The hardening distortions with respect to base body, clutch teeth and helical gears are investigated for a serial‐produced main shaft gear of a 20NiCrMoS6‐4 steel. The influences of casting geometry, annealing heat treatment and stress relief annealing of blanks, as well as vertical and horizontal loading arrangements during case hardening, are studied. The concentricity, roundness and runout of clutch teeth and helical gears are measured in the soft machined, hardened and hard‐machined conditions. The Brinell hardness is measured on blanks obtained from different manufacturing routes showing differences in hardness and scatter. Stress relief annealing lowers the hardness and the scatter for all groups, but has no significant effect on distortions. The case depth, core hardness and surface hardness are measured after hardening. The study shows that the surface hardness correlates with the oil flow measured in the quench tank. The effect of casting geometry is stronger for the clutch teeth compared to the helical gears. For the clutch‐teeth roundness and runout, significantly lower values are found for square geometry compared to rectangular. It is also seen that the major part of the runout comes from roundness errors which are mainly induced by the hardening. Horizontal loading reduces roundness errors and runout but produces conical base‐bodies with worse backplane flatness.