Diesel engine manufacturers strive towards further efficiency improvements. Thus, reducing in-cylinder heat losses is becoming increasingly important. Understanding how location, thermal insulation, and engine operating conditions affect the heat transfer to the combustion chamber walls is fundamental for future improvements. In the course of this study, the surface temperature of a steel piston and a piston with an yttria-stabilized zirconia (YSZ) coating was measured at different crank angles using phosphor thermometry. The experiments were carried out on a low-vibration, heavy-duty single cylinder engine equipped with an optical access through a removed exhaust valve. The setup included also a flow meter and two thermocouples to measure the amount and temperature of the oil which is injected into the piston cooling gallery. To be able to measure surface temperature though the flame at continuous operation, the engine was run on n-heptane to reduce soot formation. During and after combustion, the results showed a fast surface temperature variation for both the steel piston and the YSZ piston. For both pistons, the measurements were carried out at three positions on the piston surface and seven engine conditions. Using the crank-angle resolved surface temperature, the oil temperature and the mass flow of the oil, the heat flux was calculated and the heat losses of the YSZ coated piston were compared to the heat losses of the steel piston.