Drivetrain & Differential

What belongs in the drivetrain

The drivetrain moves power from the crankshaft to the ground. On a typical 4x4 that means the clutch or torque converter, the transmission, the transfer case, front and rear driveshafts, and the axle assemblies with their differentials, gears, bearings and seals. The subcategories here separate those systems so you can narrow down to complete axles and third members, performance axle components, OEM-style replacement axle parts, drive shafts, shifters or transfer case upgrades.

Identify the axle before you shop

Most axle parts are specific to the axle model, spline count and sometimes the production year. Product titles in this category refer to Dana 30, Dana 35, Dana 44, Dana 80 and AMC Model 20 housings, among others. Read the data tag on the differential housing or pull the cover and count ring gear bolts and measure the ring gear diameter. A part labeled for a Dana 44 front axle will not necessarily fit a Dana 44 rear axle, so note the axle location as well as the model.

Gear ratio and differential type

• Ring and pinion ratio: a numerically higher ratio such as 4.56 or 5.38 multiplies torque more and helps larger tires turn freely, at the cost of higher engine speed on the highway.
• Matched sets: ring and pinion gears are lapped and used as a pair, and front and rear ratios must match on a four-wheel-drive vehicle.
• Open, limited-slip and locking differentials: each handles traction differently, and clutch-type limited-slip units use clutch packs and friction additives that wear over time.

Installation and setup

Gear installation depends on correct pinion depth, backlash and bearing preload. These adjustments need dial indicators, marking compound and patience, and a mistake can destroy a new gear set within a few miles. Many owners hand this work to a driveline shop. Replace crush sleeves, seals and gaskets whenever the assembly is opened, and use the gear oil grade and capacity the axle manufacturer specifies. Torque fasteners to published values.

Confirming compatibility

Always confirm fitment for your exact year, make and model, and compare the part number against the manufacturer's specifications. Spline count, axle shaft diameter, bolt pattern and housing casting numbers all matter, and cross-referencing them avoids returning the wrong part after a weekend teardown.

Maintenance habits matter as much as the parts you choose. Check axle breather tubes so they are not clogged or collapsed, since a blocked breather builds pressure and pushes oil past the seals. Inspect universal joints and slip yokes for play, grease them if they have fittings, and look at the driveshaft angles after any lift or suspension change. Vibration at a particular speed often points to a driveshaft that is out of balance or running at too steep an angle. Gear oil that looks milky has taken on water, and metallic glitter on a magnetic drain plug is a sign that something inside deserves a closer look before it fails on the trail. Keep a record of the ratio, axle model and spline count for each axle so future orders are straightforward.

When budgeting a drivetrain project, remember the supporting parts. Bearings, seals, shims, carrier hardware and fluids are all needed alongside gears, and skipping them to save a little money tends to cost more later. If you are unsure which items belong together, a driveline specialist can review your list.