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.
That's an overview. Errors I may have made, or questions, will be appreciated. imp

 

The 9" is a monster compared to what they're using today...

 

Well, I hoped someone might add experiences gained, but what you say is true. The 8.8 today is IMO just as strong, but is heavier, hard to work with, difficult to change ratios, axle shaft is the inner bearing race (9 inch has replaceable axle (wheel) bearings, separate from the axle), but since 9" is gone we work with what we're given.

One set-up I fouled, early on, in my '55 Merc., running a 430 cu. in. Lincoln, I installed a 4.57 9 inch, with Borg-Warner H/D Overdrive out of a Lincoln. I mismatched threads putting the diff. bearing caps in place, a brand-new 4.57 gearset which within two or three blocks beyond my house screamed like hell! I think I was 20 or so. That incident resulted in my entire life being changed: hit a Pontiac, lost my DL, my parents' licenses revoked, mine also, I was just preparing to get married and drive to L.A. Wound up driving honeymoon trip Chicago to L.A. and back with no license, new wife had none (only 15!!!!).

 

How did that quote get so messed up?

The 9" has no differential case gear-ratio range restrictions; one differential accepts all gear ratios
from the 2s through the 6s. There are more gear ratios available for it than any other one out there.
There are gears for at least every tenth of a ratio change from the 3.1 to 5.1 range. The drop-out third
member makes it easy to change gears. The strongest differential out there because of the larger offset,
the hypoid distance between the ring-gear and the pinion-gear center-lines, but that also why it's less
efficient, more gear contact. You know about the large tapered roller bearings, and extra pilot bearing...
The best part, the two adjuster nuts make setting ring-gear backlash pretty easy: loosen one side and
tighten the opposite side to move the gear back and forth. So much easier, and better than using stupid
shims... You probably read and or know all that, but man, I'd like to have one, but then again, I don't
have a motor that would necessitate one.

They were used in nearly all their cars and trucks from 57 to 86... My grandfathers 77 Cougar had the
9" in it. A friend of mine bought the car from him, and beat the **** out of it. Two transmissions he went
through, not one issue with the differential. I mean, he raced this car everywhere he went, the C4 could
not take the abuse, but that rear-end did. He launched it over railroad tracks at 80, did that with me in
it... He eventually tore the motor up racing it around... It was his first car, he liked it for some reason,
and just beat the hell out of it.

You can still get trac-loc for it from ford performance.

Yeah, the 8.8 is just .2 smaller, but it doesn't have the offset like the 9" does. I once was able to get the
pinion to spin while brake torquing another car... That was a horrendous noise. Can't do that with the 9"
with the more metal to metal contact area. You'd more likely burn the trans out before you could strip
or have the pinion jump the ring gear. O'h that sound, I can still hear it...

 

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

 

I still haven't had the opportunity to work on one of these in person yet, but I remember being dismissive of them when I first saw pictures. I was totally ignorant of how clever the design was, focusing only on the lack of an easily removable inspection cover or drain plug.

 

WOW! I am proud that the vulnerable FORD 9" is getting its due respect. I have used the 9" in everything from my "Funny Car" to my oval cars. It is truly a testament to strength and durability. I am currently working with a customer on installing a 9" FULL FLOATER. This rear will be handling a 850 HP Dart 363. Yes, the 8.8 is no slouch, but for peace of mind, I prefer the 9". Oh, by the way, the Chrysler eight and three quarter is no slouch either. Also sets up like a 9".

 

With separate pinion gear retainer? Interesting. Note the idea is used in many BIG axles for trucks, the center sections are big as a bushel basket!

 

There are two of these for sale on FB Marketplace...