Autolite Xtreme Start Iridium Lawn & Garden Spark Plug Automotive Replacement, XST255DP

What It Is

The Autolite Xtreme Start Iridium Lawn & Garden Spark Plug (XST255DP) is a high-performance spark plug designed for small engines in lawn mowers, tractors, zero-turn mowers, and other outdoor power equipment. Its iridium center electrode and fine-wire design deliver easier starting, improved throttle response, and longer lifespan compared to standard copper plugs.

Fitment & Compatibility

Before buying, confirm your engine’s spark plug gap and thread size. The XST255DP has a 14mm thread diameter, 19mm (3/4") reach, and a 5/8" hex – common for many Briggs & Stratton, Kohler, Kawasaki, and Honda engines. Check your owner’s manual for the exact OEM plug number (e.g., Champion RC12YC, NGK BPR5ES). This plug is pre-gapped at 0.030" (0.76mm), but always verify with a gap tool. It replaces many standard copper plugs but is not compatible with engines requiring a resistor plug (check for “R” in the OEM number). Measure your old plug’s thread length and hex size to ensure a match.

Key Things to Know

• Installation difficulty: Easy – remove the old plug with a socket, gap-check the new one (though pre-gapped), and hand-tighten plus 1/4 to 1/2 turn with a torque wrench (recommended: 15-20 ft-lbs). Don’t overtighten; aluminum heads strip easily.
• Quality: Iridium is 6x harder than platinum and 8x harder than copper, giving 4x longer life (up to 100 hours vs. 25 for copper). Fine-wire tip reduces voltage needed for spark, improving cold starts and idle.
• Common gotcha: This plug is not for all engines – some require a shorter or longer reach, or a different heat range. If your engine calls for a Champion RJ19LM (long reach), this won’t fit. Always match the reach and hex size to your old plug.
• Performance benefit: Users report noticeably easier starting, especially in cold or damp conditions, and smoother idle. No performance gains on stock engines, but better reliability.

Should You Buy It

Buy it if you want a premium, long-lasting spark plug for a small engine that uses a standard 14mm, 19mm reach plug (common in many lawn tractors and mowers) and you’re annoyed by hard starts or frequent plug changes. Avoid it if your engine requires a different thread size, reach, or a specific resistor plug – stick with the OEM copper replacement for simplicity.