NGK 91185 Racing Spark Plug 12mm 1.043in Flat Seat 4AN Iridium

Vehicle Fitment

• Universal racing spark plug - not specific to any year/make/model
• Ideal for custom motorsport or performance engines requiring 12 mm flat-seat plug

Features

• Part number NGK 91185 (R2556G-10) - racing series spark plug
• Fine-wire iridium-tipped center electrode for high ignitability
• Platinum ground electrode for improved wear and durability
• Solid copper core ensures efficient heat transfer
• Flat seat design suited for engines with flat-seat cylinder heads
• 12 mm thread diameter and ~1.043 in (26.5 mm) reach for deep-seated applications
• 5/8" (16 mm) hex for common socket use
• Designed to withstand the high loads of racing conditions

Specifications

• Part Number: 91185 (NGK Racing Spark Plug R2556G-10)
• Thread Diameter: 12 mm
• Reach: 1.043 in (26.5 mm)
• Seat Type: Flat
• Hex Size: 5/8" (16 mm)
• Heat Range: 10 (racing-oriented)
• Resistance: ~5000 Ω (resistor type)
• Electrode Materials: Iridium center, platinum ground

Included Components

• 1x NGK 91185 Racing Spark Plug (sold individually)

Product Description

The **NGK 91185 Racing Spark Plug** (NGK part **R2556G-10**) is engineered for motorsport and high-performance applications where extreme thermal and mechanical conditions are encountered. Featuring a fine-wire iridium-tipped center electrode and a platinum-tipped ground electrode, this racing plug delivers enhanced ignitability and durability compared to conventional plugs, helping engines maintain spark integrity under heavy load or high RPM conditions. 

With a 12 mm threaded body, flat seat, and approximately 1.043 in (26.5 mm) reach, the NGK 91185 is well-suited for custom or modified engines that require this specific plug geometry. Its resistor design helps suppress electrical noise, and the robust construction makes it a solid choice for competition engines or high-output builds. Because this plug is part of NGK's racing lineup, it doesn't have standardized OEM vehicle fitment - fitment should be confirmed based on engine requirements and plug specifications rather than year/make/model listings.