This is the Outer Cylinder Head for the O.S. Max .21XZ-B Engine (OSMG2050). This part attaches to the top of crankcase and provides the cooling of the engine. FEATURES: Blue anodized aluminum 10 heatsink circular cooling fins INCLUDES: One outer cylinder head for the OS .21XZ-B REQUIRES: Cylinder head bolts to the crankcase with four 3 x 15mm cap head Inner Cylinder Head/Head Button (OSMG5051)