Briggs & Stratton 491055S Spark Plug

What It Is

The Briggs & Stratton 491055S is a standard-resistor spark plug designed for many Briggs & Stratton small engines found in walk-behind lawn mowers, riding mowers, and other outdoor power equipment. It replaces the original Champion RC12YC plug and is a direct OEM replacement for engines requiring a 14mm thread, 19mm reach, and 3/4-inch hex size.

Fitment & Compatibility

Before buying, confirm your engine’s spark plug gap (usually 0.030″ for this plug). Check that your engine uses a 14mm thread diameter and 19mm reach—common on most single-cylinder Briggs engines from the 1980s onward. This plug fits many Briggs models (e.g., 10T502, 12H802, 12A902, 19G412) but is not universal. Verify your engine’s model number (stamped on the valve cover or blower housing) and cross-reference with Briggs’ official parts lookup. It is an OEM-spec part, not a generic aftermarket plug, so it’s a direct swap for the original.

Key Things to Know

• Quality: This is a genuine Briggs & Stratton part, not a third-party knockoff. It includes a resistor to reduce radio interference and is built to OEM tolerances.
• Installation difficulty: Very easy—requires only a spark plug socket and a gap tool. Torque to about 20 ft-lbs; overtightening can strip the aluminum cylinder head.
• Common gotchas: Do not use anti-seize on the threads—most modern plugs have a plated shell that doesn’t need it, and anti-seize can cause over-torquing. Always gap the plug to 0.030″ before installing (check your engine manual, but this is standard for most Briggs models).
• Lifespan: Replace every 100 hours or once per season under normal use. A worn plug can cause hard starting, misfiring, or reduced power.

Should You Buy It

If you own a Briggs & Stratton engine that calls for a 14mm, 19mm reach plug (most common models), this is the correct, reliable replacement at a fair price. It’s ideal for routine maintenance—skip it only if your engine requires a different gap or thread size (e.g., some older models use a 13/16″ hex or a different reach). For the vast majority of Briggs mowers, this is the plug to buy.