The venerable 9 inch Ford differential has had a ton written about it over the years. Designed in the mid 1950’s Ford used these in a loads of applications from passenger cars to trucks and high performance muscle cars. The web is loaded with resources detailing the cast and nodular OE style diffs and so we’re going to ignore those here and instead focus on the differences between the typical 9 inch OE style builds and the massively stronger 10″ assemblies that are the choice for cars and 4×4’s requiring vastly more strength than a stock type Ford diff would ever tolerate.
So what’s the difference between a regular 9 inch gear and a “Pro Gear.”
To begin with Pro Gears are typically made from a softer 9310 grade alloy that tolerates shock load better than typical 8620 material used in more standard applications. The drawback with the softer material is greatly increased wear and one of the reasons 9310 Pro Gears are not recommended for street use. They simply do not last long in high mileage situations.
Size- Pro gears for Ford style housings come in 9″, 9.5″ and 10″ diameters. The big difference is the diameter of the pinion shaft and spline count. Normal 9 inch gears use a smaller 28 spline pinion shaft with the progears using a 1.5″ 35 spline pinion shaft and correspondingly larger bearings. Take a look at the images below to check out the differences.
Note the much smaller diameter pinion shaft on the OE style 9 inch gear and greatly reduced tooth size compared with the 10″ pro gear on the right.
Pinion bearing differences. On the left is a daytona style pinion support for the 35 spline pro gear ring and pinion and on the right a daytona pinion support for the smaller 28 spline ring and pinion. The much larger bearings reduce flex further on the pro gear setup.
Carrier bearing differences. Typical “big bearing” Ford style housings use a 3.250 OD carrier bearing as can be seen on the right. These are ideally suited to a maximum of a 35 spline axleshaft as the spool (or differential) bearing journal thickness is compromised with larger axleshafts. 3.812″ OD bearings are recommended for use with 40 spline axles in extreme performance applications. Note the much larger ID of the adjuster nut on the left side 3.812″ OD bearing model shown above.
Why the bearing diameter matters? Above are both 40 spline spools. The top one is built for use in a 3.812″ bearing case. The bottom one is designed to work in a 3.250″ case. The result of the 40 spline spool in the smaller bearing case is a very thin journal that is much more likely to fail under extreme loads.
The combined parts matter. When we build a complete Nitro Pro billet assembly for the Street Beast we use the best parts we have. That starts with the Nitro 6061 T6 Billet case machined from a single piece of alloy and engineered to support the pinion gear at a level never before seen. The ribbing structure greatly reduces flex where the pinion support meets the case and the 10 high strength studs make sure the components stay together under even the biggest hit at the track. Further, designed with a load bolt to support the back of the ring gear these case assemblies reduce gear wear, failure and increase overall capacity of the finished assembly. Add in the high strength thick journal 40 spline spool, quality bearings, solid pinion spacer, forged yoke and a fully “dressed” 10 inch pro gear and these units simply do not have a weak link.
Most people don’t need a monster 40 spline billet drop out, but many of the features found in these ultra high end assemblies also exist in our “regular” 3.250″ billet cases and even the budget friendly Nitro nodular units. Race it Sunday and sell it Monday is as true today as it’s ever been and pushes us to innovate for our customers whether for a Pro Mod or your hopped daily driver.
For more info on 9 inch units please visit http://www.Nitro-Gear.com or call 877-463-1054
Here’s a link to a video assembling one of these with a lot of discussion about 9 inch in general.
NITRO GEAR & AXLE