Jerry Heasley
March 30, 2017

Detailing a rear axle is a major part of a Mustang concours restoration, right up there with the engine and the front suspension in complexity. There are so many parts, colors, codes, machined surfaces, and other details to figure out that it can truly boggle the mind. So when Bob Perkins detailed a Ford 9-inch rear axle using original parts, we flew to Wisconsin to document it and show the readers what is involved.

As the subtitle to this story said, this is hardcore, in-the-trenches restoration tech that only applies to those who are looking to restore their Mustang to exactly as it was when new, and also to those who are curious what goes into a high-end restoration like those done by Perkins and select few top shelf restorers.

Every high performance Mustang from 1964½ through 1973 came with a Ford 9-inch. The goal with a restoration is to reproduce the factory procedure, which has a way of uncovering surprises. For example, Perkins showed us barely discernible “clamp marks” on top of the axle housing. Do show car judges check for clamp marks? And what are clamp marks, anyway? Apparently, Ford’s Sterling Axle Plant clamped onto the housing with a fixture that made these marks. Okay, that’s very interesting—those clamp marks came from a large tool that held the axle in place for machine work, assembly and painting. But, why do these marks matter? “That’s the type of stuff you lose if you have to sand the axle housing a half dozen times to get rid of rust pits,” Perkins said.

Elite judges know this and they look for clamp marks. Perkins also showed us stretch marks and grooves in the steel housing. He went over how to decode the galvanized steel rear axle tag. He pointed out a very rare “Rockford” bolt and special boot crimp pliers to attach a rubber axle tube vent hose clamp. He held up an original final inspection sticker, plus an orange strap made of plastic that is one of maybe a half dozen left in existence. Bob chronicled the life of this orange strap that an employee at Dearborn Assembly swept off the floor and into a dustbin in 1969, brought home in his lunch box, and finally sold to Perkins at a swap meet several years ago. Few cars have the provenance of this plastic part, which has aged into a sort of artifact of the Mustang car culture.

All this is fascinating stuff, sure to drop jaws even among Mustang connoisseurs. Although the nitty-gritty details apply to Ford 9-inch rear axles from 1969-1970 Boss and Cobra Jet Mustangs, most of the general features, such as paint, are relevant to any 1964½ to 1973 Mustang rear axle.

01. Bob Perkins slides one of the two axles into the housing with help from MCA National Director and Gold Card Judge J.P. Weber, who stopped by for a visit that day.

02. Start with a good, rust-free axle housing. Clean enough to uncover and photograph old markings, such as “T16” for reproduction later. First Bob blasts the housing (taking care to hunt old markings) to bare metal with #10 glass beads to remove old paint. Minor imperfections—gouges or dents or pits—need to be sanded out of the axle housing and then sprayed with Ditzler DP90LF. If pits remain, apply a second coat of DP90LF (which is low-gloss satin black epoxy primer). However, do not fill pits with a primer-surfacer, which is a color such as white or yellow or green. One nick in the metal during assembly (such as sliding a U-bolt over the housing) will expose color and ruin the housing’s appearance. Primer-surfacer fills in pits at the expense of covering up factory clamp marks and grooves and stretch marks—features concours judges look for. If the housing is so pitted as to require heavy build primer/surfacer, restorers should start with a better housing. Finally, cover the Ditzler DP90LF with Ditzler semi-gloss black DDL9423.

03. Ditzler DP90LF and DDL9423 are available from most major paint supply stores.

04. Perkins’ 9-inch rear axle is from a 1970 Boss 429.

05. To the left of the top of the digit 3 is a row of clamp marks that reveal the originality and excellent condition of this axle housing. Most likely, the digits “31” refer to 31-spline axles, as opposed to 28-spline axles.

06. On the passenger side of the axle we found more clamp marks.

07. There is no source that lists or decodes what an assembly line worker hand wrote on each axle. Restorers today simply reproduce the markings they find, such as the “X” and “26” and “L-7” seen in the center section of the axle housing.

08. “982A” hand written on this axle housing matches “982A” stamped into the axle tag. Starting at the bottom of the “2” and extending through the “A” are three grooves that are common to 1969-1970 9-inch Ford rear axle housings. If these grooves are missing, chances are the axle has been badly pitted or was poorly restored.

09. The original axle tag, made of galvanized steel, was present on this axle. Many of these tags are missing. Kevin Marti reproduces them, but the date code would be an unknown. Decoding information is readily available. This tag decodes as follows:

9JC: Third week of September of 1969 (9 is for 1969, J is for September, C is for third week)
3L91: 3.91 locking rear axle (L is for locking)
982A: Ford internal code for 3.91:1 Traction-Lok
WFD-D: internal code for ordering parts
9: 9-inch

The “T” enclosed in a circle indicates Traction-Lok.

The yellow daub quality control mark indicates the differential has been filled and tightened. Permatex around the edge of the filler plug is how Sterling Axle delivered the rear axles and is considered concours correct and a cool detailing feature.

10. On the left is the original “982A” paper axle tag. On the right is a reproduction of that tag.

11. Perkins prefers to install the original tag, which has survived almost 50 years. This photo shows the mounting location of the tag under the inner U-bolt.

12. You can also install a reproduction tag, which has a sticky backing like masking tape.

13. “T16” is another Sterling Axle Plant marking that restorers find and reproduce; located on the bottom of the center section of the housing.

14. The top of the center section of the axle housing has more clamp marks, plus stretch marks in the steel that reveals the originality of the metal. Highly sanded and filled metal degrades or totally removes this original look.

15. Machine the surface of the axle housing as seen here.

16. The original axle-housing gasket is Ford part number “B7A-4025-A.”

17. Installed, the original gasket does not hang over the entire machined surface of the axle housing. Gaskets available today are not trimmed and do hang over the machined surface.

18. From the rubber hose, brake fluid flows into the brass distribution block, and exits into two steel lines, one to each rear wheel cylinder. The longer metal line is armor coated (to prevent vibration when the brake pedal is applied) and supported in a phosphate- and oil-plated bracket. A special bolt labeled “ROCKFORD” screws into the housing to secure the bracket and distribution block. Notice the flared nuts on the steel brake lines are dyed red.

19. From the backside of the of the distribution block blue-green paint is visible from the mounting hole for the axle vent tube (left) and the hole (right) for the bolt that secures the distribution block.

20. The distribution block is stamped with Ford engineering part number DOZA-2A448-B and date coded “13 10 9” for the 13th day of October 1969. Ford continued to produce this brake hose assembly into the 1980s. This hose is a NOS date-coded assembly line correct part for a Boss. A later service hose made in the 1980s would probably have a later date code.

21. A hard-to-find detail on the brake line-mounting bracket is this fine thread bolt, identified by “ROCKFORD.”

22. Squeeze the clamp that secures the rubber axle vent tube with boot-style, crimp pliers as seen here. This same tool is used for some fuel lines.

23. With the drum brake parts installed, we can see the placement of the five different springs and their colors. Also, notice the two green paint daubs on top of the wheel cylinder. The two green daubs indicate the C6OZ-2282-A wheel cylinder, which indicates a 7/8-inch diameter piston. The C3OZ-2282-D wheel cylinder for 1970 has a 29/32nds diameter and two white paint daubs.

24. The hardware inside rear drum brakes is available from Mustang vendors, with the exception of the large, round brake backing plate on the right. Perkins paints these backing plates as the factory did, by dipping. He dips the plate in five gallons of Ditzler DDL9423. Dipping results in slightly more gloss than on the rear axle housing, which uses the same paint but is sprayed.

25. Inside the brake drum, the primary hold-down spring is blue, and the secondary hold-down spring is black. However, in most aftermarket rear brake kits these two short springs come painted purple. So, simply re-paint them the correct colors.

26. The yellow, blue, and green springs are usually painted correctly in reproduction kits. For reference, the secondary shoe-to-anchor spring is yellow. The primary-shoe-to-anchor-spring is green. The automatic adjustor spring is blue.

27. Original Ford brake linings are stamped FoMoCo. “PRI” stands for primary. The secondary brake shoe lining will be stamped FoMoCo and “SEC.” Original shoes are riveted, while current linings produced today are glued on the shoe and are usually dark gray rather than tan due to lack of asbestos content.

28. Brake drums are either a natural cast finish, as seen on the left, or blacked out. Ford painted drums black on cars with wheels with open areas, such as Magnums. With full wheel covers, there was no need to black out the natural cast finish.

29. The face of this original brake drum was dated October 26, 1969.

30. Every machine shop has a lathe to “turn” the inside of brake drums so they are not pitted and fit flush with new brake shoes, however most shops will not turn the outside of the drum, as seen here. But, whatever was machined originally (from the factory) needs to be re-machined for factory appearance.

31. On the passenger side, the axle housing fades from semi-gloss black to bare metal, as seen here. A fixture that secured the axle for painting (vertically) prevented paint application here. The backing plate, painted semi-gloss black, is stamped “Bendix,” but some plates are Kelsey-Hayes. (One side could be Bendix and the other Kelsey-Hayes, or both could be Bendix or Kelsey-Hayes.) Stamped in the backing plate to the left of “BENDIX” is a date code of 3029 for the 302nd day of 1969, which was October 29—very close to the October 26th paint-marking on the brake drum. The zinc dichromate bleeder is one-quarter inch, which is stock for 1964½ through 1972 Mustangs. The larger 3/8-inch bleeder is stock for a 1973 Mustang and most service replacement wheel cylinders. The T-bolts holding the backing plate and the axles are phosphate and oil.

32. The axle tube is painted semi-gloss black clear to the end on the driver’s side. Yellow paint is a typical code used on most 9-inch Mustang axle housings. The “991” stamping on the right is a date code for 1969, September, and first week. On the left side of the brake backing plate stamped and highlighted in yellow paint is 3029 for the 302nd day of 1969.

33. U-bolts are natural finish and have brown paint daubs for 1970 (light gray daubs indicate 1969). The nuts seen here are clear zinc, but zinc dichromate or zinc dichromate with red dye are also factory stock. In order to accommodate for the rear sway bar plate between the rear axle housing and the rear leaf spring’s shackle plate, these 1970 U-bolts are longer than the 1969 U-bolts.

34. Original parking brake cables look much different from current reproductions.

35. The FoMoCo logo and orange paint mark identify this rear parking brake cable as original.

36. Sterling Assembly used this small orange strap to wrap cables together for transit. One end went on the parking brake cable, as seen here, and one end went over an axle housing stud to keep the cables from dangling. Typically, assembly line workers discarded these orange straps. However, when assembly line workers bolted rear axles into cars, they frequently took one end off the stud and let the orange strap hang on the cable, as seen here.

37. This orange strap hanging on a 1969 Boss 429 parking brake cable is a rare find. Bob has seen them on only a half-dozen or so of the cars that he has restored, and has never seen one on another car at a show.

38. The center of each axle is machined. Green paint on the end of the studs and pink on the axle are identification codes. Bob Perkins has also seen blue and white on 31-spline axles. The circle with an “OK” is an inspector mark.

39-40. 31-spline axles for 1969-1970 use Ford’s C7OZ-1177-A seal. Even when pulling off the drum, there is no way a judge can check for this seal. Perkins said, “I would hate to think that somebody might take this assembly apart 50 years from now and say, ‘look, Perkins used a Taiwan seal in his killer show car.’ It’s the stuff you don’t see that makes a good car a great car.”

41. The inside of the axle is bare steel, featuring a rectangular retainer with four bolt holes.

42. After slipping the brake drum over the wheel studs, you will need three “lock tabs.” These lock tabs are available from Mustang vendors.

43. Push a lock tab over every other stud, or three of the five studs.

44. Tim and Bob (right) install the “chunk” (also called the gear set, center section, or third member) into the axle housing.

45. These zinc dichromate nuts, dyed red, secure the third member to the axle housing of 1969-1970 models. Sterling Axle also used these nuts to secure the axle to the housing. They can also be clear zinc or gold dichromate for 1969-1970. They have Ford part number 385696-S100.

46. Detail work on the bottom of the third member shows machining on the pinion support. Notice machining is visible on the surface where the axle housing bolts up against the “pumpkin” or case of the third member. Today’s reproduction gaskets hang over this machined surface. The third member case is painted red oxide. Bob uses Ditzler DP74LF to replicate this red finish.

47. The Boss 429 yoke has an orange paint ID. Sterling Axle sprayed the oil slinger yellow prior to assembly. The pinion support bracket is bare steel.

48. The rubber pinion snubber is bolted on. The engineering part number, C9OA-4906-A, is visible.

49. Sterling Assembly attached a final inspection sticker to the passenger side of the pumpkin, just forward of the pinion bracket. Of course, most of these stickers have degraded or they have been pulled off and are extremely rare. They were made of a canvas type material. They are not available as a reproduction at this time.

50. When finished, oil the entire rear end with a lubricant of your choosing. WD-40 works fine.

51. Ford’s “Sterling Axle Plant” in Sterling Heights, Michigan built each Ford 9-inch rear axle as a “sub assembly,” ready for installation on the assembly line. This Ford 9-inch rear axle and brake assembly is now restored to Thoroughbred concours condition with original date-coded Ford parts.

52. A restored rear axle is a work of automotive art and the major part of a classic Mustang’s rear suspension.