Which type of fitting is prohibited for use in fuel systems?

Cast iron fittings are prohibited for use in fuel systems due to their brittleness and the potential for failure under stress or when subjected to high-pressure conditions. These characteristics can lead to leaks or ruptures, which pose serious safety hazards in a system that handles flammable fuels.

In contrast, materials like aluminum, brass, and steel are generally considered more reliable and are often used in fuel systems. They have better resilience and can withstand the corrosive nature of various fuels, making them safer choices for maintaining the integrity of fuel lines and fittings. The prohibition of cast iron is primarily a safety measure to ensure the reliability and longevity of fuel systems.