The heat treatment used to secure these physical properties consisted in normalizing the forgings at a temperature of from 1,550 to 1,600 deg.F., followed by cooling in the furnace or in air. The forgings were then quenched in oil from a temperature of from 1,420 to 1,440 deg.F. for the No. X-3,335 steel, or from a temperature of from 1,500 to 1,525 deg.F. for No. 6,135 steel, followed by tempering at a temperature of from 1,075 to 1,150 deg.F. At the option of the manufacturer, the normalizing treatment could be substituted by quenching the forgings from a temperature of from 1,550 to 1,600 deg.F., in oil, and annealing for the best machineability at a temperature of from 1,300 to 1,350 deg.F. The double quench, however, did not prove satisfactory on No. X-3,335 steel, due to the fact that it was necessary to remove forgings from the quenching bath while still at a temperature of from 300 to 500 deg.F. to eliminate any possibility of cracking. In view of the fact that this practice is difficult to carry out in the average heat-treating plant, considerable trouble was experienced. The most important criterion in the production of aviation engine connecting rods is the elimination of burned or severely overheated forgings. Due to the particular design of the forked rod, considerable trouble was experienced in this respect because of the necessity of reheating the forgings before they are completely forged. As a means of elimination of burned forgings, test lugs were forged on the channel section as well as on the top end of fork. After the finish heat treatment, these test lugs were nicked and broken and the fracture of the steel carefully examined. This precaution made it possible to eliminate burned forgings as the test lugs were placed on sections which would be most likely to become burned. There is a great difference of opinion among engineers as to what physical properties an aviation engine connecting rod should have. Many of the most prominent engineers contend that a connecting rod should be as stiff as possible. To produce rods in this manner in any quantity, it is necessary for the final heat treatment to be made on the semi-machined rod. This practice would make it necessary for a larger percentage of the semi-machined rods to be cold-straightened after the finish heat treatment. The cold-straightening operation on a part having important functions to perform as a connecting rod is extremely dangerous. In view of the fact that a connecting rod functions as a strut, it is considered that this part should be only stiff enough to prevent any whipping action during the running of the engine. The greater the fatigue-resisting property that one can put into the rod after this stiffness is reached, the longer the life of the rod will be. This is the reason for the Brinell limits mentioned being specified.