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Locker Basics


Bronco Guru
May 23, 2004
Vancouver, WA
This FAQ is called Locker Basics however it will actually cover the most common differentials from an Open Carrier clear up to the Selectable Lockers along with the general principals behind the way each one works. For the purpose of this FAQ the word Locker is used in its strictest form to apply to one particular tupe of differential which is the most widely accepted use of the word as it applies to diffentials. The FAQ covers a large variety of differentials because the word is often used loosely to cover any carrier that is not an open carrier and therefore all of them require discussion to understand the differences.

First an overview of what the differential carrier is. The differential carrier is contained inside the axle housing and literally is what carries the differential components. The differential components are what actually transfers power from the drivetrain to the axles. In most cases the power is allowed to differentiate between axles to allow for differences in wheel speeds between the tires. (When cornering for instance) The way this is done varies between the different types and each one has particular characteristics that make it better suited to particular uses.

Carrier Options discussed below:
Open Carrier
Limited Slip (LS)
Lunchbox Locker
Lincoln Locker
Selectable Lockers

Carrier Principles and Characteristics:
Open Carrier: Open Carrier is the most common type of differential in use. It utilizes 2 side gears (which the axle attaches to) and two spider gears which ride between the side gears. As the carrier itself is rotated by the ring gear the sides and spiders direct the power to the axles. Due to the way this is set up it forces both axles to have the same amount of torque at all times. Don't misunderstand this though as meaning both sides will always have torque. Instead this means that as soon as the torque drops to one side it also drops on the other. For instance when a tire is in a slick spot of mud or when a tire gets airborn from flex. Because of this loss of torque to one side means the axle basically no longer has any traction.

Limited Slip (LS): A limited slip axle differs from an open carrier in that there are extra clutches or gears in them that force boh sides to get power when there is a difference in torque between both axles. I.E. As one axle begins to spin faster there is a difference between them which causes the clutch cogs to rotate away from each other forcing the clutches against the side gears to force power to both sides. Many manufacturers have called this a locker in the past however it really is not a locker as it doesn't fully lock the axles together. Note: LS units rely on a difference of torque between axles however there must always be some torque to them both to work. In the event all of the torque is lost on one side then neither side will get traction. (For instance after an axle breaks)

The common LS units available are as follows:
Trac Lock - This is the stock LS unit Ford normally offered as an option. These are normally considered to be fairly weak with clutch packs that wear out fast.
Power Lock - Similar in concept to the trac lock but uses 4 spider gears for strength along with all steel clutch packs that don't wear out as fast. At one time these were considered by many to be the only bulletproof option for a limited slip unit. We all know now that nothing is bulletproof regarding 4wd's.
True Trac - This is a newer LS design than the above two are. Instead of clutch packs it uses gears to provide the same thing. A bonus to gears is that they don't wear out as fast as clutches do. This desig has gained a lot of popularity and seems to be a worthy competitor against the Power Lock.
(Note to self: Be sure to work in some information here on LS fluid)

Locker: Lockers differ quite a bit from the open carrier and LS units. They instead literially lock both axles together as soon as one axle begins to run slower than the drivetrain. They will unlock and allow either axle to run the same speed or faster than the drivetrain though. Because of this the inside tire n corners always has power while the outside one usually does not. This has a slight tendacy to push the vehicle toward the outside of a curve and some people find the locking and unlocking action to be a nuisance. (See the discussion on spools for more details) This action can also be downright dangerous in some conditions. (More on this below in the section on which options are best suited to particular conditions) Since a locker never allows an axle to turn slower than the drivetrain they will still provide traction to the remaining axle after one breaks. Keep in mind though that any pieces of the axle that remain inside the carrier will still be turning and could further damage other parts.

The common Lockers available follow:
Detroit Locker - The Detroit Locker is perhaps the most popular locker available. They are very strong and stand up well to a lot of abuse. For many years these were considered bulletproof but as we know they are not. (I've personally broken one) In particular they do not handle the resulting slap to the drivetrain very well after axles or hubs break that are under a lot of torque. Therefore be wary of building in a weak point in the drivetrain if you run one. (For instance don't run hub fuses that are designed to break earlier than the rest of the components)
Detroit Soft Locker - This is the same as the above but is an upgraded version that has some adjustments to the springs so the locking and unlocking action is not as severe. The performace is identical otherwise and it is no stronger than the previous design. This design has pretty much replaced the earlier Detroit Lockers now.

Lunchbox Locker: Lunchbox Lockers differ from the regular lockers in that they are designed to fit into a stock or in some cases limited slip carrier. These operate in the same manner as the lockers above however they tend to have more severe locking and unlocking actions and are always limited to the strength of the carrier they are installed into. These also tend to develop problems when used with larger tires. 33" seems to be the most common suggested cutoff point. This seems to be hit and miss though. Some people use them with 35" or 36" tires without problems while others have them fail right away with 33" tires. My personal suggestion is to avoid using them with 33" or larger tires if for no other reason than that they rely on the strength of the carrier they are installed in. One other benifit to these is that they often can be installed without changing the gear setup.
Lunchbox Locker options:
Lockright -
EZ Locker -
Aussie Locker -

Spool: A Spool isn't really a differential as it doesn't differentiate torque between the axles. Instead it completely locks both axles together permanently so that they will always get power. This is a popular option with those wanting to save money that want power to both tires off road because they are generally a lot less expensive than a Locker. The tradeoff for this is that they will literally drag tires around corners and will bind the axle splines up which can cause premature wear to them. Some people prefer the spool to lockers and claim it is due to the unpredictable nature of the locking and unlocking action of a locker. Personally I have never had this issue with them. it may be worth noting though that this seems to be more of an issue with those running manual trannies where they are more inclined to shift while they are driving through a corner.
Spool options fall into two categories:
Full Spool - This replaces the entire carrier and is normally very strong.
Mini Spool - The mini spools are similar to Lunchbox Lockers because they both utilize the existing carrier. These are normally fairly strong but will not stand up to the same abuse as a full spool. Using upgraded cross shafts often helps because this is where they most often fail. I have seen a few though that literally split open the carrier and therefore like the lunchbox lockers they are limited in strength to the carrier they are installed in.

Lincoln Locker: I threw this one in for those that may not know what it is but that may hear it on the trails. A Lincoln Locker is named such after the Lincoln Welders and is called this because it is literally nothing more than welding together the parts inside a carrier to make it act like a spool. My personal experience with these is that trying to do this almost always causes more problems than anything. The welds often fail and when they do it often causes more damage and is often more costly to fix than it would be to just do it right in the first place. My recommendation is to just buy a mini spool instead if you are on a really tight budget.

Selectable Lockers: Selectable Lockers usually are not really lockers as such. Instead they are usually a spool combined with either an open carrier or a limited slip where either one can be selected by the driver based on the conditions. When locked they usually exhibit the same characteristics and negative effects as as spool that a regular locker doesn't have. Because of the selectable nature of them they work better in a larger variety of conditions than a single type of carrier does.
ARB Air Locker -
Eaton E-Locker -
Ox Locker -
Electrac -

Please note that some of the information above is based on opinions and is therefore subjective. In these cases I have tried to provide the opinions that are the most widely accepted and/or that are based on personal experience and have tried to make it clear that they are opinions rather than facts.

Notes to self:
Don't forget to discuss effect on steering
LS fluid
Conditions where each works best
Be sure to discuss sidehills
Describe each lunchbox option and selectable option

Please note: This FAQ is a rough draft that is not currently complete. Please wait to comment until it is complete or if you think you have something important to add please send me a PM about it.
Last edited:


New Member
Feb 22, 2007

the article was helpful getting to know the basics-i have a 93 bronco xlt-351windsor-197k mi- -no lift/33's-recently had my e4od rebuilt w/retrofit kit and shift kit-added a trans command-this vehicle is an everyday driver/toy hauler 19ft/boat 19ft/and medium offroad use(only veh. i have) my rear end -stock 3 L 55 (does this mean a locking diff. with a gear ratio of 3:55?) has to be replaced or rebuilt chewed up an axle, but found out that my spider gears were the culprit that provided the peices of shrapnel. i realize i need to replace the spider gears, i just don't know what would be the bestway to go to get the strength needed for payload and play, your suggestions will be appreciated


Bronco Guru
May 23, 2004
Vancouver, WA
The 3 and the 55 are 3.55:1 gear ratio. The L in betweeen them indicates that it is a limited slip unit rather than a locker. Presumably this would be the trac lock assuming they were still using them in 1993. Might be an Auburn though as well. Either way they are not known for standing up to much abuse.

If the previous setup worked well for the way it was/is being used then I think I would consider staying with a limited slip and just upgrade to the stronger true trac unit. If not since it is being used for towing as well I think I would suggest a selectable locker although that is a big jump in cost. (I would guess about $500 or so difference)


New Member
Sep 23, 2007
A few typos

1st paragraph:
"... the word Locker is used in its strictest form to apply to one particular tupe of differential ..."

> "tupe" vs "type"

Limited Slip (LS):
"...there are extra clutches or gears in them that force boh sides ..."

> "boh" vs "both"

True Trac:
"... This desig has gained a lot of popularity..."

> "desig" vs "design"

p.s., I tried to send this privately but "SaddleUp has exceeded their stored private messages quota" :)


New Member
Oct 15, 2009
What is the correct procedure for shifting a 1974 Bronco (automatic transmission) into 4wd? Manual hubs, J-shifter, and what gear do you put the transmission in ( nuetral, park, reverse, etc.).

9 inch ford

New Member
Oct 23, 2010
what kind of locker is in a stock 67 bronco rear 3.55

I have a 9 inch posi out of a 67 bronco with gears in the 3.50 range. Without taking it apart ,does anyone know what kind of locker is in this. It does have 28 spline axles. And what kind of good quality oil do i put in this.I am installing this in a 1931 Graham Rat Rod. Thanks for your help ! Fred


Jr. Member
Dec 22, 2012
If it is stock it is a Ford made unit. Any good brand oil will work. You must use a limited slip additive in addition to the oil.