Abstract: Disclosed is a diametral extensometer device useful for measuring diametral changes in a valve stem caused by axial loads on the valve stem used in a motor operated valve assembly. The device is useful in the field of valve diagnostics. The diametral extensometer is comprised of a semi-circular clamp member spaced from and partially surrounding the outer periphery of the valve stem. The clamp member has a reference pad that is adjustable radially with respect to the valve stem and a sensor pad having a sensor capable of measuring diametral changes in the valve stem and generating an electrical signal indicative of those changes. A stabilizer pad provides stability to the clamp member during installation of the clamp member and during movements of the valve stem. Non-stick surfaces are provided for smooth operation, and point contact with the valve stem eliminates errors associated with rocking of the traditional V-groove contacts.