A method of manufacturing an ultrasonic flow meter flow tube, wherein the flow tube comprises transducer holders (20, 22), each transducer holder (20, 22) comprises an opening and a transducer seat for mounting of a transducer housing (26, 27), and each transducer holder (20, 22) is forming one part of a pair of oppositely placed transducer holders (20, 22), characterized in that the method comprises the following steps: - providing a transducer seat-machining tool for machining the transducer seat of one of the pair of transducer holders (20, 22), the transducer seat-machining tool comprising a tool head mechanically connectable to a guide shaft, - mechanically connecting the tool head and the guide shaft, - mechanically guiding the tool head by a part (55, 109) of the transducer seat-machining tool engaging with the transducer holder opposite placed to the transducer holder to be machined, - machining the transducer seat of the transducer holder (20, 22) to be machined with the tool head; each of the pair of transducer holders (20, 22) comprises a guide hole, and the tool head comprises a guide bushing, the method further comprising the following steps: - guiding the tool head of the transducer seat-machining tool, ensuring a surface of the transducer seat is perpendicular to a coaxial axis, by ∘ positioning the guide bushing in the guide hole of the transducer holder (20, 22) to be machined, and ∘ positioning the guide shaft or the guide bushing in a position engaging the guide hole of the opposite placed transducer holder (20, 22). characterized in that the method comprises the following steps: - providing a transducer seat-machining tool for machining the transducer seat of one of the pair of transducer holders (20, 22), the transducer seat-machining tool comprising a tool head mechanically connectable to a guide shaft, - mechanically connecting the tool head and the guide shaft, - mechanically guiding the tool head by a part (55, 109) of the transducer seat-machining tool engaging with the transducer holder opposite placed to the transducer holder to be machined, - machining the transducer seat of the transducer holder (20, 22) to be machined with the tool head; each of the pair of transducer holders (20, 22) comprises a guide hole, and the tool head comprises a guide bushing, the method further comprising the following steps: - providing a transducer seat-machining tool for machining the transducer seat of one of the pair of transducer holders (20, 22), the transducer seat-machining tool comprising a tool head mechanically connectable to a guide shaft, - mechanically connecting the tool head and the guide shaft, - mechanically guiding the tool head by a part (55, 109) of the transducer seat-machining tool engaging with the transducer holder opposite placed to the transducer holder to be machined, - machining the transducer seat of the transducer holder (20, 22) to be machined with the tool head; each of the pair of transducer holders (20, 22) comprises a guide hole, and the tool head comprises a guide bushing, the method further comprising the following steps: - guiding the tool head of the transducer seat-machining tool, ensuring a surface of the transducer seat is perpendicular to a coaxial axis, by ∘ positioning the guide bushing in the guide hole of the transducer holder (20, 22) to be machined, and ∘ positioning the guide shaft or the guide bushing in a position engaging the guide hole of the opposite placed transducer holder (20, 22). ∘ positioning the guide bushing in the guide hole of the transducer holder (20, 22) to be machined, and ∘ positioning the guide shaft or the guide bushing in a position engaging the guide hole of the opposite placed transducer holder (20, 22).