Abstract: A mold core assembly for removable installation in a molding machine includes a first lock element attached to a core and a second lock element movably attached to a core retainer. The second lock element is movable between a locked position fitted in locking connection with the first lock element and an unlocked position separated from the first lock element. A slip ring surrounds the first lock element of the core, engaging and holding the second lock element in its locked position. A spring is connected between the core retainer and the slip ring, with the spring biased to oppose movement of the slip ring away from its locked position so that the core is normally held in its locked position in the core retainer.