1. Identify Your Current Rear End:
* Housing Type: Determine the size of your rear end housing (e.g., 10-bolt, 12-bolt). This is crucial for selecting compatible parts. The 10-bolt was more common in lighter-duty trucks, while the 12-bolt was used in heavier-duty applications.
* Gear Ratio: Note your current gear ratio (e.g., 3.73, 4.11). You'll want to either reuse your existing gears or source new ones that match your desired ratio. Changing the gear ratio is a significant undertaking within this project.
* Axle Shaft Size: Measure the diameter of your axle shafts. This ensures you get the correct posi-unit.
2. Options for Posi-Traction Installation:
* Complete Posi-Unit: The easiest and often most cost-effective approach is to purchase a complete limited-slip differential assembly (a posi-unit) that's compatible with your rear end type and gear ratio. This is a drop-in replacement. These are available new or used from various suppliers specializing in classic Chevy parts. Make sure to specify your rear end type and gear ratio when ordering.
* Rebuild with a Posi-Unit Carrier: You can rebuild your existing differential, replacing the spider gears and side gears with a posi-unit carrier assembly. This involves disassembling the differential, installing the new posi unit, and reassembling the differential housing. This requires more mechanical skill and specialized tools. If you decide on this option you'll also need to make sure the carrier itself is compatible with a limited-slip unit.
* Adding a Locker: Instead of a limited-slip differential, you could install a locking differential (such as a Detroit Locker or similar). This locks the axles together, providing maximum traction, but it can be rougher on the drivetrain and not ideal for on-road driving.
3. Parts Needed (general):
* Posi-traction unit (complete or carrier): This is the heart of the conversion.
* Differential cover gasket: A new gasket is crucial to prevent leaks.
* Gear oil: Use the correct weight and type of gear oil specified for your differential.
* Differential setup tools: If rebuilding the differential, you'll need a gear installation tool and a torque wrench for proper assembly. It is highly recommended to have these tools for a complete rebuild.
* Possibly new ring and pinion gears: Depending on the condition of your existing gears and your desired gear ratio.
4. Installation Steps (General Outline):
* Removal: Remove the rear axle assembly from the truck. This requires jacking up the truck, removing the wheels and brakes, disconnecting driveshafts, and unbolting the differential from the leaf springs or suspension.
* Disassembly (If rebuilding): If you're rebuilding your differential, carefully disassemble it, noting the order and position of all parts.
* Installation: Install the posi-traction unit (or rebuilt differential) into the housing. If you are installing a complete unit, this is less involved.
* Reassembly: Reassemble the differential, carefully torquing all bolts to the manufacturer's specifications.
* Installation: Reinstall the rear axle assembly back into the truck.
* Fluid: Fill the differential with the correct gear oil.
Important Considerations:
* Mechanical Skill: This is not a beginner's project. Replacing a complete assembly is easier than rebuilding, but still requires some mechanical aptitude. If you're uncomfortable working on differentials, it's best to have a professional mechanic perform the work.
* Tools: You'll need specialized tools for working on differentials, including a bearing press, gear installation tools, and a torque wrench.
* Safety: Work in a well-ventilated area and wear appropriate safety gear, including eye protection and gloves.
This information provides a general overview. For specific instructions, consult a repair manual for your 1968 Chevy truck, which will provide detailed instructions and torque specifications for your specific model and rear end type. Many online forums dedicated to classic Chevy trucks also have valuable information and discussions regarding this conversion.
