A cylinder head assembly comprising: a crankcase provided with a cylinder hole and an exhaust muffler cavity , wherein the crankcase is provided with an annular boss at the end of the cylinder hole, an end surface of the annular boss forms a first mounting surface provided with a first exhaust throttle through-hole communicated with the exhaust muffler cavity , and an outer circular surface of the annular boss is not coaxial with the cylinder hole and the outer circular surface is biased toward the first exhaust throttle through-hole ; cylinder head components comprising an exhaust valve sheet , a valve plate and a suction valve sheet , wherein each of the cylinder head components has a circular shape, an edge of each of the cylinder head components is provided with a protrusion , and each protrusion is provided with a second exhaust throttle through-hole corresponding to the first exhaust throttle through-hole ; and a cylinder cover provided with a first cylindrical mounting groove and a second cylindrical mounting groove , wherein the first cylindrical mounting groove is coaxial with the cylinder hole , the second cylindrical mounting groove is coaxial with the outer circular surface of the annular boss , the cylinder cover is connected to the crankcase and clamps the cylinder head components between the bottom of the first cylindrical mounting groove and the first mounting surface , the second exhaust throttle through-holes are communicated with the first exhaust throttle through-hole to form an exhaust passage, and the second cylindrical mounting groove is matched with the outer circular surface of the annular boss . a crankcase provided with a cylinder hole and an exhaust muffler cavity , wherein the crankcase is provided with an annular boss at the end of the cylinder hole, an end surface of the annular boss forms a first mounting surface provided with a first exhaust throttle through-hole communicated with the exhaust muffler cavity , and an outer circular surface of the annular boss is not coaxial with the cylinder hole and the outer circular surface is biased toward the first exhaust throttle through-hole ; cylinder head components comprising an exhaust valve sheet , a valve plate and a suction valve sheet , wherein each of the cylinder head components has a circular shape, an edge of each of the cylinder head components is provided with a protrusion , and each protrusion is provided with a second exhaust throttle through-hole corresponding to the first exhaust throttle through-hole ; and a cylinder cover provided with a first cylindrical mounting groove and a second cylindrical mounting groove , wherein the first cylindrical mounting groove is coaxial with the cylinder hole , the second cylindrical mounting groove is coaxial with the outer circular surface of the annular boss , the cylinder cover is connected to the crankcase and clamps the cylinder head components between the bottom of the first cylindrical mounting groove and the first mounting surface , the second exhaust throttle through-holes are communicated with the first exhaust throttle through-hole to form an exhaust passage, and the second cylindrical mounting groove is matched with the outer circular surface of the annular boss . <EOS>