STD295 - Lycoming Spark Plug Gasket

STD295 – Spark Plug Gasket, 18mm Copper Crush Washer (replaces C10S, LW-12285)

Available now on the Airparts.aero Marketplace.

The STD295 is the Textron Aviation standard part number for the ubiquitous 18mm solid copper spark plug gasket used on virtually all reciprocating aircraft engines, including those from Lycoming and Continental. This seemingly simple component performs two flight-critical functions that are absolutely essential for engine health and safety: it creates a gas-tight seal to contain combustion pressures, and it forms a critical thermal bridge to conduct heat away from the spark plug into the cylinder head.

Manufactured from soft, fully annealed copper, this gasket is engineered as a single-use "crush washer." When a spark plug is torqued to its specified value, the malleable copper deforms to perfectly match the microscopic surface imperfections of both the spark plug and the cylinder head, creating a perfect seal. This process also provides the necessary surface contact for efficient heat transfer, which is critical for preventing spark plug overheating, pre-ignition, and catastrophic engine damage.

Key Features

• Single-Use Crush Washer Design: Engineered to deform upon torquing, creating a perfect gas-tight seal that is impossible to replicate on a second use.

• Solid Annealed Copper Construction: Made from 99%+ pure, fully softened copper, selected for its exceptional thermal conductivity and malleability.

• Critical Heat Transfer Path: Serves as the primary conductor of heat from the spark plug core to the massive cooling fins of the cylinder head, regulating the plug's operating temperature.

• Gas-Tight Combustion Seal: Reliably contains combustion chamber pressures that can exceed 1,000 PSI, preventing dangerous hot gas leaks.

• Universal 18mm Spark Plug Fitment: Designed to fit all standard 18mm-threaded aviation spark plugs from manufacturers like Champion and Tempest.

Technical Specifications

• Part Number: STD295

• Component Type: Spark Plug Gasket / Crush Washer

• Commonly Interchangeable Part Numbers:

  • Champion Aerospace: C10S

  • Lycoming Engines: LW-12285

  • Continental Aerospace: 649129

  • Tempest Aero Group: RA-C10S

• Material: Solid, Annealed Copper Alloy

• Application: For all standard 18mm aviation spark plugs.

• Nominal Dimensions:

  • Inside Diameter: ~0.715 inches (18.16 mm)

  • Outside Diameter: ~0.885 inches (22.48 mm)

Aircraft Compatibility

The STD295 is a standard consumable part required for spark plug installation on nearly every aircraft powered by a Lycoming, Continental, or Franklin piston engine. It is used on a vast range of aircraft, from a Cessna 150 to a Beechcraft Baron and beyond.

Crucial Note: The applicable engine maintenance manual is the final authority for all spark plug installation procedures, including the requirement for a new gasket.

Applications

• Required for the installation of any 18mm aviation spark plug into a cylinder head.

• Used during routine maintenance, 100-hour inspections, annual inspections, and engine overhauls.

Why Choose STD295?

The decision to use a new STD295 gasket with every spark plug installation is one of the most critical and least expensive safety decisions a mechanic can make. Re-using a gasket is not thrifty; it is exceptionally dangerous. A previously crushed gasket becomes work-hardened and cannot form a proper seal, leading to combustion gas leaks and, more critically, a drastic reduction in heat transfer. This will cause the spark plug to overheat, which can lead to pre-ignition or detonation, two events that can destroy a piston and cylinder in seconds. The cost of a new gasket is negligible compared to the cost of a top overhaul or a potential in-flight engine failure.

The function of the STD295 gasket is rooted in the metallurgy of copper and the physics of plastic deformation. The gasket is supplied in a fully annealed state, a heat treatment process that creates a fine-grained, equiaxed crystal structure with minimal internal stress and a low dislocation density, rendering the copper extremely soft and malleable. When the specified installation torque (typically 360-420 inch-pounds) is applied, the compressive stress easily exceeds the copper's low yield strength, causing it to plastically deform. This deformation forces the copper to flow like a viscous fluid, conforming to the micro-topography of the steel spark plug and the aluminum cylinder head sealing surfaces. This same process of deformation creates a massive number of new dislocations within the copper's crystal lattice, a phenomenon known as work-hardening or strain-hardening. This makes the used gasket significantly harder and less malleable. If re-used, this now work-hardened gasket cannot plastically deform a second time, leaving microscopic gaps that lead to high-pressure gas leaks and insulating air pockets that severely inhibit the critical thermal conduction path.

Note: This is a single-use component. This gasket MUST be replaced with a new one every time a spark plug is installed. Re-using a spark plug gasket will lead to improper sealing, poor heat transfer, spark plug overheating, and can result in pre-ignition, detonation, and catastrophic engine damage. Installation must be performed by a certified A&P mechanic, and the spark plug must be torqued to the value specified in the engine maintenance manual.

Documentation

The definitive procedures and torque values for spark plug installation are detailed in the applicable Lycoming or Continental engine maintenance and overhaul manuals, as well as service literature from spark plug manufacturers such as Champion Aerospace (e.g., publication AV-6).