In late 1987, Ford introduced the 5speed ZF transmission in the F250, F350, & F450 trucks. They were available for gas as well as diesel models. The easiest way to identify the unit is to see that it has power take off (pto) opening plates on the lower sides of the unit. ZF transmissions are reliable units that should provide good service when taken care of properly. Keep in mind that the ZF has 6 constant mesh mainshaft gears, so there is a considerable amount of potential for heat buildup. It is never a good idea to leave the truck idling with a pto engaged, such as in tow truck applications. The unit was redesigned in '96 and had small, but significant changes. The synchro teeth were enlarged to provide longer life and more positive shifts, and the bearing retainer was designed to be replaced without having to disassemble the case. The case was beefed up also with this design.