Abstract: A ball joint of the type having a ball stud and preformed plastic socket includes means for sealing the stud head receiving socket opening and assuring the maintenance of the seal in spite of accumulated tolerance variations. A cap with an inwardly converging peripheral closure surface fits within and conforms closely to a matching closure surface in the socket opening. Deformable protuberances on the outer side of the cap are engaged and deformed by a surface of the suspension control arm to which the socket is mounted, or by a surface of a cover which fixes the socket to the control arm. The protuberances extend far enough from the cap outer side to assure that engagement and deformation in spite of accumulated tolerance variations in the components of the ball joint. The deformation forces the cap into the socket opening as the ball joint is completed to engage the closure surfaces more tightly and assure maintenance of the seal of the socket opening.