How to Rebuild a Ford 9 inch. Part 2

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As with any thread that is Thrown Together some information is left out or forgotten.

This post will add more information to the tools needed to service any 3rd member.

Basic tools:

1) First off your going to need a selection of Brass Tools.

Brass hammer

Brass punch set.

if you have a vice brass inserts would be a good thing.

The reason is brass will not damage the hardened steel of the differential unit. there is a reason gun smiths love bass tools and you should too.

the biggest mistake i see is people using cold chisels or hardened steel punches or hammers and then beating away trying to free the case or gears. making divots and dents and cracking parts and not seeming to care about the damage.

there is a place and a time for a cold chisel, but use it sparingly.

2) cold chisel, yup at some point your going to need to beat on stuff.

3) regular punch set: good for removing the pins and the main shaft for servicing the internal gears.

4) a sacrificial gasket scraper is always a good thing to have around or a thin piece of metal or even a trowel something you can jam into a slot and pound on.

5) seal puller another tool you should have, it makes cake out of removing the pinion seal.

6) set of metal files, helps fix previous repair damage.

7) big vice

If you Plan on removing any bearings:


1) Bearing separator. DO not start banging a screw driver under a bearing carrier trying to force it off the pinion shaft, Get the separator tool or go home.

2) Bearing race installer set: get one its worth it, stop searching around the shop for something just the right side to smack a seal or bearing in, just get a cheap aluminum set to get the job done right.

3) 2 claw gear puller, or whats called a differential side bearing puller:

This tool is the most dangerous if you get a cheap one you may end up in the hospital. you need to make sure its a quality tool to start but what you really want is the tool to be equipped with an anti separation rod, this keeps the 2 claws from trying to spread outwards under load.

What happens is as you apply force the claw will start to slip outwards you have 2-6 tons of force on those claws eventually the tool is going to slip the claw will pop off the side and 2-6 tons of force will be released just like a suspension spring, and then 911 will have to be called. so get a good one. i recommend CJ81D from snap-on tools, its not cheap at around 270$.

4) Shop press: here is your excuse to get one from harbor freight i spent 100$ for a 12 ton press on sale.

5) Dremal tool with a re-enforced cutter disc, your going to need to cut off the wheel bearings when the time comes.


Dials and indicators:

1) inch pound torque wrench, expensive and critical to setting the pinion bearing preload.

2) foot pound torque wrench, any style is fine, needed to torque all the cap bolts down.

3) 1" micrometer or caliper, i like calipers better

4) feeler gauges: if you have a really messed up posi unit you will need this

5) small carpenters steel ruler, a nice thick one about 5" long. you want a nice flat machined edge to measure against.

6) dial indicators you may need 2 different styles of mounts for this depending on how you need to use them


Tools you will make or find:

1) a real strong table

2) spanner wrenches that you can make from flat steel and 2 bolts, you need these to turn the carrier bearing adjuster nuts. i ended up using a REALLY big adjustable wrench that i drilled 2 holes in the jaws and put a bolts through, so i could turn the carrier adjusters, in retrospect 2 flat steel pieces and some bolts would of been much better.

3) a 5/8" X 2.5 - 3" bolt and nut and washers, good clutch hub compression tool, you need this to measure the clutch pack height if your changing the shims in the posi unit.


Expensive tools you don't really need but make things a 1000% easier.

1) Rear end holding fixture, see my brass punch set



1) traction modifier for a posi, get a few bottles of the stuff your going to need it when you soak the new clutches, and you will need one proper bottle to add to the rear end fluid when the 3rd member is all back together.

2) regular rear end fluid.


I'm sure i left some important tool out, if i remember i'll come back and fill in more info.


Technical information


Clearance, Tolerance and Adjustments 9” and 8” Axles

Description Inches

Maximum runout of back face of ring gear 0.004

Differential side gear thrust washer thickness 0.030-0.032

Differential pinion gear thrust washer thickness 0.030-0.033

Nominal pinion locating shim 0.015

Available pinion gear shims in steps of 0.001 in. 0.010-0.029

Backlash between ring gear and pinion early manuals use 0.008-0.012

later manuals use 0.008-0.015

(preferred in later manuals) 0.012-0.015

Maximum backlash variation between teeth 0.004

Carrier bearing preload (case spread) new bearings 0.008-0.012

used bearings 0.005-0.008

Torque Specifications

Description n-m lb-ft

Differential bearing cap bolt 95-115 70-85

Ring gear attaching bolt using loctite 95-115 70-85

Oil filler plug 20-65 15-50

Pinion retainer to gear housing bolts 41-60 30-45

Bearing cap nut lock bolts 17-33 12-25

Hoghead to axle housing nuts 34-54 25-40

Axle shaft bearing retainer nuts (8” rear) 28-54 20-40

Axle shaft bearing retainer nuts (9” rear) 3/8” bolts 35-55

7/16” bolts 50-75

Universal joint U-bolt nuts 8-15


Pinion bearing preload new collapsible spacer; n-m lb-in

Used bearings .9-1.5 8-14

New bearings 1.8-3.3 16-29

Note; above spec. is the same for a solid pinion spacer with a pinion nut torque

of 125 lb-ft for testing only and 200 lb-ft when finished


I will add a little more detail to rebuilding the Traction unit in this section.


I have a broken positive traction unit now that i can use for demonstration purposes.

Lets start with looking at the damaged caused by metal tools on hardened steel.

this is my original ford gear set that was destroyed by improper removal and installation methods.

Your looking at about 350$ worth of damage caused by a previous person.

a pry bar and hammer was used to get the ring gear off the original open differential my car came with. When the ring gear was later installed on to a traction unit further damage occurred.

Since the back side of the ring gear was used to pry it off the ring gear was actually bent outwards more hammer marks on the back again warped the ring gear further ruining it. If your 3rd member is sent out to a transmission shop and comes back looking like this, sue the transmission place.

my ring gear in the photos above is warped and beyond run out spec listed in the manual, it is so far out of spec then when original discovered installed in my original rear end, you would turn the yoke on the pinion shaft and you could actually feel it binding when turning.

now lets look at 450$ worth of damage.

my busted posi traction unit.

note the divots in the metal.

I put a Little H mark here indicating a High spot in the machined surface.

all this damage again caused by incorrect use of tools.

the machined surface was damaged .008 out of runout spec, in addition i discovered the entire case of the posi unit was bent, causing an oval gear mesh between the ring gear and pinion gear.

this was not the original traction unit to my car it was most likely taken from a junk yard and installed in my car at some point. but the unit is in fact scrap metal as i found out.

I will be using this ruined posi unit for demonstration purposes in how to service one.


There are a few different types of ford posi traction units.

they vary from 28 or 32 spline, thin walled, thick walled, nodular walled, you can have a 4 or 5 or 6 clutch type depending on year.

there are really early ones that have a special clutch pack style that can no longer be found on Earth and are impossible to service today.

and there are 2 gear and 4 gear type units.

the strongest would be a nodular 32 spline with 4 gears.

this one is the weakest one ford made, a 28 spline 2 gear unit from the mid 60s.


word on getting parts: RATECH is your only hope for getting parts. they are one of the few manufacturers of parts for the ford 9" since ford no longer stocks most parts for them:

lets get started.

Tip for removing the ring gear, after taking out all 10 bolts from the back side install 2 longer bolts back into the gear, take a hammer and work back and forth hammer the sacrificial bolts, the ring gear will pop off the diff unit with out damage.


First take your misc nuts and bolts and insert them next to the Philips or hex head style counter sunk screws

tighten them down just a little more then finger tight.

the posi case is under pressure from the heavy springs inside.

the bolts will help take pressure off the counter sunk bolts and prevent the case from popping apart sending parts flying all over your work space.

loosen the screws and remove them

now loosen the the bolts you installed, allow for 1/4-1/2" of play

how to correctly pop the case halves:

use a brass punch and bass hammer strike the edge of the lower case you do not need to put the unit in a vice or anything to do this, just strike it on the table top like in the photo. work around the lip of the lower case half, strike and turn, strike and turn.

POP the case will come apart.

your first look inside.

and lets take it apart.

again use a brass punch and hammer


At this point you inspect everything, anything that looks worn or you can feel a lip with a fingernail against it has to be replaced.

check your side gear washers for wear make sure they are in spec

the thrust gear washers take a beating they are conical shaped, you can't really measure them too well, usually they are worn paper then or have pieces missing from them or have DISAPPEARED. in short get new ones anyway.

ok you inspected everything it looks good, lets re-assemble. have a little motor oil handy.

rub a little motor oil on the side gear washer coat both sides.

rub a little oil on the back of the side gear also.

coat the thrust washers in a little oil.

springs back in

pressure plate: the orientation matters not much in a 2 gear but in a 4 gear you have to be careful to install it correctly, the sides on the pressure plate that go down around the main shaft must be on the machined sides of the center spring block. Only 2 sides of the spring block are machined, the other 2 are rough so make sure the pressure plate is inserted so the ears go onto the machined surfaces.


Clutch side


Inspection: look for wear anything you can feel with your finger other then perfectly smooth means the traction unit needs to be replaced.

the clutches

case inspection: Look for any cracks in the case in these areas around the case.

You find anything that looks like a crack take the entire posi unit and throw it in the garbage.

how to use the bearing puller

again measure that thrust washer

Lets assume you looked over your unit everything looks good and you suspect the unit was never serviced or the service done to it was great

that means you can trust that the correct shim was installed in the clutch pack.if that is the case then we can replace the clutch pack if worn

look at the clutch pack it should have a waffle pattern in it, if you find the waffle pattern is worn or missing in areas its time to replace the pack.

Ok so you have a new clutch pack, remember to keep any original shims you find and its time to put it all back together.

Shims first

start with steel then alternate

now the special top clutch that is metal on one side and fiber on the other.

note it has a special coating on it as well

this goes on top

fiber side up

do not forget this thrust washer.

clutch hub: make sure you inspected the surfaces of the hub for any wear, if you find the thrust gear area is worn and you feel ridges with your finger you will most likely have to replace the entire posi unit. ford made tons of different clutch hubs with different depths over the years it will be impossible to find a correct replacement.

you will have to wiggle it around to get it fully seated and to line up all the teeth on the clutch pack.

lets get the clips in

Side thrust gear: again make sure the machined surface of the back side is not worn that you can feel ridges with your finger. the surface may look discolored or worn but feel smooth, that is fine.

Now at this point i would recommend installing a Axle shaft in your vice.

take the clutch side of the case and place it onto the axle shaft like this.

you must make sure that the clutch hub splines match the thrust gear splines PERFECTLY. when you reassemble the case halves.

if these splines are out of alignment you will never be able to install the axle shaft all the way in when the 3rd member is back on the axle case.

the reason is once the case is put back together the spring pressure plate holds against the thrust hub and gear with over 150 Foot pounds of force preventing the traction unit from acting like an open gear differential.

so if these 2 splines are not aligned and you put everything back together you will discover your mistake later and have to take the entire axle and traction unit apart again to fix it.

now as a demonstration imagine the lower case is installed on an axle shaft as the picture above shows.

put the case back together.

install the nut and bolt and the counter sunk screws for alignment.

Tighten down the nut and bolts evenly to press fit both cases together, when fully seated, tighten the counter sunk screws into the case.

no need for any thread lock, remove the nuts and bolts and the case will be under a lot of pressure keeping the countersunk bolts in place.

plus the ring gear sits on top on them keeping them from ever falling out.

remember to always use new bolts for the ring gear and remember to use the washers with the bolts.

use red thread lock on them and torque to 77 Ft pounds, work around in a star pattern tightening the bolts. then go back and re-torque them 2 or 3 more times again in a star pattern.

the reason is the case halves for the posi will actually seat a little more once all the 10 bolts are tightened.

your ready to move on to backlash and case spread setup.

in part 1 of this topic.


What happens if the wrong shims are in the traction unit?


This method I'm about to show you can also be used to confirm if you already have the correct shim in your old clutch pack and can just reuse it.

Step one.

remove all shims from the clutch pack and remove the thrust washer from the center of the clutch hub area.

assemble the clutch pack WITHOUT any shims.

insert the clutch hub into the pack and fully seat it.

now take your depth gauge and measure out the depth from the top of the machined surface of the clutch hub, and the machined surface of the bottom of the clutch hub like this:

every single clutch hub is pretty much unique and you will get a range of different ones. .500" is common.

for educational purpose the measurement of the clutch hub depth in the photo is .490" just under 1/2"

next place a bolt with washers through the assembly as so.

tighten the bolt squeezing the clutch pack together with the hub to simulate the compression of the springs inside the traction unit. tighten to 10-15 Foot/pounds. some of the traction lock additive fluid will leak out don't worry about it.

when compressed take a carpenters square ruler or some flat machined surface at least 6" long and lay it on top of the hub, then take your feeler gauges and start slipping them under the end of the ruler. like this

make sure your feeler doesn't start dragging on the ruler bottom.

my measurement was .012", your measurement will differ.

Now we do some math.

Take the clutch hub depth subtract it from the shim thickness.

.490-.012 = .478

take that result and subtract it from fords magic number of .455

.478-.455 =.023

so mathematically the result is the correct shim size for your traction lock.

now the shims only come in whole numbers in .005 steps, so in this case you can either install a .025 and go for a more aggressive traction lock feel or go with a .020 shim and error on the side of caution.

low an behold i have a .025" shim in my traction lock.

what is the danger if i decide to install a .010 shim?

the traction lock will be too loose burn up the new clutch pack and act like an open rear end.

what is the danger if i decided to install a .035 shim?

the traction lock will be extremely tight and when you put the traction lock case back together and tighten it down the traction lock can crack or shatter, or it may break or shatter when installed in the car for the first burn out.

so take care this method is a good way to double check your shims.

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