The effects of the orientation of a bileaflet mechanical heart valve on the viscous and turbulent stresses in the flow past it and on the flow rate in the right coronary artery were investigated in vitro in a mock circulation loop, using a fluid that matched the kinematic viscosity of blood and the refractive index of the aorta model. Measurements were made past the valve mounted in three orientations at the base of an anatomical aorta model, within physiological aortic flow conditions. At peak flow, the turbulent stresses were on average 21 % higher and viscous stresses exceeding 10 Pa (namely of a level that has been associated with blood cell damage) were 30 % more frequent when the valve was oriented with its plane of symmetry normal to the aorta’s plane of curvature than when it was parallel to it. This was attributed to the impingement of a lateral jet on the concave wall of the aorta and to steeper velocity gradients resulting from the geometrical imbalance of the sinuses relative to the valve’s central jet when the valve was in the ‘normal’ orientation. Very high levels of turbulent stresses were found to occur distal to the corners of the valve’s lateral orifices. The bulk flow rate in the right coronary artery was highest when the valve was positioned with its central orifice aligned with the artery’s opening. The coronary flow rate was directly affected by the size, orientation and time evolution of the vortex in the sinus, all of which were sensitive to the valve’s orientation.