Two-Speed Rear Axles
By Tony Pascarella
For those hardy souls restoring larger trucks, the siren song of the two-speed rear axle, first offered in the '39 Chevy 2-tons*, is hard to resist. Maybe you're upgrading your ton and a half with one of these handy units, or perhaps your truck already has one. Either way, here's some handy info from Tony Pascarella. For those contemplating searching out a 2-speed unit, please keep in mind that the Advance Design 2-speed has a 8:10 "low-range" ratio and a 6:10 "high-range." The regular rear axle is a 6:17. Thus, the 2-speed rear won't give you a better top speed, it gives you more "grunt" to start a heavy load moving.
*Prior to the '39 model year, 2-speed rear axles were available as after market add-ons from Timken and other manufacturers.
Obviously the biggest thing different about the Advance Design and other older 2-speed rear axles is the vacuum shift unit itself. The easiest way to check the operation of the two speed (especially if the driveshaft is disconnected or the axle is out of the truck)is to use a vacuum source to shift the axle from one range to another while turning the pinion yoke by hand. You can feel the unit shift. A small hand operated vacuum pump is useful. On some units, the vacuum diaphragm can be removed and the unit shifted mechanically.
To check your truck rear brakes, Jack the axle up until the wheels are free of the ground, secure with jack stands. First the brakes must be backed off. There are two adjusting nuts on each backing plate. Turn the nuts in a counter clock wise direction to loosen. If you take off the wheels, there are a few screws holding on the drum. No need to pull the heavy hub. Just remove the screws, slacken the brake adjusters, pull off the 40 lb drum.
Place an old can under the hub to catch any gear oil leaking out and remove the 8 bolts on the axle flange. Hit the axle flange squarely with a heavy hammer. The axle should pop out enough to grasp it and pull it free. You will be looking at some large nuts retaining the hub. Using an old screw driver ,pry away the locking tab.
Either using a proper wrench or a proper punch and hammer, slacken the right hand thread nut, pull off the lock tab and remove the inner nut. The hub and wheels are ready for removal.
If you don't have a wheel dolly, gently lower the axle and wheels onto a good creeper, taking some of the wheel weight on the creeper. Place a piece of plank or plywood on the creeper for reinforcement.Try to pull off the assembly by wiggling slightly from side to side or up and down as you pull. You might have to raise or lower the axle slightly to get it in the best position. The assembly weighs maybe 300 lbs, so be cautious.
Remember, the inner axle seal is easily damaged, so be as careful as you can.
An alternate method is to remove the wheels leaving the hub in place. With the help of an assistant try pulling off just the hub and drum. The hub and drum weigh maybe 120 lbs.This second method is sometimes easier for the home mechanic without a dolly.
When you replace the drum, be mindful of that new or existing oil seal. Steady as she goes! The inner nut gets tightened fairly snug, to put a pre-load on the bearings. Make a new paper gasket for the axle flange, as sealer alone will usually leak.