Spin the clock back forty to fifty years ago, and you’d find that anything with a Ford blue oval on the side and a big honking engine under the hood would have come with only one rear end choice: a 9 inch. If that Ford had one of the nasty engines like a 427 or a 428 CJ then that 9-inch was filled with a 31-spline carrier and matching axles. Most ultimate power Fords also came equipped with a nodular iron center section (and most of those were marked with a big “N” cast directly over the pinion).
Ford center sections are manufactured with a separate bolt-in support for the pinion. Cars with nodular center sections were regularly fitted with what was called a “Daytona” pinion support. These supports make use of the same size outer bearing as the more pedestrian supports, however the inner bearing is much larger, and the inner webbing is much beefier. The majority of nodular iron, 31-spline muscle-Fords came equipped with Ford’s clutch pack equipped Traction Lok differential (which is a conventional limited slip arrangement). With the dawn of the 1970 model year, a positive locking (gear driven) differential was manufactured by a company called Detroit Automotive Products Corporation, and was made available in high horsepower cars with gear ratios of 4.30:1. These are the legendary “Detroit Lockers”, and we’ll dig deeper into the Locker in a later segment of the series.
Obviously, Ford did something good with the design and layout of the 9-inch. It’s pretty much the standard go-to rear end for drag racing. A big plus is the fact the center section can be removed easily and serviced on your workbench; that definitely beats wrestling with a complete housing underneath the car. It is possible to track down most of the stock hardware to piece together a factory-style nodular iron 31-spline 9-inch, complete with a Daytona pinion support and even a Detroit locker. The truth is, what you’ll find is old, used up hardware. Be prepared to drop some serious coin in order to whip the old stuff into shape. This is less than ideal and not race effective. What is good is the fact there are plenty of choices out there (in the racing aftermarket) for updated aftermarket brute force Ford center sections. The setups we’ll show you are head and shoulders above the original parts.
Several companies manufacture and sell nodular iron cases. In case you’re wondering, nodular iron is a type of cast iron that first saw the light of day in 1943. While most varieties of cast iron prove brittle, nodular iron is much more ductile, because of its “nodular graphite” inclusions. When you consider aftermarket cases, think about the availability of upgrades. Case-in-point: Mark Williams has a reinforced nodular iron case that is stronger than stock, but is comparable in weight to a stock Ford assembly. These 9-inch cases come complete with billet steel rear end caps that have been precision alignment bored. They also include special billet steel adjusters and studs to secure the pinion assembly. They’re available with 3.062-inch or 3.250-inch bore sizes (the larger the bore, the larger the axle diameter/spline you can use – the larger the axle diameter and spline, the stronger the axle).
For a light, small displacement, lower horsepower application, Williams offers a special “Light Weight” aluminum case. The case is based on their nodular iron assembly, and is cast with the same material as their high horsepower (capable) “Thru-Bolt” cases. Unlike the “Thru-Bolt case”, the Light Weight case does not have additional reinforcement and heavier walls (and obviously, doesn’t have though bolts – more on them below). The light weight case is 15 pounds lighter than an OEM Nodular iron case and is also 5 pounds lighter than the “Thru-Bolt” case. This setup can also be used on the street and on certain oval track applications, but it’s particularly compelling for something like a normally aspirated 4-cylinder or dragster application.
The “Thru-Bolt” aluminum case is manufactured by Mark Williams. However, this is no ordinary aluminum; this is a highly refined, extreme-duty component that has become the standard in virtually all NHRA Pro Stock cars. It’s also used with regularity in slower class drag race cars, “pro street” cars and any number of seriously quick street machines (the center section you see here is going into a strong LS-powered, small tire machine). It weighs 11-pounds less than Williams’ comparable nodular iron carrier or a stock Ford carrier. Cast from an ultra strong aircraft alloy (30% stronger than 6061T6), the case is engineered with special “Thru Bolts” that go completely through the center section to secure the main caps.
The actual main caps are machined from 7075 aluminum and include billet steel carrier adjusters. The pinion pilot-bearing bore incorporates an extra length bearing that is completely captive and retained by fasteners. Meanwhile, the pinion support is held in place by way of large diameter 7/16-inch studs. The case is manufactured in three different bore sizes (3.062-inch, 3.250-inch and 3.812-inch, although for anything short of an all-out drag car, the 3.812-inch piece isn’t necessary). All thru bolt case configurations have clearance for 9-1/2-inch gears (9-1/4-inch actual diameters – more on these down the road). Fluid passage ports for external lubrication systems are pre-drilled. Additionally, the case is setup so that you can add a load bolt if necessary. What’s a “load bolt”? Essentially, a load bolt is positioned in close proximity to the backside of the ring gear, but not in contact with it. It only makes contact with the ring gear if there is deflection under severe load. FYI, Load Bolts were originally used on heavy duty truck differentials. They prevented chipped teeth if the driver’s foot slipped off the clutch when backing up. Bottom line here is, it might be necessary in extreme power applications, but for us mere mortals, it’s not required.
Next issue, we’ll look at the carriers – spools, posi and lockers.
Source: Nine Lives
Mark Williams Enterprises
765 South Pierce Avenue
Louisville, Colorado 80027