RC12YC Spark Plug 491055 (Pack of 10) Fit for Briggs & Stratton Spark Plug Replace 491055S 491055T 72347GS 72347 805015 M78543

What It Is

This is a pack of 10 RC12YC spark plugs, a direct replacement for Briggs & Stratton engines commonly found on walk-behind mowers, riding mowers, and small utility engines. It’s a standard copper-core plug designed for reliable ignition in 4-stroke engines.

Fitment & Compatibility

Before buying, verify your engine’s exact spark plug number—this set replaces OEM numbers 491055S, 491055T, 72347GS, 72347, 805015, and M78543. It fits most Briggs & Stratton single-cylinder engines (e.g., 450, 500, 550, 625, 675 series) but not all—check your engine’s model number sticker or owner’s manual. The plug has a 14mm thread diameter, 0.750” reach, and requires a 13/16” socket. Gap is preset at 0.030” but always verify with a gap tool. It’s aftermarket, not OEM Briggs & Stratton, so quality can vary—use only for non-critical or older machines.

Key Things to Know

• Quality: Copper core means good conductivity but shorter lifespan (25–50 hours) compared to platinum or iridium plugs. Expect to replace more often.
• Installation: Hand-tighten then 1/2 turn with a socket—overtightening can crack the insulator. Use anti-seize if you live in a humid area.
• Gotcha: These plugs are pre-gapped, but shipping can knock them out of spec. Always check and adjust gap before installing.
• Pack value: A 10-pack is cost-effective if you own multiple mowers or change plugs seasonally, but wasteful if you only need one.

Should You Buy It

Buy if you maintain several small engines and want a bulk, budget-friendly supply for frequent swaps. Skip if you need a single plug for a newer or high-performance engine—you’re better off with an OEM or premium plug for longer life and guaranteed fit.