Autolite AR51 Spark Plug

High-Performance Racing Spark Plug

Autolite AR51 is a high-performance racing non-resistor spark plug built with OE-style form, fit, and function. It is a direct choice for classic performance applications that call for a flat-seat plug with a 14 mm thread and 0.750-inch reach.

Compatible With Porsche 911, Ferrari 365 GTB/4, and Alfa Romeo Giulia Sprint applications listed below, this plug uses a copper center electrode, nickel ground electrode, and ceramic insulator. The tapered ground electrode tip and 0.025-inch pre-gap support consistent ignition characteristics in performance-focused engines.

Key Features

• Non-resistor design for racing and performance ignition setups
• OE-style spark plug construction with standard/regular center electrode design
• Copper center electrode core with nickel tip and nickel ground electrode
• Flat seat, 14 mm thread diameter, and 13/16-inch hex size
• Pre-gapped to 0.025 inch for listed applications
• Autolite AR51 part number for catalog matching and service lookup

Detailed Application Notes

Autolite positions the AR51 as a racing plug with OE-style fit and function for foreign and domestic applications. The non-resistor construction is suited to performance ignition systems where resistor suppression is not required, while the tapered ground electrode and ceramic insulator support stable spark delivery. This part number, AR51, appears across multiple classic European applications including Porsche 911, Ferrari 365 GT series, and Alfa Romeo models.

This part cross-references Interchanges (e.g., 41XLS, AG092C) for the same application.

Common Replacement Uses

• Restoring ignition performance in classic carbureted engines
• Replacing worn plugs that no longer hold the specified gap
• Addressing misfire, rough idle, or hard-start conditions caused by plug wear
• Serving as an OE-style replacement for performance and vintage applications

Installation Notes and Quality

Verify the specified gap for the exact application before installation. Confirm seat type, thread diameter, reach, and terminal style match the engine requirements. Use the vehicle service specification for final gap setting and torque procedure.

Autolite supplies ignition products to the aftermarket and original equipment sectors and markets this plug as an OE-style replacement with performance-focused construction. The AR51 part number is used for catalog identification across the listed fitments.