1956 Dodge Coupe

Ford 8.8 Rear End Swap

Ford 8.8 Rear End Swap 1956 Dodge Plymouth
The 2001 Ford Explorer 8.8 Core Rear End

Introduction

While building the poly 390 stroker and push-button 727 TorqueFlite are two things, planting the power is yet another, especially for a car that is going to see 1/4-mile passes on sticky cheater slicks. As with the factory front suspension with a stock 190 HP Dodge poly 270 engine and PowerFlite two-speed transmission, there is nothing inherently unsafe about the factory rear axle, but there are severe weaknesses, antiquated design features, and parts are more difficult to find and more expensive than more modern axle assemblies. Rather than spending time discussing all the issues with running the factory rear end or a 1956 Chrysler/Desoto rear end, I’ll just say it’s not an option. To start the debate for a swap, the 1956 Dodge and Plymouth have a 60″ axle width drum-to-drum with 41″ spring perch centers. The spring perches aren’t much of an issue for me since I can weld on new ones, but shortening the axle is something I prefer to not have to do.

A popular swap for a 1956 Dodge and Plymouth that will see track time putting out up to 600 TQ is a 1965 – 1970 B-body 8.75″ (pre-1965 have tapered axles). While once a cheap and easy option, those days are long gone despite what some say online. I run across the occasional forum post where someone asks for budget rear end swaps and someone replies claiming they purchased a complete, working posi 8.75″ for $400, but those steals are very few and far between if they are even true as of 2022. From all the swap meets and online ads within 200 miles of my house that I have scoured over the last five years, a complete posi 1965 – 1970 8.75″ assembly core that needs rebuilding costs between $800 – $1,000 and almost always have the smaller 741 carrier and gears taller than 3.55. The core will also need new forged axles for my purpose. By the time I buy such a core, purchase a ring and pinion and install kit, purchase forged axles, and go through the unit including bearings and brakes, I’ll be into an 8.75″ $2,000 if not more. 

Another option is a new Ford 9″ built to whatever width I want. Speedway Motors either manufacturers or carries everything needed to build a Ford 9″ with a new housing, new 3.70 posi third member, and new forged 31-spline axles. Factory 9″ drum brakes can be bolted on, or Ford Explorer 8.8″ disc brakes can be adapted. Here again, similar to the 8.75″, this Ford 9″ assembly and after locating and rebuilding brakes will run about $2,700 after shipping and tax.

8.8″ Ford Explorer Option

Now for where I have settled on the matter: 1990 – 2001 Ford Explorers use a strong 8.8″ rear axle that measures 59 -1/2″ drum to drum with a 38-1/2″ spring perch center. It also has an offset pinion like the 1956, so driveshaft to tunnel clearance will not be an issue. In multiple tests, the Explorer 8.8″ is comparable in strength to the Mopar 8.75″ and weighs about the same. The Mustang and Ranger also used a version of the 8.8″, but the Explorer has some benefits over the others. All Explorer 8.8″ assemblies feature a heavy gauge 3.25″ diameter housing, thick cast center section, forged 1.31″ diameter 31-spline axles, pinion flange for a bolt-on yoke, and 5 on 4-1/2″ bolt pattern. 1990 – 1994 have 10″ drum brakes like the Ranger and Mustang versions, whereas 1995 – 2001 have 11″ disc brakes and a slick cable-operated emergency brake setup with brake shoes that use the inside of the rotor as a drum. While many came with open differentials, many–particularly the Sport/Sport Trak models and those with tow packages–came with 3.27 (door label code D1), 3.55 (code D5), 3.73 (code D4), or 4.10 (code D2) posi differentials. Unless removed, there will also be a tag on one of the cover bolts with the ratio and L/S (limited slip). Salvage yards are ripe with the correct years, and complete units are under $200. Parts are cheap and readily available at any parts house. The downsides of the 8.8″ are that they use the axle shaft as the roller bearing inner race and don’t have a drop-out differential, but these are not critical for my build since the bearing design is strong/dependable, I don’t plan on swapping gear ratios, and I don’t mind the extra hassle if I have to change ratios. With the strength, gearing I want (3.73:1 posi), disc brakes with easy emergency brake hookup, almost exact width, and under $200 price tag, the Ford 8.8″ is an excellent choice for our 1956 Dodge.

Pulling the Rear End

I showed up to the same salvage yard where I picked the 1996 Dodge Dakota front clip after watching their online inventory until they had a handful of 1995 – 2001 Explorers in stock. Showing up to the first car, I found a 2001 wrecked on the front side and 160K miles on the odometer. I was pleased to find the D4 door label code I sought–a 3.73 posi unit. One downside of this car was that the yard had placed the jack stands underneath the axle to where I would need to jack up the car and reposition them to pull it. Moving down the list, the next car was an open differential, then a 3.55 posi, followed by one missing an axle, and the last one was a 4.10 posi with only 130K miles on the odometer. I paused long and hard at this low-miles 4.10 posi because it was such an easy pick with someone having already pulled the driveshaft and shocks, but 4.10 would be just too low for what I’ve designed, so I proceeded back to the first car.

After carefully inspecting the assembly for any damage and verifying the differential listed on the cover tag, I found that the forklift driver at the salvage yard had stabbed one of the shocks with the fork and clipped the brake hose, but everything else was fine. Using a drain pan, I pulled the cover and inspected the gear oil and housing floor for debris but found nothing unusual. I unbolted the driveshaft and rolled the ring and pinion over inspecting all the teeth, which looked fine. After jacking up the rear and repositioning the jack stands, I cut the emergency brake cable where it exited the floorboard to where I have plenty of cable to join to the 1956 cable using an adjustable bracket I will make. I pulled the shocks, the trailing arm front bolts, u-bolts, sway bar links, and the rear leaf spring shackle bolts. I lowered the rear of the leaf springs onto the ground and slide the axle assembly back and out from under the car. Looking at the sway bar, I am not sure if I can make it work if I have to relocate the spring perches out 1-1/4″ to match the 1956 springs, but I decided to spend the extra money to keep the sway bar, links, and the trailing arms in the event I can use them on this or another project. $140 later, I loaded everything into my truck and headed home (Figure 1). From what I can tell after more closely inspecting the assembly at home, I’ll spend a little more money on new brake pads (the rotors are usable), new emergency brake shoes, and a new brake hose.

In the next addition to this article, I will detail installing the rear end into the 1956 Dodge. I will include measurements, photos, and descriptions for anyone else interested in the swap, just as I am doing with the 1996 Dodge Dakota front clip. I will include details and likely write a tech article about properly measuring and setting pinion angle in relationship to the transmission output shaft and driveshaft angles. I am holding off on these suspension projects until I complete the engine build since I will need to clip the chassis, install the rear end, and install the engine and transmission all before I can measure up the driveshaft to get the car back on the road. I have a suspension shop that is going to build me a slip-yoke driveshaft to mate to the flanged 1964 727 TorqueFlite and the 8.8″ pinion flange, thus eliminating the 1956 ball and trunnion dinosaur.  

FIGURES

Ford 8.8 Rear End Swap 1956 Dodge Plymouth
The 2001 Ford Explorer 8.8 Core Rear End Awaiting Prep and Collecting Leaves