A method of manufacturing a venturi tube 10′ having a certain fluid flow direction is provided. The method provides a first core member 11 in a mold. The first core member has a periphery and an end 21 of a certain size. A second core member 17 is provided in the mold upstream of the first core member. The second core member has a periphery and an end 22 with a size that is less than the certain size. At least a portion of the end of the second core member is disposed adjacent to at least a portion of the end of the first core member such that a step is defined between the first and second core members. Material is molded about the peripheries of the first and second core members such that the peripheries define a stepped fluid flow surface of a venturi tube, and so that any resulting flash occurs only in the certain flow direction.