2021 Subaru Crosstrek: Spark Plug Type & Gap

Quick Reference: 2021 Crosstrek Spark Plugs

Why The Right Plug Matters

Choosing the correct spark plug is critical for engine health, performance, and fuel economy. It’s not just about finding a plug that fits; the engineering behind it matters. For the direct-injected Subaru Boxer engines in your 2021 Crosstrek, only an OEM-equivalent Iridium-tipped plug should be used.

• Heat Range: A spark plug must get hot enough to burn off carbon deposits, but not so hot that it causes pre-ignition (detonation). The NGK plugs specified for the 2.0L and 2.5L engines have different heat ranges (the "7" and "8" in their part numbers, respectively) tailored to each engine's specific operating temperatures. Using the wrong one can lead to poor performance or even engine damage.
• Electrode Material: Your Crosstrek came from the factory with Iridium plugs for a reason. Iridium is an extremely hard, dense precious metal with a very high melting point. This allows for an ultra-fine center electrode, which requires less voltage to create a spark. The result is a more focused, consistent spark, leading to better throttle response, improved fuel efficiency, and a very long service life. While platinum and copper plugs exist for other vehicles, they are not appropriate for this application and would suffer a drastically shorter lifespan and reduced performance.

How to Gap a Spark Plug

"Gapping" a spark plug means setting the precise distance between the center electrode and the ground electrode. This gap must be exact to ensure a strong, healthy spark. For the 2021 Crosstrek, the specified gap is 0.031 inches (0.8 mm).

However, a crucial point for modern Iridium plugs like those used in your Subaru is that they come pre-gapped from the factory and should generally not be adjusted. The fine-wire Iridium center electrode is extremely brittle and can be easily snapped or damaged by traditional gapping tools, especially the common coin-style "ramped" gappers. If you must check the gap, use a wire-style or feeler gauge with extreme care. Do not apply any pressure to the center electrode. If an adjustment is needed, only ever bend the ground electrode (the strap of metal over the top) with a dedicated tool that hooks the strap, never by prying against the delicate tip.

Replacement Procedure Tips

Replacing the spark plugs on a Subaru Boxer engine is more involved than on an inline-four or V6. The plugs are located on the sides of the engine, low in the engine bay, making access the primary challenge.

• Coil-on-Plug Removal: The 2021 Crosstrek uses a modern coil-on-plug ignition system, meaning each spark plug has its own ignition coil sitting directly on top of it. There are no old-school distributor caps or spark plug wires. Each coil is held in place by a single bolt. Once the bolt is removed, the coil and its integrated boot can be gently twisted and pulled straight off the spark plug.
• Access is Tight: On the passenger's side, the air intake box will need to be removed. On the driver's side, the battery and its tray must be taken out to create enough room to work. You will need a good socket set with various extensions and a universal joint to reach the spark plug tubes.
• The Anti-Seize Debate: Historically, technicians applied a light coat of anti-seize compound to plug threads to prevent them from getting stuck in the cylinder head. However, modern plug manufacturers like NGK apply a special trivalent (zinc-chromate) plating to their plug shells that serves the same purpose. They specifically recommend not using anti-seize, as the lubricating effect can cause you to accidentally over-torque the plug, potentially stretching the threads or damaging the cylinder head. Our advice: Follow the plug manufacturer's guidance. If using NGK plugs, install them dry.
• Dielectric Grease: A small dab of dielectric grease applied to the inside of the coil boot before reinstallation is highly recommended. This grease is a non-conductor that helps seal the boot against moisture, preventing spark-arcing, and it keeps the rubber from vulcanizing to the plug's porcelain, making removal much easier next time.

Known Issues

Unlike some other well-known engine families (such as the Ford 3-valve Triton engines infamous for their two-piece plugs breaking apart), the Subaru FB-series engines in the 2021 Crosstrek do not have any widespread, notorious issues related to spark plug failure or removal. The design is robust and conventional. As long as you use the correct high-quality Iridium plugs, torque them to spec, and follow the proper procedure, you should not encounter any unusual problems.

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