Actually the reasons are two-fold. You need to heat treat the bearings and the metal gear face needs to harden under heat and pressure. If you don't break them in properly, you can actually get to the softer metal underneath and then all hell is going to break loose in there.My mechanic explained the reason for the need to break the gears in. He spoke with a tech from Ford racing who explained that even though the gears are made of hardened steel. Going through several heating and cooling cycles keeps the metal from flaking. Anyway after the install he ran it through cycles and then told me to take it easy. The cost to have it done was $400.