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Early Bronco

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Early Bronco Axles

Types of Differentials

There are numerous types of axles.. A stock Bronco comes with either an open differential or a 2 or 4 pinion trac lok. The gear ratios available were 3.50, 4.11 and 4.56 (only with 6 cylinders). Of the above carriers (type of differential) the 4 pinion trac lok is the most desirable for a stock Bronco. However, many people have cracked the actual carrier housing of the trac loks, making them undesirable for HD off road use. However, if you are a street driver and don't do much extreme four wheeling and are not running a tire over 33 inches, they'll do fine.

How the Differential Works

The open diff is simply that, open. It is sometimes called a peg leg because only the wheel that is easiest to spin will spin. This means that if one tire is in the mud it will spin and the other will not. Also, if one tire is in the air, it will be the one spinning. Hence open diff is not very desirable in low traction situations. They are fairly reliable though, and some people like them for the snow...but plan on finding yourself stuck in no time. They may be the best option if you do NOT do any four wheeling and rarely find yourself in sticky situations. They're nice in the front if you live in snow country.

The trac lok is considered a limited slip (l/s) differential. It utilizes a clutch pack that will send traction to both wheels. However it is not very good at distributing power and normally just functions as an open differential. These can be built up "tighter" than stock, but this will only last for a shirt time, and you will again ultimately end up with an open diff. If you are planning on putting one of these in, the 4 pinion design is the stronger of the two, but normally the weak point is the case of either. These normally run about $150 for a rebuilt unit, though you could pay considerably more depending on where you buy it.


There are two kinds: The Detroit and the Lock-Rite. These are automatic lockers. This means that both wheels get full power: They must both turn at the same speed when coupled together inside the carrier.
They do lock and unlock for turning though. The Detroit is by far the most popular and reliable carrier for the 9 inch if you are doing HD four wheeling. However, the added benefits of the locker come with some drawbacks. They can sometimes have a "mind of their own." They can really act up on the streets, locking and unlocking and can also be quite noisy. Even with all these drawbacks, most hard core enthusiasts will elect to install one. They do act up more with a manual transmission than with an automatic. Dyneer, the makers of Detroit lockers, have come out with a new soft locker which is suppose to be a lot more cooperative. The price of Detroit is normally around $400.

The Lock Rite is a locker which can only be installed if you have an open carrier. It takes the place of your spider gears. These have some of the same characteristics as the Detroit, but most say it is not as bad. The benefits of the Lock Rite are its ease of installation. The average backyard mechanic is capable of installing it, whereas a Detroit, or any other type of carrier, necessitates special skills and expensive tools. These run around the $300+ mark, though they can be found for considerably less. If you are into hard core stuff, try one up front if you don't want a Detroit. Shop around for the $300 price. Bottom line? The Detroit is stronger.

For the Front End

All of the above exist for the front. There is also the Power-Lok (priced at around #350) which is similar to the trac lok, just much better. If you have your heart set on a l/s diff, then go this route. It will work for much longer and it will be a l/s for much longer. You can also build these real tight.

There are also some other various types of l/s out there. There is one called the True Trac, which is also made by Dyneer. This is an all gear driven unit, which means it won't wear out and will always function as a l/s type unit, making it perhaps the best l/s you can get. These run in the high $300's.

Another option is the ARB air locker. As of this writing it is NOT available for the 9 inch, but is available for the D-44. This locker gives you an open differential until you hit a button on your dash, which, by way of air pressure, turns your diff into a spool. A spool essentially LOCKS together the two axle shafts. This is more extreme than the Detroit or Lock-Rite as there is no "ratcheting" or give on turns. Some have complained about problems with the compressors that come with it and problems with the solenoid valves. Though others have nothing but good things to say. If you decided to go with the ARB, run a steel type line for the air. If you run a plastic line you WILL break it....and it is useless with no air.

The main drawback of this system is price, which normally runs around $650 with the compressor. Again, many folks love this carrier as you get the benefits of the open diff on the streets (if in the rear of a vehicle) and a locked whenever you want on the trail. It is also nice to be open up front when doing easy four wheeling.

Ring and Pinion

Due to the conical shape of the pinion, the ring actually WEAKENS with the addition of more teeth. Simply put, the more teeth you put on the smaller they get. Although you now have a larger contact surface, the teeth, by getting smaller, actually weaken dramatically. This is one reason why you should not go with low axle ratios with a strong engine.

The lowest one would want to go is 5.13 in the D-44, though the 9 inch can be reliably built with lower gears. Remember, lower gears are higher numerically.

Ford 9 Inch Facts

Why is the Ford 9" so popular among racers? Aside from the ease of working on a removable carrier, the Ford 9" has several features that make it preferable to the counterparts from Chevy and Dodge.

oLight weight
The 9" has a lower weight/strength than either the Chevy 10-12 bolt or the Dodge.
oLarger Diameter
A 9" ring gear lessens the torque load on the gear and provides a wider range of available gear ratios. The 9" has the widest range available of all the popular differentials.
oMore teeth in contact
Having a lower pinion center line, the 9" keeps more gear teeth in mesh than the other brands. This means higher load capability. However, one drawback is that the losses are slightly higher.
oThree pinion bearings
Most rears have only two bearings supporting the pinion gear. The 9" has the usual two bearings plus a smaller pilot bearing that allows much higher loads to be applied without deflection of the pinion. Stronger gear sets are available (the Dana 60), but the weight penality is severe.

Axle Shafts

The 9 inch utilizes 28 spline axles. There are four types of axles that came with the 9 inch:
oThe big bearing from 66-73
oThe big bearing for the 74-75
oThe big bearing for the 76-77
The '76-'77 are the most desirable, with the '77 housing being the most desirable as it is more HD. There are also a few variations of brakes available. Again the '76-'77 have the best ones: 11-1/32 x 2-1/4. The other years utilize smaller ones.

One must remember that the axles in your Bronco are at least 18 years old, assuming you have a '77. And they could be almost 30 years old if you have a '66. Therefore, they will all be fatigued to some extent. Therefore, if you install bigger tires, you are likely stressing them more than they can handle and run the risk of breaking one. Now if you go ahead and add a locker as well, you are definitely over stressing them and can count on breakage.

If you go even further and add a granny low 4 speed (which will multiply the torque incredibly) you will be lucky to pull out of your driveway without breaking an axle. Obviously, this is a slight exaggeration, but the more you do the higher the likelihood of breakage. Also, the smaller bearings are more prone to breakage than the big bearings.

So what is the answer? Upgrading to 31 spline shafts is the best thing to do! This necessitates changing your carrier (locker or l/s). When you upgrade to the 31 spline it is also recommended that you get the tapered roller bearing, which helps distribute load better. Therefore, if you plan on upgrading axles you must do the carrier as well, and if you plan on upgrading carriers you are better off doing the axles at the same time. Otherwise you'll have to do both later on.

Buy a new axle (if you don't want to go 31) and they will last for a long time.
FYI, you can install a Lock Rite now in your rear end with the 28 sp shafts and then when you get the extra money upgrade the Lock Rite with some new 31 sp side gears (around $30) and then buy your new 31 sp shafts. You can get the 31 sp for the big or small bearing, but the tapered roller bearing ONLY works for the big bearing. If you are planning to dump that kind of money into shafts, gears, and a carrier, then you may as well only do it in a big bearing housing. Though even with the small bearing you will likely NOT break the 31 sp shafts. It is just much more relaxing to know that you have the big bearing (tapered roller).

Front Shafts

These seem to break most often at the U-Joints. Again, all the additions for added traction and tire size will increase breakage. These are 30 sp shafts if you have the 44. Most offroaders will scrap the D-30; however, if you are not applying a lot of horsepower and do not offroad very often, by all means keep it, but DO NOT dump a lot of money into it. You can just as well get a 44. In other words, if you break a carrier or ring and pinion, don't fix it, just get the D-44.

The EB came with 1-1/16 OD u-joint caps for the front shaft u-joints. Many people break them, so they upgrade their inner and outer shafts to a different shaft so as to be able to run the larger 1-3/16 OD joint (Spicer 297). Inners and outers can be had from many different vehicles. BUT you will need to shave the short shaft a little at the splines (you can do this at home) and the long side shaft will need resplining. Remember, here you are replacing with used equipment. Many of the EB houses sell these upgraded axles in new versions and some sell them in used versions.

All are sold ready to bolt in. If you are breaking shaft u-joints then this may be the answer but if you are breaking the yokes, then this may actually weaken the assembly. In order to accommodate a larger cap it is necessary to have less material on the yoke. Also when replacing these joints use a good quality SOLID joint, made by Spicer, TRW, Neapco, Rockwell, etc. This advice holds true for all joints on the drive shafts too.

One last note is on axle gear oil. The differentials require that you use a GL4 rated gear oil. It is recommended, however, that you buy gear oil rated one level higher (e.g. GL5) than the requirements to allow for degradation. Some of the synthetics are a GL6 rating, such as Torco, which Currie Enterprises suggests for your diffs.

Additionally, if you have a l/s then you want to install a friction modifier whenever you change gear oil. Many of the higher quality gear oils come with it premixed. Friction modifier will not harm an open diff.

Axle Specifications

Rear Axle Specifications
'66-'75, 28 spline, big bearing short side: 26 15/16"
long side: 29 1/4"
'66-'75 28 spline, small bearing short side: 27 1/8"
long side: 29 5/8"
'74-'75 28 spline, big bearing/small brakes short side: 27 1/4"
long side: 29 3/4"
'76- '77 28 spline, big bearing/big brakes short side: 27 3/16"
long side: 29 11/16"

Brake Specifications
66-75 small bearing 10 x 2.5
74-75 medium duty 10 x 2.5
66-77 large bearing 11 x 1.75
76-77 11 x 2.25

Rear Axle Bearings
Application Seal Bearing I.D. O.D.
69-78 Bronco 2780 and 2900lb 9568 rw-207-ccra 1.3780 2.8346
69-78 Bronco 2780 and 2900lb 9568 rw-207-ccra 1.3780 2.8346
69-77 Bronco 3300lb 51098 514003 1.5312 3.1496
Tapered Roller Bearing 3195 a-20 1.5625 3.1483

Front Axle Bearings
Application location Seal Bearing I.D. O.D.
'71-'79 Bronco inner 4250 a-37 1.7812 3.0625
outer N/A a-37 1.7812 3.0625
'69-'71 Bronco inner 4250 a-37
'66-'69 Bronco inner 442380 a-37

Axle Shaft Seals
71-77 Bronco 5131
66-71 Bronco (from 3/18) 480570
66 Bronco (to 3/17) 40652s

Note: The tapered roller bearing was not available on the EB but is a common retrofit and improvement when upgrading to the HD 31 sp axle shafts.

All these part numbers are for Federal Mogul National Oil Seals and BCA bearings.

For additional Axle related information see the following Tech Notes:

o Front Disc Brake Conversion
o Power Steering Conversion

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