for example the combustion chamber neck and to enlarge the distance a in areas where the heat flow is less such as for example in the area of the injection head the axial depth d v of a given depression is identical in each of the circle segments shown in fig3 the depth d v of the depression however can vary along the longitudinal axis 7 however in order to keep the groove effect of the depressions from becoming too large the circle segment of a depression should not exceed a semicircle the depth d v has to be chosen as a function of the selected cooling medium and of the flow velocity axial depths in the range from 0 1 mm to 0 2 mm have proven to be favorable in order to improve the heat transfer by up to 50 only two bores are needed in order to create the four circle segments that is to say the four depressions in the web walls 10 of the cooling channel of fig3 in contrast a total of four bores have to be made in the embodiment of fig4 whereas the drilling axis 8 of a given bore is situated on the longitudinal axis 7 in the example of fig3 each drilling axis 8 in fig4 is arranged offset by an offset distance d relative to the longitudinal axis 7 the radii of these bores are smaller than the previous embodiment resulting in a greater groove effect as far as the depth d v of the depression is concerned like in the above mentioned embodiment it is sufficient if it is in the range from 0 1 mm to 0 2 mm whereas the depressions are arranged symmetrically.