How many splines does a Ford 9-inch rear end have?
There isn’t a single fixed number. The Ford 9-inch rear end typically uses either 28-spline or 31-spline axles, with higher-spline options available from the aftermarket for high-horsepower builds.
Factory spline counts
The two most common spline counts you’ll encounter on factory Ford 9-inch rear ends are 28-spline and 31-spline axles. This distinction matters for compatibility with axles, lockers, and related hardware.
- 28-spline axles: Found on older factory configurations and some entry-level builds.
- 31-spline axles: Became more common in later production and are standard in many performance-oriented setups.
Knowing which count you have helps determine compatible parts and potential upgrade paths for strength and reliability.
Aftermarket and high-horsepower options
For builds that exceed typical street power, aftermarket 9-inch axles are offered in higher spline counts to increase strength and reduce the risk of axle failure. Availability varies by vendor and intended use.
- 33-spline axles: A common mid-range upgrade for higher durability.
- 35-spline axles: Popular in high-horsepower drag race and heavy-launch setups.
- 40-spline and other counts: Available from specialized manufacturers for extreme power levels, often paired with custom carriers and hardware.
When upgrading, verify compatibility with your carrier, hubs, and wheel studs, as not all components fit every spline count.
How to determine your spline count
What a spline is and why it matters
Splines are the ridges on the end of the axle shaft that engage the hub and carrier. The number of splines determines what axles and hardware will fit your rear end. The count can vary by production year, model option, and aftermarket modifications.
To identify your spline count, you can:
- Inspect the visible end of the axle shaft and count the ridges from the shoulder to the end.
- Look for stamping or part numbers on the axle or axle housing that indicate the spline count or model.
- Consult the vehicle's factory service manual or build sheet for the original axle specification.
Accurate identification ensures you source compatible axles, lockers, and upgrade components.
Summary
The Ford 9-inch rear end does not have a universal spline count. The most common factory configurations are 28-spline and 31-spline axles, with higher-spline options available in the aftermarket for enthusiasts pursuing greater strength. Always verify your exact spline count by counting splines on the axle shaft or checking official documentation, especially before purchasing upgrades or replacements.
Are all Ford 9-inch rear ends the same?
We should point out there are three distinct 9-inch axle ends: the late big bearing (Torino) and the early big bearing both take a 3.150-inch od wheel bearing (the difference is in the bolt spacing for the backing plates), the small bearing ends take a 2.834 od bearing with yet a different backing plate bolt pattern.
Is 28 or 31 spline axles better?
31 spline axles which are about 30% stronger than the 28 spline axles making them a popular aftermarket upgrade for Fox, SN95, and New Edge Mustang owners.
How do you tell if you have 28 or 31 spline axles?
Measure the axle housing if it's a 2.5 inch diameter it's a 28 spline axle if it's a 31 spline it a 3.25 inch diameter.
What spline is a Ford 9-inch?
Allstar Ford 9in Axle 31 Spline 29-1/2in ALL67500
| Alternate Description | FORD 9IN AXLE 31 SPLINE 29-1/2IN |
|---|---|
| Inside Spline | 31 |
| Outside Diameter | 5.94 in |
| Spline Length | 10 in |
| Gun Drilled | Yes |
