The Whole 9 Inches- Installing a Ford 9-inch Rear End in a 1966 Mustang
The Ford 9-inch rear is by far the most heralded of all rear differentials. Factory installed in practically everything since 1957, there are millions hanging out in salvage yards everywhere, waiting to be slid under your ride. That being said, it isn’t just a simple bolt in procedure, especially if the housing you have isn’t a perfect fit.
When looking for a 9-inch housing, careful measuring is necessary for a proper fit. There are some tolerances that can be fudged a little; the overall width of the unit does not have to perfectly match the existing rear-end unit, it can be narrower (the most common choice) or wider, as long as your wheels will still fit. The spring perches are the most critical measurement, as there is less than a 1\4” of play available on the alignment pins. You certainly don’t want a rear end that has half of the spring perches sitting off the springs. While the perches can be cut off and welded in place, it is much easier to keep looking for a closer fit.
The third-member or center section of the 9-inch is also available in thousands of configurations with different gear ratios and limited slip or open differentials. For a performance unit, a limited-slip unit is the obvious choice, and with OEM and aftermarket units available, the decision is easy. The nice thing about the 9-inch is that you can buy a housing that fits the car and scavenge a third-member from another unit, and most salvage yards allow you to swap parts in the yard so you only have to buy one rear-end. Just make sure you keep the axles to match the housing, which is an important part of the puzzle.
Once all the parts have been secured, the installation can begin. While most of the install will be fairly straightforward, there will be the typical snags. We installed a 1957 9-inch housing with a 74-Ford brakes and an unknown year 3.73-limited slip third-member into our 1966 Mustang Fastback. Our biggest problem (besides a mental fatigue-induced brake-bleeding fiasco that resulted in a 3 a.m. finish time) was a need for a set of cross-sized U-joints. We didn’t have the time or money to swap driveshafts, so we sourced a set of U-joints from the local NAPA auto parts store with a pair of 1 1\8” caps for the rear-end yoke and a pair of 1” caps to fit the original drive shaft. This type of unit is available pre-made, but we needed ours that day, so we bought 2 joints and swapped caps. The center section of the joint is the same diameter, so no problems there, and we have a spare should we ever snap one.
We also ran into a problem with the original shock mount plates. These are the pieces that the U-bolts bolt to under the springs and clamp the rear end in place. The stock 8” rear axle tubes measure 2.5” in diameter, while the 9” tubes are 3”. This required a set of 3 1\8” U-bolts and modification to the shock plates. We had to drill 4 new holes in each plate in order to keep the shock mounting point the same. Just make sure you mark the plate in the correct orientation as they are really easy to turn 90 degrees and drill in the wrong place, this is knowledge learned from experience.
While our install had a few bumps in the road, the install was still fairly simple and should only take a few hours with all the right tools and parts in hand. Just be sure to get your parts a few days in advance, as some may need to be ordered, and can really put a kink in your plans. Especially if the project vehicle is your daily driver.
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