A releasably locking dilator and sheath assembly and methods for releasing the dilator from the sheath and longitudinally splitting the sheath. The assembly includes a dilator having a dilator hub and a sheath having a sheath hub. The sheath hub may have a valve and has two opposing winged tabs, each tab having a perpendicular portion and an angled portion as well as a female threaded portion. The dilator hub has a male threaded portion designed to engage the female threaded portion of the sheath hub. The dilator is released from the sheath by rotating the dilator 90° in relation to the sheath and pulling the dilator out of the sheath. The sheath is longitudinally split by creating a couple on each of the winged tabs thereby forcing the sheath and the sheath hub to split longitudinally. With the sheath and sheath hub split longitudinally, the sheath is removed from around a catheter while leaving the catheter in place.