Top head gasket.
Gaskets provide a seal between the pump’s components, such as the oil pan, crankcase, cylinder, valve plate(s) and head (as well as the intercooler & aftercooler). Gaskets can be made from metal, cork, silicone, rubber, fiberglass or Teflon. It’s common for gaskets to “blow out” and leak, as debris in the head and back-pressure can damage them. When replacing, clean all old gasket material from the components, and install dry. Tighten the bolts to the correct torque setting for your unit. Having a clean air filter will also minimize gasket damage.