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Ford 9-Inch, What I've Learned


Explorer Addict
November 12, 2009
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West-Central AZ along the Colorado River
Year, Model & Trim Level
59 Ranchero F250 D'Line
Here per @gmanpaint .
I probably have learned more about Ford's 9-inch axle than anything else they built. Everyone seems to like them, myself included. Might say I "cut my teeth" learning by doing on cars, about when Ford introduced it's now-famous 9-inch, with 9" ring gear, though they built a 9-3/8" which is not important here.
Most significant about the 9 is that unlike most other axles, which have a solid-cast center-section with axle tubes welded in, the 9 has a pressed-steel, hollow shell with axle tubes. The gearing itself is secured to a separable center-section, a "carrier" of heavy cast steel, held in place within the center section by ten 3/8-24 grade 8 studs and nuts.
The second neat thing is that the pinion gear and it's bearings is retained in another, smaller carrier which is bolted to the center section by five 3/8-16 grade 8 bolts. Even more unusual is the fact that the pinion gear itself is supported by 3 bearings instead of the customary 2. The third bearing, a straight roller type, is located at the inner tip of the pinion, riding on a ground stub protruding from it's end. Thus, the gear itself is supported on both of it's sides, instead of just one; they called it "straddle-mounted".
Third thing is that a number of carriers may be kept at hand, each having a different gear ratio within them. Changing from one ratio to another then becomes a much simpler job: slide the two axle shafts clear of the differential, remove the 10 nuts securing the center, and pull it out. The gears and all come out with it. At one point in my "career" I had at least 15 or 20 such carriers, many in half-completed shape, and piles of ring and pinion gearsets. Here's a stripped center carrier, of the famed "N" type, nodular iron, very strong.

The small hole visible above locates the pinion straddle bearing. The big 5-bolt hole receives the pinion bearing carrier. The two differential carrier bearing caps are obvious. The pinion retainer is sealed by a 5-inch O-ring, while the carrier itself is sealed by a gasket. Assembled with gears, these carriers are pretty heavy. I spent years hefting them in and out of center-sections using only my two hands. Below is an assembled, sectioned view of the pinion retainer with retainer, gear, bearings, seal, and U-joint yoke. Compared to conventional hypoid axles, 9 inch pinions are shorter, their two back to back tapered roller bearings being closer together. Best design practice: bearings as far apart as possible. The straddle bearing takes care of that.

Nestled between the 2 tapered bearings is either a "crush sleeve", or solid spacer. Either's purpose is to establish correct bearing preload, very important, while allowing adequate tightening torque on the nut to prevent loosening in service. All these parts are made to manufacturing tolerances, which means no two parts are absolutely identical in dimensions. The crush sleeve allows for ease of obtaining preload over a range of size tolerance. The solid sleeve, though, is more foolproof, but requires very careful attention to assembly. Such sleeves were used by Ford at the factory for High-Performance applications. The 9-inch, introduced in 1957, was produced until 1986, being last used in F-150 trucks. Note the 9 was apparently not used in Rangers beginning 1983, though the axle was used until 1986, last in Granadas, far as I know. Since it WAS used in many early Mustangs, before the 8.8, those were set for leaf springs, just as were Gen I and II Explorers, with their non-independent rear suspensions.
That's an overview. Errors I may have made, or questions, will be appreciated. imp

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Anyone who has had to press cone and roller assemblies off a mounting to change an incorrect shim thickness (as this shade-tree guy had to as a teen-ager) can really appreciate the set-up of the 9-inch. The design uses NO shim behind any bearing. The differential case bearings are adjusted side to side by cup-shaped "nuts" threaded into the carrier bearing support and caps. They also serve to add preload. The pinion bearings have NO shim behind them, but rather the entire pinion retainer assembly is located for correct pinion depth by steel shims placed between the retainer and center section. They ranged from, like, 0.012" to 0.030" in thickness and looked like this:


At one time, I had a drawer full of them! Easy as pie to arrive at correct pinion depth, remove 5 bolts, slip assembly out of carrier, remove shim and replace with another. Some of the ratios have enormous pinions, just fitting through the hole in the carrier. imp

Some arguments against use of the 9-inch exist, of course! In 60 years of messing with drive axles, I guess I've heard almost all of them, as well as the good. "A good, solid casting containing the pair of gears is stronger than the removable pinion concept: only 5 bolts hold it in".

Really? Five 3/8 inch bolts clamping that baby shut with a force of about 25,000 lbs total. Take a competing set-up integral center section and put 25,000 lbs. on the nose of the pinion bearing, supported from the yoke area: what do you think will happen? Any one experienced with axle work will wince when they recognize the extent of this destruction, how the parts no longer "look right":


Could you fix this catastrophe?


The reality of these big gears is such that absolute precision is the prerequisite to good and long-lived operation. The design, called "hypoid gearing", means the pinion gear is set below the centerline of the ring gear. Like this:


Reason for doing this I've always heard, to lower the floor pan and tunnel design, I always questioned; too simplistic. Figured it out after battling my way through middle-aged college attendance, finally got my Engineering Degree, and understood better those foibles I had been seeing in practice building my cars the previous 20 years (got the degree at 34). Look at the curl and length of the teeth on the pinion: MUCH bigger than a simple bevel design:


Bigger teeth = bigger contact area = less unit loading = less tooth stress = less friction = less heat build-up. But, the hypoid design is very complicated to make, since the tooth surfaces of both ring and pinion gears are curved in three planes, sort of like the curvature of a fireman's hat.

All the above is true IF the mating surfaces of the teeth do not experience much RUBBING friction. Hypoid design approaches rolling surfaces, like a ball bearing or roller bearing, where the two curved surfaces carrying the load "roll" over one-another, rather than "pushing against" one-another.

Explorer gears tend to hum. A common complaint, and I've wondered about it. For one thing, the Ex rear differential carrier, a Ford 8.8" design, while a strong S.O.B., is made of aluminum. Strike 1. Aluminum deforms far more readily than steel or cast iron, so the gear mating surfaces more easily experience varying tooth contact patterns in service. My '04 hums a bit under load uphill sometimes, at 164K, most of the time is utterly quiet. No worries.
Ball 1: Aluminum reduces the "unsprung" weight by a significant amount, unsprung being real important to the "quality of ride" guys. Ball 2: Aluminum reduces vehicular weight, important to the "vehicular police politicians".

So popular is the 9-inch, many aftermarket makers offer entire assemblies, with the "banjo" (the center section, hollow, with axle tubes welded in) made up as weldments, separate pieces, stronger than Ford's original design. Specialized pinion retainers are available. Whole host of other options. The damned 9" may be the most popular drive-train component ever!

Strong? You bet! Yet, my buddy Grubby Jack Brochler, lucky recipient of one of the first built 63-1/2 Ford 427 cars, split his 9" center section CLEAN IN HALF, top to bottom! After that, the Nodular edition came along.

Just recalled: Ford used 4.57 gears in 9-inchers from the factory in certain vans and taxicabs, which brought the replacement part cost under $50 for a brand new ring and pinion gearset! 'Course, I bought a set, stuffed them into my 9 in my 1955 Mercury, my first REAL car. One of the first new gear installs I did, and I screwed up BIG TIME! Wanna know, ask me, glad to blab more, this has gotten long enough. Thanks for even readin' this far! imp

Thanks for the overview. I've loved the 9" since my first cars, the 70's, 72 Gran Torino and Rancheros. I've swapped just three chunks, never built one, had my last one put together by a friend for a trade. I removed two from local JYd's, a 3.50 LS 9" from a work truck, and a 4.11 from an old van. I have that 4.11 LS in my Ranchero now, but that was intended for a mild 351C and AOD in the late 80's. Now I think it needs to be a 3.50 if I get that far, with a 351 Clevor and 4R70W.

I didn't know the 9" pinion had three bearings, that is cool.

Do you know if the 06-10 8.8 chunk is any better than the 02-05 versions?

How about some info on limited slip carriers? I've got 2 trucks with a 9" right now and both are open diff. I've got a nodular center section sitting on the shelf with 3.50 gears/open carrier I want to build up with a little lower gear and L/S. Any suggestions?

Tons of Mustangs used 9-inch, yet few answers resulted on one of my Mustang Forums, where I originally posted this thread. Lots of interest shown here is rewarding of the fact that quite a bit of time and thought goes into this technical business. Short of time right now, I'll be back tonight. Thank you all for your interest! imp

@imp ,

Hurry up!

How about some info on limited slip carriers? I've got 2 trucks with a 9" right now and both are open diff. I've got a nodular center section sitting on the shelf with 3.50 gears/open carrier I want to build up with a little lower gear and L/S. Any suggestions?
Yes, several. I favor the original all-gear design nowadays called "Eaton Truetrac", elsewhere called "Torsen" for all but the most highly demanding applications. Beyond those, no differentials at all, axles mechanically locked together, usually by a "spool". I actually welded the diff. gears together in my first real car, a '55 Merc; in those days it raised eyebrows and interest whenever any vehicle drove through our Bob's Big Boy restaurant, rear tires squawling around the turns, it was "badass"!

If you are wanting real limited slip, that's usually done by clutch designs, which have a definite "break-away" torque between the two axles. New clutches provide breakaway somewhere between 100 and 200 ft-lbs, meaning with one wheel off the ground, a strong guy can slip the clutches by grasping the outside of the tire. Usual way to check for worn-out clutches. Wear-out is inevitable, takes longer if little tight-turning, constant parking lot go-arounds are done.

For your 9-inchers, Ford used what they originally called "Equa-lok", later called Trac-Lok:

Thinking about a whole coverage of this subject, and wonder if I should do it as a separate thread? Regarding ratios, my 1965 "K" Mustang came with a 3.89 9-inch rear. I finally wound up with 4.57s and a B/W H-D overdrive grafted to my Top-Loader 4-speed. imp

@CDW6212R..."Do you know if the 06-10 8.8 chunk is any better than the 02-05 versions?"[/QUOTE]
Sorry, no idea! AFAIK, the 8.8's only major change over the years was that of going to an aluminum alloy casting instead of the usual iron, that being done I think with the intro of the independent rear suspension. Also alloy, the fronts, from some point in time. Real early, Gen II have no experience at all. Owned a '96 Ex, first one, surprised to find torsion bars and a disconnector built into the front axle. Then a 2wd '99, but manual trans, finally Gen III 2004 with more warning bells and headaches to boot! Suspected after much complaint about gear howl that the alloy housing may be part to blame. imp

@gmanpaint Am wondering about a thread dedicated to differentials and gears only, non-specific as to make or model vehicle. 9's were of course Ford specific.

It has to be mentioned, anyway, that ring and pinion gears are ALWAYS serviced as a matched set. To try to replace a chipped ring gear for example, using the existing pinion, is pretty nuts, as the gears, after being machined to correct shape and tolerance, are heat-treated, then lapped-in on special run-in machines, making each pair a uniquely mated set. Both gears are identified by a number painted on them, as well as paint marks on the teeth identifying which teeth are to be meshed together on set-up.

That last thing brings up "hunt". Gearsets are either "hunting", "partially hunting", or "non-hunting". The non-hunting type have the same teeth meshing each time the ring makes one turn: 30 teeth and 10 teeth would be "non-hunting", as the numbers evenly divide, 30/10 = 3.00. That type MUST adhere to the correct tooth inter-mesh by the paint marks. When disassembling a gearset, first pay close attention looking for paint marks: churning for thousands of miles in hot oil often wears them off. I have seen non-hunting sets marked by scribe marks on the teeth, from the gear factory.

39 and 12 teeth, 3.25 ratio, would be a partial hunting ratio, as the same teeth would mesh together every so often: actually every 156 turns of the pinion gear. Or, every 4 turns of the ring gear, stated another way.

39 and 14, 2.79 ratio, would be a hunting or full-hunting type: the same teeth almost never come back into contact with each other: about every 546 turns of the pinion, 14 turns of the ring gear.

So what, ya say? Hunting types spread the wear more evenly among the gear teeth, partially-hunting a little less, non-hunting tends to wear a bit faster. Equalization of the wear is aimed for. There are no teeth mesh indicators, generally, on hunting gearsets. imp

FWIW, here's a little animation of a standard ring and pinion with it's standard "open" differential, nice for those (like me!) who cannot fully visualize how all those gears work!


I think a specific to Ford would be best for the forum, as a non specific, general factoid thread, would be no different then a Wiki page.

I believe that once all the baseline technical info is laid out, the modification sharing info can begin. This thread alone, has the potential to be huge over time, with others sharing info / ideas, on past / present / future builds.

Keep at it, it's looking great!

Here's a pic of my Nodular housing with 3.50 gears for reference. It's going in one of my project trucks once built. You can see the difference in webbing between it and the one installed in the truck.





I once tried to do a reverse burn-out with a 180-turn around on top of State Street bridge over I-94 in Ann Arbor, Michigan . . . . shredded that 9" to spaghetti.
My buddy way back was a Ford Mechanic, managed to buy one of the first 1963-1/2 427 fastbacks. I actually saw his 9-inch carrier cracked in half in front, from top to bottom! Rare, but it happened; the N type carrier was not news yet then. imp

FYI, am not dead. IP down since yesterday. Writing in restaurant, hard to work here. imp

A few years ago, I happened to spot this manual at an antique mall, of all places, $ 15. My wife frowned on my buying it!

It contains info difficult to obtain nowadays. Since 9-inchers were extensively used in the later gen 1 Mustangs, it's particularly relevant to working with these axles. Here's a list of all the axles used in those 1968 small and mid-sized cars:

Notice there were a couple actually called "8-3/4".......this I never knew before.

Here's how to decipher the rear axle code for the year 1968. The letters and numbers changed some from year to year, unfortunately. A quick search can usually clarify. As @gmanpaint pointed out to me, a later example involves a bit more deciphering.

I will next go into what Ford called the "Integral Carrier" type rear axles, and compare them to the "Removable Carrier" axles. In the old years like 1968, integrals were typically smaller, lighter, less strong, cheaper to manufacture, and were used with the smaller, less powerful engines. They of course offered an advantage: lower "unsprung" weight, a factor important to vehicle design.

It might be mentioned here that integral carrier does not necessarily mean inferior: the famed 8.8" is an integral type, for example.

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Here are a few diagrams helpful when setting up the gears. First, the 9" in cut-away view:

Exploded view showing all the parts and locations with respect to each other:

Location of identifying teeth location as-lapped. Important in installing or re-installing ring and pinion:

Ford-provided tooth contact patterns on the ring gear. The 2 LH columns represent the DRIVE side of the teeth, which are much narrower than the COAST side, since the face of the tooth rises up from the gear at almost a right angle. The 2 RH columns represent the COAST side of the teeth.The COAST side rises up at a shallower angle. COAST loads the gears far less than DRIVE, of course, and the very steep DRIVE tooth angle works to lessen "spreading" of the teeth in service. The top four impressions would be ideal. I have never in all my years seen as wide a pattern, especially with brand-new gears. Note the teeth are narrower on the "toe" end (inside diameter) than the "heel" end, outside diameter. I've heard it said if a really good pattern is not obtainable, getting the DRIVE impression inwards towards the toe is better than the opposite.

When I set up gears, I've found through trial and error that grasping the non-lubricated ring gear carefully with gloved hands (the teeth are SHARP!) and rotating the assembly against the friction of the bearings should feel smooth and quiet, providing no hint of any "raspy" feel. If smooth upon rotation, that baby will run quiet as a mouse! To obtain patterns, I use Prussian Blue, available in tubes, as a grease-based deep blue color. Some have recommended black markers, some paint. I prefer my old ways. imp