1956 Dodge Coupe

Ford 8.8 Rear End Swap

1956 Dodge Factory Rear Suspension
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 Ford 8.8″ 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. 

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

To prepare the 1956 Dodge chassis for the 8.8″ rear end, I performed the following steps:

  1. Place the front and rear chassis rails on jackstands high enough to work underneath the car. I place the rear jackstands on the side rails as far back toward the rear bumper as possible to allow room to work on the leaf spring shackles.
  2. Apply penetrating oil to the rear end u-bolt nuts/threads, shock mount nuts, front leaf spring mounting bracket nuts, front leaf spring eye bolts, leaf spring shackle nuts.
  3. Remove the brake hose from the chassis brake line.
  4. Remove the u-bolts, mounting plates, and shocks.
  5. With a floor jack placed under the left leaf spring until it just touches the spring, remove the front leaf spring mounting bracket. Lower the floor jack to the ground and pull out the jack so the left leaf spring and left side of the rear end rest on the ground.
  6. Repeat step 5 on the right side.
  7. At this point, the rear end may be slid/rolled forward on the ground to release any pressure on the leaf springs.
  8. Remove both shackles and leaf springs. The shackle bolts are usually extremely corroded and require prying and possibly using a hammer and drift to punch them out. Silicone spray can help.
  9. Remove the front leaf spring eye nut, bolt, and mounting brackets from the leaf spring.
  10. My shackle bolts were heavily corroded, so I rebuilt them (Figure 2). I explain this process in the article on leaf springs and shackles.
1955 1956 Dodge Plymouth Leaf Spring Shackles
Figure 2: 1956 Dodge Rebuilt Shackles
Prepping the Ford 8.8″ Rear End

The 8.8″ leaf spring perch locations are too inbound and need to be removed. The link bar brackets interfere with the new spring perch locations and need to be removed. Using a cut-off wheel in an angle grinder, I cut off the spring perches and link bar brackets being careful not to cut into the rear end housing. Switching to an 80-grit sanding disc on the angle grinder and clean up the cuts until the housing is smooth and at down to bare metal from the original perch location toward the brakes to where the housing tapers to allow for welding on the new spring perches to clean metal (Figures 3 – 4).

1955 1956 Dodge Plymouth Ford 8.8 Rear End
Figure 3: Ford 8.8″ Rear End Original Spring Perch and Link Bar Bracket
1955 1956 Dodge Plymouth Ford 8.8 Rear End
Figure 4: Ford 8.8″ Rear End Prepped for New Spring Perches
Installing the 8.8″ Rear End

I acquired the following parts for the installation:

  • Pair of new leaf springs (Figure 5). I special-ordered from ESPO 1956 Dodge convertible spring that are the highest ride height, had an additional leaf added for increased load capacity, and had the springs built 1-1/2″ shorter eye-to-eye to correct the error in factory shackle geometry. I detail leaf spring selection and the shackle problem in another article.
  • Pair of heavy gauge 3″ housing leaf spring perches. The 8.8″ Explorer housing is 3-1/4″ diameter, so I had to grind the perch bird’s mouth cuts slightly until they fit snugly around the 8.8″ housing.
  • Eight Moog K7308 shackle bushings.
  • Pair of Monroe 66859 gas shocks, which I selected after installing the 8.8″ rear end and measured the needed extended and compressed length. This particular shock was used on 1970 Dodge P300 vans for those wanting to cross reference.
  • Eight new 3/8 bolts (Grade 8) and nylock nuts for the front leaf spring mounts.
  • (2) axle seals, (1) pinion seal, (1) new pinion crush washer, (1) differential cover gasket, new rear end oil, new parking brake shoes and hardware, and brake pads.

I placed the 8.8″ under the chassis up on jackstands high up to where I could fit the leaf springs below them without the axle interfering. I installed the front leaf spring mounting brackets and installed the shackles to the leaf spring and chassis. I swung the front of the leaf spring up where the bracket contacts the chassis flange and secured the bracket with two of the four 3/8″ bolts. I didn’t use all four bolts per bracket because I would be removing the mounts again.

At this point, the leaf springs are installed and hanging. I placed the spring perches over the leaf spring studs and lowered the rear end onto the perches. I attached a plumb bob to the outside of each frame rail just behind the rear axle in order to center the axle. For my chassis, I found that the outside frame rail plumb line to the inside of the axle flange is 3″ to center the axle (Figure 6), although this measurement should be checked on the specific vehicle. I installed the u-bolts, mounting brackets, and nuts snuggling down the nuts enough to hold the rear end from rotating under its own weight but loose enough to move the pinion up or down using a mallet. Confirming the axle is still centered, I moved on to setting the pinion angle. The pinion angle is a complex measurement that I detail in another article that can be reviewed. After setting the pinion angle, I confirmed the axle was still centered using the plumb lines and placed a strong tack-weld at each corner of the spring perch to secure the perch to the housing. I will remove the housing to complete the final welds, which I describe below. At this point, I measured the fully extended length needed for the shock from the chassis stud down to the factory 8.8″ mounting bracket hole. Figure 7 shows the factory 1956 Dodge shock in comparison to the 8.8″ shock mount that I will use.

1955 1956 Dodge Plymouth Leaf Springs
Figure 5: New Custom 1956 Dodge Leaf Springs with Extra Arch and 7 Leafs

1955 1956 Dodge Plymouth Ford 8.8 Rear End

1955 1956 Dodge Plymouth Ford 8.8 Rear End
Figure 7: 8.8″ Shock Mount Compared to the 1956 OEM Shock higher up
Welding the 8.8″ Spring Perches

After I removed the rear end and placed it high up on jack stands, I removed the axles, pinion flange, and differential. I installed a jig bar through both ends of the axle housing and finished welding the spring perches. I welded one half of each side of the perch at a time, jumping from side to side, and allowing enough time in between welds for the material to cool so as to not overheat the material. After the housing cooled to ambient temperature, I removed the jig bar, cleaned all the components including the axles, bearings, differential, and inside of the housing. I reinstalled the differential including a new pinion crush washer and installed new axle seals in the housing. I installed the axles and differential cover. At this point, before installing the emergency brakes, rotors, and calipers, I prepped the housing for paint, primed, and painted it.