This is a 6061 Aluminum Cylinder Head for Tokyo Marui Next-Gen AEG's. Because the Piston assembly is constantly slamming against the Cylinder Head during a gearbox cycle, the Cylinder Head endures a lot of stress. Plastic Cylinder Heads tend to crack and fail over time as a result of these stressors. Unlike stock plastic Cylinder Heads, this unit is built from high-grade aluminum and can withstand the strongest of springs.