The strength of expansive cement grout borehole plugs cast in rock cylinders is investigated theoretically and experimentally as a function of curing and testing temperature of the plugged rock cylinder and of borehole size. The distributions of thermally induced stresses and displacements, shear stresses along the plug-rock interface due to an axial stress applied to the plug, and resultant interfacial shear stresses due to a combination of uniform elevated temperature and axial load are studied and analysed. Conservative in this analysis is the assumption of elastic behaviour and the fact that the strength tests were conducted after a cement grout curing period of only eight days. Details on experimental procedure regarding sample curing conditions, experimental apparatus and sample loading are presented. The axial strength decreases with increasing curing and testing temperature and with increasing plug radius. The results of the theoretical analysis lead to a recommendation to design plugs with a length-to-radius ratio of at least 8 in order to reduce the thermal shear stresses at the plug-rock interface to acceptable levels.