No expert but I have a similar problem.
Series 2, 3 carriers are common and there is a break where things just don't fit.
I have A 456 Dana that I want to switch to a 3.73.
Done some research and found this article answered a lot of questions for me.
http://www.crawlpedia.com/index.htm
[ CRAWLpedia ]
[Off-Road Encyclopedia]
Thick Gears and Carrier Breaks
Car and truck axles come in a variety of gear ratios depending on their application and many aftermarket gear manufacturers extend that range even further with extra high and extra low gear ratios.
The gear ratio in an axle determines how many times the drive shaft needs to spin in order to turn the wheels one rotation. For example, a 3.73 gear ratio requires the drive shaft to spin 3.73 times to turn the tires around once.
In the diagram below, the drive shaft turns the pinion against the ring gear that is connected to the wheels via the axle shafts. The number of teeth on the pinion and the ring gear are what determine the gear ratio.
The higher or "taller" a gear ratio is numerically, the more the engine needs to turn to spin the tires. Higher gear ratios work great for trucks that need more torque to get heavy loads moving while lower or "shorter" gears are better for racing and for improving highway gas mileage because the engine can turn slower at higher speeds.
Find the correct gear ratio for your vehicle
with our Gear Ratio Finder
Calculate your existing gear ratio
with out Gear Ratio Calculator
NEW! -
Need Help with Carrier Breaks and Thick Gears? Check out the new comments section below!
Ring and Pinion Gear Sizes and Alignment
The gear ratio of any axle can be calculated by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear. For example, an axle with 41 ring gear teeth and 11 pinion gear teeth would have a 3.73 gear ratio (41/11 =3.73).
Low gear sets have more teeth on the pinion gear than high ratio gears do and as such the overall diameter of the pinion gear tends to change significantly across the scale of ratios. Ring gears, on the other hand, maintain their size because the teeth are cut on the face of the gear and not along the circumference. As a result, and because axle housings are machined with a fixed position for the pinion and differential carrier that holds the ring gear, a gap is created between the two gears as the gear ratio increases.
The gap between the ring gear and pinion gear is corrected in two ways: The first way is to make the ring gear thicker and the other is to use an offset differential carrier that moves the ring gear closer to the pinion. While it would be nice to avoid changing the carrier and use thicker gears for all ratios, there is a point at which the rotating mass of the additional material begins to negatively affect performance. This is why many axle manufacturers use two different carriers for high and low gear ratios.
What is a Carrier Break?
Axles that are designed for use with a large range of gear ratios often have two different differential carriers, one for lower gears and one for higher gears. The point at which the axle switches from one carrier to the other is called the
carrier break. For example, the Dana 60 axle has a 4.10/4.56 carrier break that requires one carrier for 4.10 and numerically lower gears and another carrier for 4.56 and taller gears.
When changing gears in an axle, it is important to determine if the axle has a carrier break and if so, which carrier it has installed. If the axle does not have a carrier break, then any aftermarket gears will fit just fine. If the axle does use a carrier break, then the first thing that needs to be checked is if the new gear ratio is on the same side of the carrier break as the existing gears or if they will be "jumping the break".
For gear changes where the gear ratio crosses the carrier break, the installation will require the use of the other differential carrier. Luckily, aftermarket differential and locker manufacturers are aware of this and almost always offer models for both lower and higher carriers.
Some aftermarket gear manufacturers also offer
ring gear spacers and
thick gears that are designed to jump a carrier break without the need for changing the carrier. These gears are not available for all applications or all gear ratios, however, whenever possible, they are a great option.
How to Select The Right Gears and Carrier
Step 1: Determine If Your Axle Has A Carrier Break
Find your axle in the table below to see if it has a carrier break. If so, then you need to check and see if your current gear ratio is on the low carrier or the high carrier. If not, then all gear ratios will fit on the stock carrier and you can't go wrong.
Step 2: Find Your Existing Gear Ratio
If you don't know your existing gear ratio, you can use our
Gear Ratio Calculator to figure that out.
Step 3: Select Your New Gear Ratio
Using our
Gear Ratio Finder and
Engine RPM Calculator, select the appropriate gear ratio for your application.
Step 4: Check If The New Gears Jump the Carrier Break
If your new gear ratio is within the same carrier range as the stock gears, then standard gears will work fine. If your current gear ratio is on the low carrier and your new gears are on the high carrier, then you will need to change the carrier, use a ring gear spacer, or find thick gears. If your existing gears are on the high carrier, and your new gears require the lower carrier, then you will need to change to the lower carrier.
Current Ratio New Ratio Installation Options
Lower Carrier Lower Carrier Standard Gears on Lower Carrier
Lower Carrier Higher Carrier Standard Gears on Lower Carrier with Ring Gear Spacer
Standard Gears on Higher Carrier
Thick Gears on Lower Carrier
Higher Carrier Lower Carrier Standard Gears on Lower Carrier
Higher Carrier Higher Carrier Standard Gears on Higher Carrier
Tip: Aftermarket Carriers, Limited Slips, and Lockers
Most aftermarket differential manufacturers offer both carriers so if you plan on upgrading your differential with your new gears, make sure that you order the correct one so that you can run standard gears and avoid needing thick gears or spacers.
Popular Axle Carrier Breaks
Axle Low Carrier High Carrier
AAM 9.25" All / No Break Same
AAM 9.25" IFS All / No Break Same
AAM 10.5" All / No Break Same
AAM 11.5" All / No Break Same
AMC Model 20 2.73 & Down 3.08 & Up
AMC Model 35 3.31 & Down 3.54 & Up
Chrysler 7.25" 2.47 & Down 2.76 & Up
Chrysler 8.25" 2.47 & Down 2.76 & Up
Chrysler 8.75" (All) All / No Break Same
Chrysler 9.25" All / No Break Same
Chrysler 10.5" All / No Break Same
Chrysler 11.5" All / No Break Same
Dana 27 3.73 & Down 3.92 & Up
Dana 30 3.54 & Down 3.73 & Up
Dana 35 3.31 & Down 3.54 & Up
Dana 36 ICA 2.73 & Down 3.07 & Up
Dana 44 3.73 & Down 3.92 & Up
Dana 50 All / No Break Same
Dana 60 4.10 & Down 4.56 & Up
Dana 61 All / No Break Same
Dana 70 4.10 & Down 4.56 & Up
Dana 80 3.73 & Down 4.10 & Up
Ford 7.5" All / No Break Same
Ford 8" All / No Break Same
Ford 8.8" All / No Break Same
Ford 9 Inch All / No Break Same
Ford 9.75" All / No Break Same
