How to Choose, Replace, and Maintain the Craftsman Lawn Mower Spark Plug for Optimal Performance​

2025-10-15

If your Craftsman lawn mower struggles to start, runs rough, or loses power unexpectedly, a worn or faulty spark plug is often the culprit. This small but critical component ignites the air-fuel mixture in your engine, and keeping it in top condition directly impacts your mower’s reliability, fuel efficiency, and longevity. Whether you’re a DIY enthusiast or new to lawn care maintenance, understanding how to select, replace, and care for your Craftsman spark plug will save you time, money, and frustration. In this guide, we’ll break down everything you need to know—from identifying when it’s time for a replacement to choosing the right model and mastering installation—ensuring your mower performs at its best season after season.

Why the Spark Plug Matters More Than You Think

Before diving into replacement steps, let’s clarify why the spark plug deserves your attention. In a gas-powered Craftsman lawn mower, the engine relies on a controlled explosion of gasoline and air to generate power. The spark plug creates the electrical spark that ignites this mixture. Over time, heat, carbon buildup, and wear can degrade its ability to produce a strong, consistent spark. A weak or misfiring spark plug leads to:

  • ​Hard starting: You might crank the engine repeatedly without success, especially in cold weather.

  • ​Poor performance: The mower may sputter, stall, or lack power when cutting thick grass.

  • ​Reduced fuel efficiency: Incomplete combustion wastes gas, costing you more at the pump.

  • ​Engine damage: Severe misfires can overheat components or leave unburned fuel in the cylinder, corroding parts.

For Craftsman mowers—known for their durability and value—maintaining the spark plug isn’t just about fixing problems; it’s about preserving your investment. A well-maintained plug ensures your mower starts reliably every time and operates efficiently, extending the life of the engine.

Step 1: Know When It’s Time to Replace Your Craftsman Spark Plug

How often should you replace the spark plug? Most Craftsman mower manufacturers recommend checking it annually and replacing it every 1–2 years or after 50–100 hours of use, whichever comes first. But don’t rely solely on the calendar—watch for these warning signs:

1. Difficulty Starting

If you have to pull the recoil starter 10+ times or charge the battery repeatedly to get the engine running, the spark plug may be failing to ignite the mixture properly.

2. Rough Idling or Stalling

A sputtering, uneven idle or sudden stalling when you release the throttle often points to inconsistent spark delivery.

3. Excessive Smoke or Soot

Black smoke from the exhaust or a sooty spark plug (visible when removed) indicates incomplete combustion, which the plug may be contributing to.

4. Visible Wear or Damage

Remove the spark plug (we’ll cover how later) and inspect it. Look for:

  • ​Eroded electrodes: The center electrode (tip) or ground electrode (side) may be worn down, shortening the spark gap.

  • ​Carbon buildup: A black, crusty coating can insulate the plug, reducing spark strength.

  • ​Cracks or corrosion: Physical damage to the ceramic insulator or metal threads means the plug can’t function safely.

Step 2: Identify the Correct Spark Plug for Your Craftsman Mower

Craftsman offers dozens of lawn mower models, from push mowers to zero-turn riders, and each may require a specific spark plug. Using the wrong model can lead to poor performance, engine damage, or even void your warranty. Here’s how to find the right one:

1. Check Your Owner’s Manual

The easiest way is to consult your mower’s manual. Look for the “Specifications” or “Maintenance” section—it will list the exact spark plug part number (e.g., NGK BR6S, Champion RC12YC).

2. Decode the OEM Number

If you don’t have the manual, find the OEM (Original Equipment Manufacturer) spark plug number printed on the old plug. Craftsman often uses plugs from brands like NGK, Champion, or Autolite. For example:

  • ​NGK BR6S: Common in Craftsman push mowers with Briggs & Stratton engines (e.g., XT Series, 247cc engines).

  • ​Champion RC12YC: Used in many Craftsman models with larger engines, like ST Series walk-behinds.

  • ​Autolite AR6: A budget-friendly alternative for smaller, entry-level Craftsman mowers.

3. Match Key Specifications

If you can’t find the OEM number, match the plug based on three critical parameters:

  • ​Thread Size and Length: Most Craftsman mowers use 14mm thread diameter with a 19mm length (M14x1.25). Incorrect length can cause the plug to protrude into the cylinder, damaging pistons.

  • ​Heat Range: The heat range (e.g., “6” in BR6S) indicates how quickly the plug dissipates heat. Craftsman engines typically use “cold” to “medium” heat range plugs (numbers 5–7) to prevent overheating in warm climates.

  • ​Gap Size: The distance between the center and ground electrodes (usually 0.030–0.040 inches). Craftsman mowers often require a 0.035-inch gap, but always check the manual—too small a gap weakens the spark; too large causes misfires.

Step 3: Gather Tools and Prepare for Replacement

Replacing a spark plug is a simple DIY task, but having the right tools ensures safety and precision. Here’s what you’ll need:

  • ​Spark plug wrench or socket: A 5/8-inch (16mm) deep socket works for most Craftsman plugs. Avoid adjustable wrenches—they can strip the hex head.

  • ​Gap tool: A feeler gauge to check/adjust the spark gap (optional if using a pre-gapped plug).

  • ​Rag or wire brush: To clean debris around the spark plug hole.

  • ​Anti-seize lubricant: Prevents the plug from seizing in the cylinder head over time (use sparingly—too much can foul the plug).

  • ​Gloves and safety glasses: Protect hands from sharp edges and eyes from debris.

​Safety First: Always disconnect the spark plug wire before removing the old plug. This prevents accidental ignition if you accidentally crank the engine.

Step 4: Replace the Spark Plug in 6 Simple Steps

Follow these steps to swap your Craftsman spark plug correctly:

1. Locate the Spark Plug

On most Craftsman mowers, the spark plug is on the side of the engine, near the cylinder head. It’s connected to a thick rubber wire—follow this wire to find the plug.

2. Remove the Old Plug

  • Use the spark plug wrench to loosen the plug counterclockwise. If it’s stuck, gently tap the wrench with a rubber mallet to break corrosion.

  • Once loose, unscrew it by hand and pull it out. Avoid dropping it—damage to the ceramic insulator can ruin the plug.

3. Inspect and Clean the Area

Use a rag or compressed air to remove dirt, grass clippings, or oil from the spark plug well. Debris falling into the cylinder can cause serious engine damage.

4. Check/Adjust the New Plug’s Gap

If your new plug isn’t pre-gapped, use a feeler gauge to measure the distance between the electrodes. Gently bend the ground electrode (the curved one) until it matches the manufacturer’s spec (often 0.035 inches for Craftsman).

5. Install the New Plug

Apply a thin layer of anti-seize to the threads (don’t get any on the electrode or insulator). Screw the plug into the cylinder head clockwise by hand until tight, then use the wrench to snug it—do not overtighten (this can crack the ceramic or strip threads).

6. Reconnect the Spark Plug Wire

Push the wire firmly onto the plug’s terminal until it clicks. Give it a gentle tug to ensure it’s secure.

Step 5: Maintain Your Craftsman Spark Plug for Long-Term Performance

Replacing the plug is only half the battle—proper maintenance extends its life and keeps your mower running smoothly. Here’s how to care for it:

1. Annual Inspection

At the start of each mowing season, remove the plug and check for wear, carbon buildup, or cracks. A clean, light-tan electrode means it’s healthy; heavy black soot suggests rich fuel mixture (adjust carburetor if needed), while white ash indicates overheating.

2. Clean Gently When Needed

If the plug has light carbon buildup, use a wire brush to scrub it. Avoid sandpaper—abrasive particles can damage the insulator. Never clean a cracked or eroded plug; replace it instead.

3. Store Properly

If you remove the plug for storage, keep it in a dry, cool place. Wrap it in anti-static plastic to prevent moisture absorption, which can cause misfires.

4. Pair with Fresh Fuel and Air Filters

A spark plug works best with clean air and fresh fuel. Replace the air filter annually and use ethanol-free gasoline (or add a fuel stabilizer) to prevent varnish buildup that can affect combustion.

Troubleshooting Common Spark Plug Issues

Even with proper care, spark plugs can act up. Here’s how to solve common problems:

  • ​Plug keeps fouling (getting sooty)​: This often means the engine is running rich (too much fuel). Check the air filter (clogged filters let less air in, increasing fuel ratio) or adjust the carburetor.

  • ​Plug won’t fire: If the gap is correct and the plug is clean, test its resistance with a multimeter (should be 5–15k ohms). High resistance means the plug is bad—replace it.

  • ​Engine backfires after replacement: You may have overtightened the plug, cracking the insulator, or installed the wrong gap. Recheck the gap and installation.

Final Thoughts: Invest in Your Spark Plug, Invest in Your Mower

The spark plug is far more than a tiny metal piece—it’s the heartbeat of your Craftsman lawn mower. By understanding when to replace it, choosing the right model, and maintaining it properly, you’ll ensure your mower starts reliably, cuts evenly, and lasts for years. Don’t overlook this small but mighty component; a little attention goes a long way in keeping your lawn care routine stress-free.

Now that you’re equipped with the knowledge, grab your tools, check your spark plug, and get your Craftsman mower ready to tackle another season of lush, well-manicured grass.

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