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.