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Author Topic: Rear Axles: Chrysler 8 3/4" Rear Axle & Swap Information  (Read 67834 times)

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Reggie

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Rear Axles: Chrysler 8 3/4" Rear Axle & Swap Information
« on: January 02, 2008, 16:38:43 »

Chrysler 8 3/4" Rear Axle & Swap Information

Chrysler Corporation's 8.75" rear axle assembly first appeared in 1957.  The measurement refers to the 8-3/4" diameter ring gear. This gear, which is turned by the pinion gear, changes the direction of the power being transmitted to it. The pinion gear is turned by the driveshaft. It is a banjo-type (Hotchkiss) axle.  The term "banjo" means that the differential is contained in a removable carrier (or center section) assembly. When the carrier is removed, the empty housing resembles a banjo. The carrier is removed from the front of the housing, and is retained by 10 nuts on studs in the housing.  The rear of the housing is smooth with a non-removable rear cover that is welded onto the main housing. The housing and axle tubes are made from stamped and welded steel.

The 8-3/4" axle was the primary axle assembly used in most non-compact car lines through 1972.  It was also used in some high performance compact cars.  Of the many axle types that Chrysler used since 1960, it was also the only axle assembly developed by Chrysler. It was phased out because it was more expensive to manufacture than using the outsourced Spicer 7.25", 8.25" and 9.25" units.

There are three basic types of 8.75" carriers available, which are distinguished by their drive pinion stem diameter. The choice of axle pinion assembly was determined based on a combination of vehicle horsepower and weight. Below are the 8.75" axle carrier types.

8.75" Axle Carrier Types and Years of Production

  • ''741' Casting - 1 3/8" (1.375" Small Stem Pinion.
    Carrier casting numbers were 1820657 (1957-1964) and 2070741 (1964-1972). This unit was usually used in low weight/medium horsepower and high weight/low horsepower applications. Although it is the weakest of the 8.75" units, it is still a stout unit, and is stronger than the Spicer-built Chrysler 8.25" rear. The 1 3/8" pinion is also larger than the pinion in the Ford 9" rear.
  • ''742' Casting - 1 3/4" (1.75" Large Stem Pinion.Carrier casting numbers were 1634985 (1957-1964) and 2070742 (~1961-1969). This assembly was replaced by a phase-in of the 1-7/8" pinion '489' casting starting in the 1969 model year. This assembly was typically used in high weight/medium horsepower applications through high weight/high horsepower applications. Although not generally considered to be stronger then the '489' carrier, the '742' carrier is extremely stout, and is favored by many Mopar enthusiasts because they are easier to set up. Noted Mopar enthusiast John Kunkel mentioned that the biggest advantage of the 742 carrier is that it has the largest rear pinion bearing of all the 8 3/4" carriers. The larger bearing helps to prevent pinion deflection better under a load than the others.
  • '489' Casting - 1-7/8" (1.875" Tapered Stem Pinion Carrier casting numbers were 2881488 or 2881489 (1969-1974). This assembly was introduced in 1969 and was phased-in to relace the 1-3/4" pinion '742' unit through 1970. Note:  the 1-3/4" pinion also appeared in some '489' carriers during this period.  By 1973, the '489' was the only unit available in passenger car applications, and was typically used in high weight/medium horsepower applications through high weight/high horsepower applications.  This carrier is considered to be the strongest by some Mopar enthusiasts.  Because it requires a crush sleeve to set bearing preload, some Mopar enthusiasts prefer the '742' casting.  This is not really an issue now because there are now parts available to eliminate the crush sleeve.

Carrier Identification:
All 8-3/4" carrier assemblies can be identified externally by the casting numbers.  The casting numbers are cast onto an ID pad on the driver's side of the carrier just behind the 'snout' of the case where the yoke attaches to the pinion. In addition to the casting numbers:

  • The '741' will usually have a large 'X' cast on the left side
  • The '742' will usually have a large '2' cast on the left side
  • The '489' will have a large '9' cast on the left side.

Gear Ratio and Sure-Grip Identification:
Through 1965, the factory ratio was stamped on the identification boss, followed by an 'S' if it had a Sure Grip.  After 1965, a tag was affixed under one of the carrier mounting nuts to identify the ratio.  If it had a Sure Grip, an additional Sure Grip lube tag was sometimes added.  Later year versions sometimes had the filler plug painted orange.

Gear Selection:
Because of its popularity, a wide range of gear selections is available for the 8.75" unit.  Gear ratios available on the 8-3/4" axle through the years include, but are not limited to: 2.76, 2.93, 3.23, 3.31, 3.55, 3.73, 3.91, 4.10, 4.56, 4.89, 5.17 and 5.57. 

Interchange Notes:
Any complete 8.75" carrier will interchange with another 8.75" carrier in any 8.75" housing. The Sure-Grip assemblies also interchange.  However, none of the other separate parts such as cases, pinions and bearings will interchange.

Swapping an 8.75" Unit Into an M-body

There Are Three Ways To Do The Swap:
  • Modify the 8.75" spring perches to fit your stock iso-clamp setup. This is the easiest way, because you won't have to chase any extra parts. You can either reuse your original springs and iso-clamp hardware, or upgrade to heavier springs with M-body cop car shock plates, rear sway bar and the correct sway bar brackets.
  • Replace the springs with B-body, HD, or SS springs. If you go this route, you will need the MP spring hangers (part # P4120081) to mount them. You will also need B-body lower shock plates (Right P4876518 and left P4876519), and different shocks. There are two variations of this method.
    • One is to enlarge the spring locator holes out about 1/4" on each side and weld/fill them in on the inside the same amount.
    • The second (less precise) method is to pull out the springs a bit so that the rear drops onto the locator pins. This method can potentially speed up the wear on the front spring bushings over time.
  • Modify the rear spring hangers and change the spring bolts to 8.75" size: This alternate method posted here, has been successfully done by Farley's member S-Type: "Remove the shackles and the 3 bolts that hold the rear hanger to the subframe and remove the rear hanger. The hanger is made up of three parts, a flat piece of steel bent into a channel shape, a large tube runs through it and there is a small rod. Nick the 1" of weld off each side of the tube and rod with a cutoff wheel so you can relocate them. The tube is now placed flush to the outside edge of the bracket in relation to the car and rewelded, center the spring stop rod to the tube and reweld (looking from the rear, the tubes will move in toward each other). You are now 44" center-to-center front and rear. Now replace the big diameter bolt that holds the springs togeather using a C-clamp on each side to keep everything aligned, with the small one that would have come with your 8 3/4 axle. Any spring shop will have them, maybe a buck apiece. Bolt in your 8 3/4 with the plates, U bolts, shocks that came with the axle like God intended. Of course you need to fabricate a drive shaft for it but that's another story. I recommend the big joint and balance it! You will pickup a little more tire clearance at the rear of the tire, bigger tires. You will lose those giant rubber biscuits that ruin traction/handling. And the pinion snubber will work much better. And it looks stock!"

Best 8.75" Donors:

The 1965 to 1970 B-body 8 3/4" rear axle is the closest fit to the F/J/M platform because the spring perch centers are 44" compared to F/J/M bodies 44.46".  Although 1962-1964 units will also fit, they should be avoided because the brake drums are pressed on, requiring a hub-puller to remove them. 

B-Body models to look for are:
  • 1965-70 Plymouth Belvedere, Plymouth Satellite, Dodge Coronet
  • 1966-70 Dodge Charger
  • 1968-70 Plymouth Road Runner, Dodge Super Bee
   
For swap reference:

  • Axle flange to flange:
    • 65-67 B-body is close to 55"
    • 68-70 B-body is about 5/8" wider than the 65-67 units
    • F/J/M is a little less at 54.34"
  • Perch center to perch center:
    • 65-70 B-body is 44"
    • F/J/M is a little more at 44.46" (or just under 1/4" farther out on each side)

Procedure with stock Iso-Clamps - Courtesy of Mark Mullins

The B-body rear has much smaller locator holes in the spring perches than the iso-clamp setup, so Mark and his dad used a hole saw to enlarge the holes to fit the M-body springs. He used the extra two holes in each B-body perch to temporarily bolt steel plates to them, so he could properly locate the pilot holes for the hole saw. The perches still rested entirely on the springs, since the springs were 2 1/2" wide, and the difference between perch centers for the two housings was less than 1/4" on each side.

Mark is running a 4.10 geared, Sure-Grip 8.75" rear in his 360 equipped 1987 Fifth Avenue. Other goodies are Eddie heads, 9.5:1 forged Speedpro pistons, a Comp Cams XE 274 cam, Hooker headers, exhaust cutouts, an Edelbrock RPM Performer intake, a 670 CFM Holley Street Avenger carburetor, and an A500 4-speed automatic transmission with a 2500 stall converter, deep pan and a TFOD shift kit.  Can you say "nasty"?

  • Buy some 1/8" X 2" flat steel stock from any hardware store. You will need less than one foot, so buy the shortest length that you can find.  Cut two sections approximately 4 1/4" each.
  • You will also need a 1 9/16” hole saw with a 7/16 arbor to drill the perches out.
  • Flip the housing upside down on a pair of jackstands so that the spring perches are facing up. You will see three holes: Two outer holes and the center spring locator hole.
  • Slide one of the cut pieces of flat steel UNDER each spring perch.
  • Hold the steel piece firmly up against the inside of the perch.
  • With your drill and a small bit, insert the bit through the CENTER of each outer hole on the perch to start pilot holes in the steel.
  • Remove the steel piece and using the pilot holes, drill a 1/4" hole for each outer hole.
  • Repeat the process for the other side.
  • Use an assortment of bolts and washers through the outer spring perch holes to secure the flat steel pieces under the spring perches.
  • With your drill and a small bit, insert the bit through the CENTER of the spring locator hole on each perch to start CENTER pilot holes in the steel. These are only for reference, and they will NOT be drilled through. BE CAREFUL TO FIND DEAD CENTER.
  • Use the centerline of the two outer bolts holding the steel plate as a reference to determine the amount of offset for the pilot hole. Locate the center reference hole on the steel and measure OUT about 1/4" (BUT NO MORE THAN 1/4" on the steel underneath. Also be sure to measure out EXACTLY AT RIGHT ANGLE from the edge of the spring perch.
  • Start the pilot hole for the hole saw.
  • Follow through with the 1 9/15 hole saw.
  • The hole saw has a pilot bit that will go through the steel plate.The hole saw blade will cut through the spring perch using the pilot hole in the steel plate as a guide. The trick is to make sure that you ADJUST THE DEPTH OF THE PILOT BIT, SO THAT YOU DON'T DRILL THROUGH THE HOUSING with it.
  • Use plenty of cutting oil while drilling the large hole.
  • The 8.75" unit will then drop right down on the M-body spring locators.
  • The 10 inch brakes on the 8 3/4 are also swappable with the 10 inch brakes on the stock 7 1/4 rear. You can also upgrade to 11" M-body cop car brakes. The brake backing plates will bolt right on.
  • You'll have to measure for the correct driveshaft length and have your driveshaft cut.
  • There may be an issue with the rear brake hose that runs from the hard line to the differential housing. Mark retained the stock piece when he swapped to the 8.75" by taking the bracket off of the frame rail and bolting it up somewhere closer to the rear axle. He has plenty of play in the line, and it doesn't even get tight when the rear of the car is in the air supported by the frame rails. A 72 Dodge pickup-truck brake hose is also said to work without requiring any modification.

Sources:
  • A Chrysler 8-3/4" Rear Axle Guide by Gary Lewallen (aka. Vaanth)
  • Mopar Chassis Manual
  • Autohobbydigest.com
  • Posts by Farley's member Mark Mullins
  • Posts by Farley's member S-Type
  • Various posts by other Farley's board members
  • Post by Moparts member John Kunkel

Links to 8.75" swap information on other F/J/M sites:

Photos:
  • 8.75 ID Pad Location (Courtesy of Chrysler Corporation)
  • A '741' casting carrier with the large "X" on the side. (Courtesy of Moparts member DAMOPARS)
  • 8.75" axle spring perch modified to fit an M-body (© 2010 Reggie)
  • 8.75" axle installed in Mark Mullins 1987 Fifth Avenue ((© 2010 Mark Mullins)
« Last Edit: February 19, 2011, 00:04:08 by Reggie »
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Reggie

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Rear Axles: Chrysler 8 3/4" Rear Axle & Swap Information - Page 2
« Reply #1 on: January 24, 2009, 20:20:16 »

Chrysler 8 3/4" Rear Axle & Swap Information

Below are photos originally posted by Farley's member S-Type showing how he reduced the rear spring offset by modifying the rear spring hangers.  The reduced rear spring offset relocated the axle locating pin centers to match up to B-body dimensions of 44" rather than the factory 44.46". 

Photos courtesy of S-Type
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"There is only one success - to be able to spend your life in your own way." - Christopher Morley 1890-1957, American author and editor

Library format and content © 2007- 2012 Reginald A. Royster, Sr., unless otherwise authored or noted.
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