In 1989 while refitting his mud truck / monster truck “Mud Hog” from GMC 2.5 tons to 2.5 ton Rockwell, Russ Gearhart made his first set of larger than stock axle shafts.  They were 1 7/8 diameter – 36 spline out of 300M aircraft grade chrome moly steel.

After beating on them with 66″ Terra tires, and a roller cam, blown 460 Ford, he eventually broke the factory 2.5 ton axle u-joints and had other numerous problems with spindles and various other components.  These failures would eventually lead to Russ rolling the truck and thinking there must be a better way to strengthen the weak areas of the 2.5 ton Rockwell’s by mating the much stronger 5 ton steering knuckle and spindle to the 2.5 ton housing and differential was the best solution.  The 5 ton knuckle has a larger cavity allowing you to use the much stronger 5 ton u-joint and axle shaft.

Over the years Russ would find and perfect the solution by building out different hybrid axles for “Mud Hog” which was one of the first real “Monster Trucks” in the world.  This was the beginning of a life long journey to design, create and test some of the most advanced and durable hybrid-driveline components in the world.

The three options when using factory 5 ton axle shafts are the Bendix (ball and claw), Rezeppa and U-joint style or the Gearhart Engineering 4340 chrome moly U-joint style. Using any combination of long or short axle shafts can be used to achieve the desired axle width, as well as, a centered pinion shaft or off-set pinion shaft.  Wheel hub options include factory 5 ton hubs or factory 2.5 ton hubs; both are available in aluminum.  When using 2 1/8 diameter 16 spline 5 ton shafts there are 2 differential options; a spool or a Detroit locker.  When using the 1 7/8 36 spline shafts there are 3 differential options; the spool, Detroit locker, or spider gears.  All Gearhart Engineering hybrid axle assemblies include Gearhart Engineering innovation bearing cap girdle kit.

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