In the finishing of highly stressed parts for aviation engines, extreme care must be taken to see that all tool marks are eliminated, unless they are parallel to the axis of strain, and that proper radii are maintained at all changes of section. This is of the utmost importance to give proper fatigue resistance to the part in question. GEARS The material used for all gears on the Liberty engine was selected at the option of the manufacturer from the following standard S. A. E. steels, the composition of which are given in Table 12, TABLE 12.--COMPOSITION OF STEELS NOS. X-3,340 AND 6,140 Steel No X-3,340 6,140 Carbon, minimum 0.350 0.350 Carbon, maximum 0.450 0.450 Manganese, minimum 0.450 0.500 Manganese, maximum 0.750 0.800 Phosphorus, maximum 0.040 0.040 Sulphur, maximum 0.045 0.045 Nickel, minimum 2.750 Nickel, maximum 3.250 Chromium, minimum 0.700 0.800 Chromium, maximum 0.950 1.100 Vanadium, minimum 0.150 All gears were heat-treated to a scleroscope hardness of from 55 to 55. The heat treatment used to secure this hardness consisted in quenching the forgings from a temperature of 1,550 to 1,600 deg.F. in oil and annealing for good machineability at a temperature of from 1,300 to 1,350 deg.F. Forgings treated in this manner showed a Brinell hardness of from 177 to 217. RATE OF COOLING At the option of the manufacturer, the above treatment of gear forgings could be substituted by normalizing the forgings at a temperature of from 1,550 to 1,600 deg.F. The most important criterion for proper normalizing, consisted in allowing the forgings to cool through the critical temperature of the steel, at a rate not to exceed 50 deg.F. per hour. For the two standard steels used, this consisted in cooling from the normalizing temperature down to a temperature of 1,100 deg.F., at the rate indicated. Forgings normalized in this manner will show a Brinell hardness of from 177 to 217. The question has been repeatedly asked as to which treatment will produce the higher quality finished part. In answer to this I will state that on simple forgings of comparatively small section, the normalizing treatment will produce a finished part which is of equal quality to that of the quenched and annealed forgings. However, in the case of complex forgings, or those of large section, more uniform physical properties of the finished part will be obtained by quenching and annealing the forgings in the place of normalizing. The heat treatment of the finished gears consisted of quenching in oil from a temperature of from 1,420 to 1,440 deg.F. for the No. X-3,340 steel, or from a temperature of from 1,500 to 1,540 deg.F. for No. 6,140 steel, followed by tempering in saltpeter or in an electric furnace at a temperature of from 650 to 700 deg.F.