Abstract: A handpiece of a dental scaler includes an outer sheath and an inner housing received in the outer sheath. An inner surface of the outer sheath forms an inclined section converging toward an end of the sheath. An end of the inner housing is reduced and is complementary in shape to the converged inner surface of the outer sheath whereby a surface contact is formed between the outer sheath and the inner housing. A circumferential groove is defined in the reduced end of the inner housing to receive an O-ring. An inner-threaded fastener is threadingly fixed to an opposite, threaded end of the outer sheath and has an inner shoulder engaging a step formed in the inner housing thereby inducing an axial linear movement on the inner housing with respect to the outer sheath to compress and deform the O-ring for securing watertightness between the outer sheath and the inner housing.