A tool for attaching valve keys to a valve stem includes an elongated holder with a longitudinal axis, an inner hollow space and an end opening. An elongated core is arranged in the hollow space of the holder coaxially to the longitudinal axis. An elongated recess for receiving a substantial upper portion of the valve stem is provided in the end region of the core coaxially to the longitudinal axis. The recess of the core issues into the end opening and has a depth which is greater than or at least equal to the axial distance between the end and the annular groove of the valve stem. The core at least partially covers the annular groove, such that the valve keys can be supplied to the lateral surface of a valve stem underneath the annular groove and incorrect engagement can be avoided.