A pump includes a cap that is fit into an opening of a pump body. The pump body has an external thread formed on an outer circumference close to a top face thereof for engagement with and coupling to a rotatable fixing ring having an internal thread so as to retain a cap from top side thereof. The threading engagement along the circumference fixes the cap and the pump body together. The cap has an underside from which a projecting ring extends downward. The projecting ring has a thread. A piston cylinder has an end forming a thread engageable with the thread of the projecting ring to complete the assembling of the piston cylinder.