What is the dimension that is used to identify a 9 inch axle?

When it comes to automotive components, the dimensions of an axle are crucial for ensuring proper fit and functionality. In the case of a 9-inch axle, the dimension that is used to identify it is the overall width or diameter of the axle housing.

What is a 9-Inch Axle?

A 9-inch axle is a type of automotive axle that is commonly used in various vehicles, particularly in the rear-wheel-drive configuration. The "9-inch" refers to the approximate diameter of the axle housing, which is a critical measurement that helps determine the compatibility of the axle with other vehicle components.

Identifying a 9-Inch Axle

The primary dimension used to identify a 9-inch axle is the overall width or diameter of the axle housing. This measurement is typically taken from the outer edge of one axle tube to the outer edge of the other axle tube, excluding any additional components such as brake assemblies or differential housings.

While the exact measurement may vary slightly due to manufacturing tolerances, a 9-inch axle will generally have an overall width in the range of 8.75 to 9.25 inches. This dimension is crucial for ensuring the axle fits properly within the vehicle's frame and suspension components.

Importance of Accurate Axle Identification

Correctly identifying the dimensions of an axle is essential for several reasons:

• Compatibility: The axle must fit the vehicle's frame, suspension, and other related components to ensure proper installation and operation.


• Performance: The axle's dimensions, such as the gear ratio and differential housing size, can impact the vehicle's power delivery, traction, and overall performance.


• Safety: Improper axle installation or the use of an incompatible axle can compromise the vehicle's handling, stability, and safety.

Accurately identifying the axle dimensions, including the 9-inch specification, is crucial for ensuring the proper fit, performance, and safety of the vehicle.

How to tell Ford 9 inch from 8 inch?

The easiest way to tell the difference between a Ford 8-inch and 9-inch rear end is to check the bottom two bolts on the center section: on a 9-inch rear end, a deep well socket will not fit, so you must use a wrench; on an 8-inch rear end, a deep well socket will fit. Other methods include examining the housing case shape, as the 9-inch often has a more distinct, flatter-sided shape, or checking the axle tag for specific codes. 
Primary method: Checking the bottom bolts

• Ford 9-inch: Use a wrench to remove the bottom two bolts on the center section housing because a standard deep well socket will not fit over them.
• Ford 8-inch: A deep well socket can be used on the bottom two bolts because they are spaced farther apart. 

This video demonstrates the difference in the bottom bolts between a Ford 8-inch and 9-inch rear end: 15sHotRodGuyGarageYouTube · Sep 1, 2024
Other identification methods

• Housing shape:
  • 9-inch: After about 1961, the 9-inch housing case has a raised center section with flat spots on either side, as shown in this YouTube video. 
  • 8-inch: The 8-inch case is more rounded and does not have these flat spots. 
• Axle tag: Check the ID tag on the rear axle. It contains information about the gear ratio and rearend style, including an ID number that can be used for confirmation, as detailed in this HOT ROD article. 
• Ring gear diameter: Although more involved, the 9-inch has a larger, 9-inch diameter ring gear, while the 8-inch has a smaller, 8-inch diameter ring gear, according to JustAnswer. 

How do you identify a Ford 9 inch?

To identify a Ford 9-inch axle, look for a raised center section with two flat spots on the back of the housing, distinguishing it from the 8-inch's rounded case with two dimples. Other identifiers include checking for socket fit on the bottom bolts, identifying the casting number on the center section, noting the axle tube's appearance (early ones neck down, later ones don't), or finding the Bill of Materials (BOM) tag on the axle tube.
 
This video demonstrates the visual differences between Ford 8 and 9 inch housings: 57sAutorestomod Manic Mechanic Gasoline MediaYouTube · Sep 8, 2016
Visual and physical checks

• Back of the housing: A 9-inch housing has a raised center section with two flat spots, while an 8-inch housing is rounded with two dimples. 
• Bottom studs: Try to fit a socket on the two bottom studs just below the pinion. If a socket fits, it's an 8-inch housing; if it doesn't, it's a 9-inch housing. 
• Axle tubes: Early housings have a "neck-down" shape where the tubes narrow towards the flanges. Later housings maintain a consistent diameter all the way to the flanges. 
• Casting numbers: Look for specific casting numbers on the center section (often starting with "C" or "D") and on the pinion support. A "nodular iron" case, which is very strong, can often be identified by the "N" in its casting number or specific numbers like C2AW-A, C4AW-B, and D0OW-B. 
• Axle end markings: To determine if you have a 28 or 31-spline axle, inspect the center of the axle hub. A rectangular depression usually indicates 28 splines, while a small round depression with two smaller outer holes/depressions points to 31 splines. 

You can watch this video to see how to check for limited slip: 56sBlu-Grit GarageYouTube · Jun 19, 2024
Tag identification

• BOM tag: The tag on the axle tube has a Bill of Materials (BOM) number that can be used to identify the axle's specific details. 
• Ford tag: Some tags have a factory tag with a code like the one shown in the example (e.g., WDHM C1 / 3.70 5CC 302) which includes the gear ratio and other information. 
• Reading the tag: You may need to scrape the surface to reveal the numbers, as they can be faint. Wire-wheeling is not recommended as it can make them harder to read. 
• Online resources: Websites like FORDification.com have indexes of these codes to help with identification. 

How to measure 9 axles?

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How to measure Ford 9 axle shaft length?

To measure for Ford 9-inch axles, first determine the total housing width by clamping straight edges to the ends of the housing and measuring the distance between them. Then, measure the brake offset (also called axle offset), which is the distance from the face of the housing end to the face of the axle flange. Finally, measure the axle length itself by measuring from the inside of one housing stud to the inside of the corresponding housing stud on the opposite side. 
Before you begin

• Install the third member: It is crucial to have the third member installed and bolted in to get an accurate measurement. 
• Use a tape measure and straight edges: A tape measure and a straight edge or ruler are essential tools for accuracy. 
• Take measurements in a straight line: Avoid measuring at an angle or bending the tape measure, as this will lead to an incorrect, longer measurement. 

Key measurements

1. Total housing width:
   • Clamp a straight edge to the outside of each housing end. 
   • Measure the distance between the two straight edges. 
   • This measurement is the overall width of the rear end housing. 
2. Brake offset (Axle offset):
   • This is the distance from the face of the housing end to the face of the axle flange. 
   • Measure from the outside of the housing end flange to the surface where the drum or rotor mounts. 
   • Common values are 2.52.52.5 inches for a large bearing/Torino housing or 2.362.362.36 inches for a small bearing housing. 
3. Axle length:
   • You can get the length by using your total width and brake offset measurements: 
     • Total width / 2 = Half of the total width. 
     • Half of the total width - brake offset = Length of each axle. 
   • For example, a 58-inch wide housing with a 2.52.52.5-inch offset would have axle lengths of approximately 26.526.526.5 inches on each side (29−2.5=26.529 minus 2.5 equals 26.529−2.5=26.5). 
   • Another method is to measure the distance from the inside of one housing stud to the inside of the corresponding housing stud on the opposite side. 
   • It is best to subtract about 1/81 / 81/8 inch from the measurement to ensure the axle does not bottom out on the differential pin.