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Introduction. A decrease of damaging loose grains by threshing tools when combing standing plants is an urgent scientific problem. The aim of the research is to develop a technical solution and experimentally confirm the possibility of loose grains pre-separation from the stripped grain heap before it enters the threshing chamber of the combine harvester. Materials and Methods. The most promising technical solution to minimize the damaging of loose grain is an inclined chamber containing an oblong-hole lattice bottom, under which inclined augers are mounted. The experimental studies had two stages. In the first series of experiments, in laboratory conditions there were determined the optimal sizes of the lattice bottom holes to ensure the maximum passage of loose grains. In the second series of experiments, the degrees of separating and damaging loose grains were evaluated in real field conditions. Results. According to the results of laboratory studies, it was found that the maximum passage of loose grains through the holes of the lattice bottom is 68.7% with the width of holes 8 mm and their length 160 mm. According to the results of field experiments, it was found that the average passage of loose grains through the holes of the lattice bottom is 90%. At the same time, damaging grains in the inclined chamber did not exceed 0.5%, and in the hopper 1.75%. Damaging grains in the bunker was in the range of 5.25% when the combine operated without pre-separation of the combed grain heap on the lattice bottom of the inclined chamber. Discussion and Conclusion. Pre-separation of loose grains from the combed heap makes it possible to reduce damaging grains at least three times, and also reduces the power consumed by the drive of the threshing machine by 11–12%.