pumpkin

A pumpkin is the cast iron or aluminum housing that contains the differential gears in a vehicle's axle assembly. It bolts to the axle tubes and holds the spider gears, side gears, and bearing races that allow the drive wheels to rotate at different speeds during turns. The term is pure shop slang, born from the round, bulbous shape of the rear axle housing on many vehicles, especially trucks and heavy equipment.

The housing itself must be strong enough to handle shock loads and torque while remaining sealed against mud and water. Most pumpkins are bolted together in two halves along a horizontal or vertical centerline, allowing technicians to separate them for gear service or replacement. The ring gear bolts to the inside of the pumpkin, and the pinion gear meshes with it to transfer power from the driveshaft to the axle shafts. Drain and fill plugs, usually magnetic, are threaded into the housing to contain hypoid gear oil.

Pumpkin size and strength vary by application. A light-duty pickup truck may have a pumpkin rated for 3,500 to 5,000 pounds per axle, while a heavy-duty semi rear axle pumpkin is built for 20,000 pounds or more. The shape and bolt pattern differ between manufacturers and model years, so pumpkins are not readily interchangeable. A cracked pumpkin or a pumpkin with stripped threads will leak oil, cause noise, and eventually fail the differential entirely.

Mechanics refer to rebuilding a pumpkin when they disassemble it, replace worn bearings and gears, and reassemble it with fresh seals and oil. This differs from simply swapping out a pre-built differential unit. The term also appears in drag racing and hot rod culture, where enthusiasts swap pumpkins from high-performance axles into lighter frames to gain better gear ratios or locking differentials. A welded pumpkin, in which the spider gears are locked solid by welding or other means, is favored in racing because both wheels turn at the same speed regardless of traction.