The evolution of certain drivetrain components and how they earned a way into becoming the hot rodder's all-time standard for which other drivetrain components are judged is simple. The formula is what holds up the best, gives one the most bang for their buck, and what's the easiest to service. I can already hear the Ford guys crying and whining, but take the small-block Chevrolet engine for example. It was Chevrolet engines that proved their superiority dethroning the Offenhauser-powered cars at Indianapolis during the mid '60s, and what about Chevy's dominance winning at Le Mans in 1966 with two Bow Tie-powered cars tying for first place against Europe's best?
OK, so my facts are a little sloppy. It was Ford engines that achieved those milestones, but you guys get the idea that it's an evolutionary process which determines who the top dog will be. Moving to the rearend of a vehicle, we've got the differential, and although the proven winner is a Ford design, it's not a fact that makes the Chevy guys pee and moan. Irregardless of where it goes, the Ford 9-inch rearend introduced in 1957 is hands down the hot rodder's No. 1 choice to transfer power to the ground. As opposed to the third member being an integral unit as found on Chevy 12-bolt or Dana designs, the 9-inch Ford third member drops out easily to service, change gear ratios, or install a limited-slip unit.
In 2006 I spoke with Frank Currie about the venerable Ford 9-inch rearend and he told me that about the only thing one could do to hurt a 9-inch is run it without gear oil, and even that would take a while to occur. From there Frank and I talked about his early days back in 1959 when he struck out on his own building 9-inch Ford rearends to handle the needs of specialty builders seeking a bulletproof differential.
Frank's company started remanufacturing 9-inch rearends and grew exponentially with his product finding many niches, but in 1986 the Ford Motor Company ceased production of 9-inch rearends. Ford built millions of examples, so it took nearly two decades for Currie to exhaust its core pile, but the company knew it was time to manufacture a brand-new 9-inch differential that offered all of the upgrades that were learned during half a century of competition and street use. Beyond evolutionary upgrades, Currie incorporated better materials into its product such as ductile iron to replace the cheaper lighter-duty iron used by Ford.
In the near halfway point of the 21st century's second decade, Currie no longer mass remanufactures Ford produced 9-inch rearends, but as a special feature for CLASSIC TRUCKS' valued readers we have step-by-step coverage of what's involved. The subject differential is a 9-inch I pulled from a 1956 Ford F-100 rolling chassis I've had stashed for 10 years. I believe it's pretty typical of something one would find listed on Craigslist or eBay. Ultimately after almost $2,000 worth of repairs this rearend is headed for my Y-block-powered 1956 Ford big-window. The premise of this story is to help readers decide between rebuilding an existing unit or opting for new. We hope it's useful.
1. The teardown bench is where it's discovered if a used rearend is complete junk or will only require minor refreshing. This is a mid-1980s Ford E-150 narrowed to fit a 1953-1956 F-100 track width.
2. Buying a used 9-inch rearend is a crap shoot. Represented as rebuilt, this differential was done wrong. Notice wear pattern. When it was narrowed, the axle was cut too short, barely engaging the ring and pinion gears.
3. The third member internals are identified and inspected for condition: Richmond 3.5:1 gears, Ford Trac Lock limited-slip, and no damage from too short an axle. Although one good burnout would have twisted the axle and blown the rearend.
4. Currie's hot tank is dedicated to steam-cleaning differential housings. Parts or setup not included; Currie charges $219.95 to disassemble, hot tank, and assemble a 9-inch.
5. Slide-hammer pulling (knocking out) the bearing race. Previous damage from a bad bearing made doing this a difficult job.
6. An extremely important step and a tool few rearend rebuilders possess. This fixture (jig) is used to ensure the differential housing isn't bent. If bent, Currie straightens the end flanges.
7. My differential had brackets for non-floating disc brakes, a bad design that needed to be carefully removed (torched) from the housing. Currie offers custom installation of every bracket configuration imaginable.
8. After the high spots were meticulously ground smooth, the housing appeared as if the cheesy disc brake brackets were never there.
9. This gauge is used to confirm the axles are not too long or too short.
10. First-timers beware; removing the bearing retainers requires an extraordinary amount of effort. Here the bearing retainers are undermined with a torch.
11. Next, the axle assembly is placed into a multi-ton hydraulic press and pressed off with an explosive reaction. Ever heard a bullet zing, that's what it's like.
12. Here, Brian Sheppard is handing the work order to make my new axle. Currie manufactures its axles from scratch in-house for new assemblies or to make a replacement part.
13. The axle is cut to length and its end is beveled. A new axle sells for $179.95.
14. The outside diameter of the axle is centerless ground on a vintage Landis machine to conform with what's needed for a 28-spline axle.
15. The axle splines are cut on a CNC Haas mill. The mill meets with resistance due to the axle's case-hardened surface.
16. After hot-tanked and checked thoroughly for alignment, the differential housing is placed into this rotisserie shot peen blaster and final cleaned with fine metallic beads.
17. When the shot-peening process is complete, the differential housing appears as if it were new steel. No further preparation for paint is needed after this stage.
18. The Ford Trac Lock was disassembled and cleaned. Next the clutch plates were checked and didn't need replacement. A new clutch pack costs around $70.
19. Labor to rebuild the Trac Lock is around $110. The final step is to adjust the Trac Lock to a specific torque setting to ensure no slippage will occur.
20. Currie manufactures and has new hardware custom made to its specs. The pinion shaft seal has the Currie logo where one would usually find Chicago Rawhide molded.
21. A setup kit with new carrier bearings ($136.95) is installed and the backlash is adjusted. All the way through this procedure the gears are spun to feel for bind.
22. Currie uses Precision brand gears exclusively. These are Richmond gears. The white paste is gear-marking compound used to give an indication of tooth contact. Improper gear lash can result in the rearend “howling.”
23. Absolutely no heavy lifting is performed by Currie technicians. From start to finish an overhead boom is used to gently and precisely lower components such as this third member into place.
24. For $495 Currie offers 11-inch Ford drum brakes that are new, except for remanufactured backing plates. The axles go into place and four bolts are tightened through an access hole on the axle flange.
25. New drums go on and the rearend is complete. Currie emphatically states to use a non-synthetic gear oil in its differentials such as Currie 9-Plus racing gear oil. For limited-slip add Currie 9-Plus posi-traction additive.