Low specific speed centrifugal pumps can operate as turbine in micro hydropower plants with relatively high head and low flowrate. Improving the hydraulic and mechanical characteristics of reverse pumps in such applications is highly appreciated. In the present study, the effect of tongue geometry on the performance and radial force of a low specific speed centrifugal reverse pump (10.3 [rpm, m(3)/s, m]) was studied experimentally. By measuring the flowrate, torque, inlet and outlet pressure as well as pressure distribution around the impeller periphery, hydraulic and mechanical characteristics of reverse pump for tongues of different stretchings and angles were obtained. Results showed that medium stretching can be used for reducing the radial force while further stretching of tongue is not beneficial. However, the longest tongue can boost efficiency the most. At nominal and high flowrates, tongue with medium stretching of angle -5 degrees can rise the efficiency and output power at the highest level comparing with other angles, while that of -10 degrees angle gives better condition at low capacities. The longest tongue of -10 degrees could rise efficiency and output power up to 8% and 32% respectively in off-design conditions which makes it the best geometry for the reverse pump. (C) 2019 Elsevier Ltd. All rights reserved.