A twist-lock clamp to attach a solar module to a rail system includes a spring between a top clamp and a t-nut, with features on the spring and top clamp to restrict rotation relative to each other. The t-nut includes features allowing rotation of about 90 degrees when the t-nut is inserted in a rail, but restrict rotation beyond about 90 degrees. The spring serves to hold the top clamp and t-nut apart while allowing the twist-lock clamp to resist sliding in the rail during installation.