A trial implant system includes a stem component and an articulating component forming part of a joint of a patient. A trial mounting assembly mounts the articulating component to the stem component at variable relative angular positions. The trial mounting assembly includes an expandable ball portion and an expansion element configured to expand the ball portion upon rotation of the expansion element. In an expand state, the ball portion contacts a cavity within the stem at three points of contact to fix the center of rotation of the ball portion at a pre-determined location within the stem. The stem cavity defines an inwardly projecting circumferential lip that bears against ball portion as it expands to ensure the three-point contact.