Our own research and analysis of the literature cited in the work have identified a number of different factors that affect the quality of the harvested grain. The paper provides an assessment of the influence of such indicators as the physical and mechanical properties of the harvested crop, the design features of the threshing-separating devices, the relative humidity of the processed heap, the size of the openings of the screens of the separating device. Studies carried out for machines with various types of threshing and separating devices have shown the advantage of rotary machines. The heap obtained on them with minimal contamination contains 2.2 times less crushed grain and 1.5 times more grain with the film. The sowing qualities of seeds, both germination energy and laboratory germination, are higher than those of seeds obtained from combine harvesters with a classic threshing-separating device. Investigations of the distribution of the components of the grain heap coming from the combine harvesters were carried out at five different moisture levels of grain on sieves with a hole width of 2.0 to 3.2 mm. The experimental results showed that the relative humidity of the grain does not significantly affect the distribution of the grain heap. At the same time, studies have shown that the bulk of the heap entering post-harvest processing in a grain cleaning machine is allocated on sieves with a hole diameter of 2.4–3.0 mm. The largest amount of whole grain is allocated on sieves 2.4–3.0 mm. With an increase in the grain moisture content of the initial heap from 14.1 to 18.9% and the size of the sieve openings from 2.0 to 3.2 mm, an increase in the amount of grain with the film is observed. The results presented can be used in organizing the activities and substantiating the design and technological schemes for post-harvest grain cleaning.