The ring groove areas of squeeze-cast Al-12%Si alloy pistons can be selectively reinforced with Saffil (Al2O3) fibres or SiC whiskers to provide local high temperature strength and wear resistance : Since the reinforced region and the unreinforced alloy typically have different coefficients of thermal expansion, cyclic residual stress may occur at the macro-interface between them when it experiences thermal cycling. This could conceivably result in fatigue induced damage at the macro-interface, making it susceptible to failure. To investigate this, the strength of the macro-interface has been measured before and after thermal cycling using bimaterial tensile samples. Prior to thermal exposure, samples typically failed at the macro-interface with an average strength less than that of the unreinforced alloy alone. The low initial strength has been attributed to several factors, including poor alloy-reinforcement bonding and an accumulation of brittle particles or other material at the macro-interface. After being thermally cycled 1000 times between 50 degrees C and 275 degrees C or given an equivalent isothermal exposure, samples typically failed in the unreinforced alloy or at the macro-interface with average strengths less than those measured prior to thermal exposure. However, there was no clear evidence that fatigue induced damage had occurred as a result of thermal cycling and the strength drop associated with thermal exposure has been attributed to alloy overageing.