I rember learning that work hardening can not be perfomed post heat treating and does not require heat. Thats why I am asking. When we cut bevel gears at my old job (gear cutters are cool) we had them sent to alferd heller in patterson for heat treating and oil quench to 50-60 rockwell so that their physical proberties could not be altered in use. Its tough to learn and be personaly involved in certain things and see and hear statements that dont make sence and question them. But if I am missing something I do want to learn. I never change the oil in my differentials but after reading what mark said I am curious. Until i see evidance of gawled gear sets due to lack of so called break in I hate to offer advise suporting the proccedure. In the end its really a moot point i suppose, it only takes a few bucks and an hour of time for peace of mind. I just feel it to be a waist unless i learn otherwise.

i didn't see anything in all data for you but it did say lubricate everything in gear oil before installing.

Just got off of the phone with a metallurgist/mechanical engineer. Here's what he said:


When you are breaking it in you are setting the right balance of hardness on the outer surface of the gear. There is no real geometry changing.

When the gears are manufactured there's a hobb setting which defines the exact environment that they were manufactured under. This is what is inscribed on the gear in tenths of thousands. At GM they probably have this setting and can spec the rear setup to minimize the need for a break in. In a shop you don't have their type of setup and a break in is needed to make up for the lack of industrial environment. Street gears for non high performance cars are harder than high performance gears and aren't as sensitive to the lack of a break in period.

He mentioned the cross sections of the gears and their associated Rockwell hardness however I do not recall enough of that to post it completely. Something along the lines of the whole gear set being hard enough but the alignment of molecules being key for wear protection.