Type & Gap

Quick reference

Why the right plug matters

Choosing the correct spark plug for your 2021 Ford F-350 Super Duty is crucial for optimal engine performance, fuel efficiency, and longevity. Spark plugs are not a "one-size-fits-all" component; various factors, including heat range, electrode material, and overall design, are tailored to specific engine requirements.

Heat Range: This refers to the plug's ability to dissipate heat. A plug that's too "cold" (dissipates heat too quickly) can lead to fouling, while a plug that's too "hot" (retains too much heat) can cause pre-ignition and engine damage. The factory-recommended plugs are engineered with the perfect heat range for your F-350's engine to maintain optimal combustion temperatures.

Electrode Metal: Modern engines, like those in the F-350, typically use iridium or platinum spark plugs. These precious metals are much harder and have higher melting points than traditional copper, allowing for finer electrode tips. This finer tip concentrates the spark, leading to a more consistent and powerful ignition. Iridium plugs, in particular, offer the longest lifespan and best performance due to their extreme durability. Platinum plugs are also excellent long-life options, while copper plugs, though offering good conductivity, have a much shorter lifespan and are rarely found as original equipment in newer vehicles designed for extended service intervals.

How to gap a spark plug

Even with pre-gapped plugs, it's good practice to verify the gap before installation. An improperly gapped plug can lead to misfires, reduced fuel economy, and poor engine performance.

1. Obtain a spark plug gap gauge: You'll need either a wire-type or coin-style gauge. Wire gauges are generally preferred for iridium and platinum plugs as they are less likely to damage the delicate center electrode.
2. Check the gap: Carefully slide the appropriate thickness wire from your gauge 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 adjustment tool on the gauge (or a gentle pry with a small, non-marring tool) to slightly bend the ground electrode outwards. If the gap is too large, gently tap the ground electrode on a clean, hard surface to close it. Always adjust the ground electrode only, never apply pressure to the center electrode.
4. Re-check: Verify the gap again after adjustment.

Replacement procedure tips

Replacing spark plugs on your 2021 Ford F-350 Super Duty, especially with its coil-on-plug ignition system, requires a bit of care.

• Coil-on-plug removal: Each spark plug has its own ignition coil mounted directly on top of it. To access the spark plug, you'll need to disconnect the electrical connector from the coil and then unbolt and gently remove the coil pack. Take care not to damage the wiring harness or the coil itself.
• Anti-seize debate: There's ongoing debate about using anti-seize on spark plug threads. Most modern spark plugs (especially nickel-plated ones) do not require anti-seize, and applying it can lead to over-torquing and damage the cylinder head threads. If you choose to use it, apply a very small amount to the threads, avoiding the electrode, and reduce the specified torque by about 10-20% to account for reduced friction.
• Dielectric grease on boots: Applying a thin layer of dielectric grease inside the spark plug boot (the part that connects to the coil or wire) can help prevent moisture intrusion, improve electrical connection, and make future removal easier. Be careful not to get grease on the tip of the spark plug or inside the coil spring.
• Cleanliness: Before removing the old plugs, it's crucial to clean around the spark plug wells with compressed air to prevent dirt and debris from falling into the combustion chamber.
• Torque specs: Always use a torque wrench to tighten spark plugs to the manufacturer's specifications. Over-tightening can damage the cylinder head threads or crack the spark plug, while under-tightening can lead to combustion gas leakage and overheating.

Known issues

Fortunately, both the 6.2L V8 and the 7.3L V8 "Godzilla" engines in the 2021 Ford F-350 Super Duty do not have widespread, notorious spark plug issues like the earlier 3-valve Ford engines (known for two-piece plug breakage). As long as you use the correct spark plugs and follow proper installation procedures, you should experience reliable ignition performance.

Need plugs replaced? Find a shop on Repairius.