Part 4: Working on the Rear Diff

Building a heavy-duty rearend for your Mopar used to mean either buying a Sure Grip 83/4 or going for the famed Dana 60. Often, due to cost or availability, neither is realistic. Fortunately, there is a low-buck solution to maximizing rearend traction.

We contacted National Drivetrain and uncovered all the hardware needed to turn our open differential into a genuine locker without breaking the bank.

A locker? That's right. National Drivetrain has the parts you need to give your red light brawler all the traction it craves.

Our axle assembly featured a 741 casing (which we upgraded to a 742 during the course of this buildup) with an open differential and 2.76 gears--definitely a weak link for anything other than casual highway cruising. To fix that little problem, we received from National Drivetrain a 3.91 ring-and-pinion set. So much for the torque-multiplication dilemma. To transfer that torque equally to both axle shafts, we selected a Powertrax Lock-Right unit.

The Lock-Right replaces your existing spider gears with an automatic locking gear set. This is a great upgrade for do-it-yourselfers who want to keep their street car...well...streetable, but are looking for a little straightline advantage.

It works something like this: During typical driving conditions, a Lock-Right-enhanced differential allows your axles to turn at different speeds (or differentiate), much like an open or limited-slip differential gear set. It's a bit clicky-sounding during sharp, low-speed cornering (when the meshed teeth overrun each other), but this is perfectly natural. Since the unit is spring-loaded, under straightline operation (both axles turning in unison) the axles are continually locked together. That's the key to full power transfer to the rear wheels and a solid hole shot.

Installing a Lock-Right is a fairly straightforward job, and the best part is that it doesn't require you to replace the differential or go through the tedious gear setup process. All you need to do is replace the existing spider gears with the Lock-Right. If, however, you need to change gears like we did, a complete gear setup sequence will be necessary.

The Lock-Right installation instructions, included with the kit, are easy to follow, but you may want to have a service manual for the rearend you are working on. With the exception of a dial indicator and magnetic base (if you're changing gears), the required tools to install the Lock-Right are probably sitting in your garage.

This particular rearend housing came with the engine we bought for our Valiant project. It is an A-Body rear, complete with big bolt pattern axles, although the open differential and highway gears definitely needed help.