An improved protective storage cap for a rod-shaped device or instrument, particularly a fountain pen, wherein the female threads for securing the device within the cap are disposed within an axially and rotationally moveable ring. The ring is held stationary in the cap by the axial thrust of a compression spring while the male threads of the device are being rotationally engaged with or disengaged from the female threads of the cap. The cap cannot be overtightened and thereby damaged because further rotation of the device after full engagement of the threads serves only to overcome the restraining thrust of the spring and the moveable ring simply rotates in its cap with the device. The axial freedom of the ring and spring also permit the device to enter the cap to a depth sufficient to form a snap seal between the working end of the device and a chamber recessed within the cap, whereby the tool of the device, for example the writing nib of a fountain pen, sealingly is isolated from the outside environment. The pen body can be secured in the cap either by the snap seal alone or by the combination of the seal and the engaged threads. The cap can be stored on the non-working end of the device while the device is being used. Since the threaded ring is able to slide axially, its threads cannot be forced onto the non-working end, and thereby marring of the surface of the pen is prevented.