The question has been asked by many engineers, why is the comparatively low scleroscope hardness specified for gears? The reason for this is that at best the life of an aviation engine is short, as compared with that of an automobile, truck or tractor, and that shock resistance is of vital importance. A sclerescope hardness of from 55 to 65 will give sufficient resistance to wear to prevent replacements during the life of an aviation engine, while at the same time this hardness produces approximately 50 per cent greater shock-resisting properties to the gear. In the case of the automobile, truck or tractor, resistance to wear is the main criterion and for that reason the higher hardness is specified. Great care should be taken in the design of an aviation engine gear to eliminate sharp corners at the bottom of teeth as well as in keyways. Any change of section in any stressed part of an aviation engine must have a radius of at least 1/32 in. to give proper shock and fatigue resistance. This fact has been demonstrated many times during the Liberty engine program. CONNECTING RODS The material used for all connecting rods on the Liberty engine was selected at the option of the manufacturer from one of two standard S. A. E. steels, the composition of which are given in Table 13. TABLE 13.--COMPOSITION OF STEELS NOS. X-3,335 AND 6,135 Steel No. X-3,335 6,135 Carbon, minimum 0.300 0.300 Carbon, maximum 0.400 0.400 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 connecting rods were heat-treated to show the following minimum physical properties; Elastic limit, 105,000 lb. per square inch: elongation in 2 in., 17.5; per cent, reduction of area 50.0; per cent., Brinell hardness, 241 to 277.