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1970 Ford Bronco - Swiss-Watch-Precision C4Westminister Transmission Service Rebuilds Ford's Venerable Auto Box From the August, 2005 issue of Custom Classic Trucks By D. Brian Smith
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Automatic transmissions are mysterious marvels of the Industrial Revolution. Every mechanical, hydraulic, electrical, vacuum-operated, and electronic component of a modern-day automatic transmission must function in perfect harmony to do its job properly. Like a Swiss watch that keeps perfect time, automatic transmissions are the precision instruments that effortlessly propel many of our custom vintage pickups. Recently, we heard of a friend's off-road misadventure. While playing in the mud with his son in their '70 Ford Bronco, Steve Shattuck lost the reverse gear in his Ford C4 Cruise-O-Matic automatic trans. We saw Steve's misfortune as a great opportunity to watch how the pros rebuild a C4. By doing so, we could convey the rebuild process and demystify these complex creations of the Industrial Age to CCT's readers via words and photos. Manufactured 1964 through 1987, Ford's C4 has powered everything from the puny Pinto up to the burly Broncos. Because C4s are compact, well-engineered, and reliable, they back many Ford-powered Ford custom vintage pickups today. Steve selected Westminster Transmission Service (WTS) in Westminster, California, to renew and improve the C4. Owned by Joe Piraino since 1987, Joe and his talented crew have rebuilt GM, Ford, and Chrysler automatic transmissions for street, dragstrip, race, and off-road applications. Indeed, Joe has been working on transmissions at this same shop since 1977. Follow along with us, as Joe and his son Michael, a Mechanical Engineering student at Cal Poly Pomona, rebuild a C4 for off-road fun in a 351W EFI-powered '70 Ford Bronco.  As they await assembly, cleaned,...  As they await assembly, cleaned, renewed, rebuilt, and new components and subassemblies of the Ford C4 Cruise-O-Matic almost look like the mechanisms comprising a fine Swiss watch.  Though the C4's cast aluminum...  Though the C4's cast aluminum case and torque-converter housing haven't been prettied up with paint or polish, remember this purpose-built trans will be going back in a '70 Ford Bronco that sees much street and mud fun. Find out how Westminster Transmission Service introduced additional reliability and performance to the proven trans by reading on.  One of the ways Westminster...  One of the ways Westminster Transmission Service beefed up the C4 was to replace the flexible bands with stouter, non-flexible bands. For improved clamping force, the Second-gear band on the right also has a higher coefficient of friction, as indicated by the red lining. The high-performance relined bands were procured from The Converter Shop in Chino, California.  From the Ford factory, Second-gear...  From the Ford factory, Second-gear servo pistons are 2.822 inches in diameter. In addition, they're cast aluminum. WTS replaced the Second-gear servo piston with...  ...a billet aluminum Second-gear...  ...a billet aluminum Second-gear servo piston from Sonnax Industries that is 3.052 inches in diameter. Since the servo piston is displacing hydraulic fluid, the more fluid pushed, the more hydraulic pressure. More hydraulic pressure means that more clamping force will be applied to the Second-gear band.  Joe points out where the Sonnax...  Joe points out where the Sonnax Second-gear piston goes in the transmission case.  With repeated use, the original...  With repeated use, the original cast aluminum Second-gear servo piston will wear, creating a gap or air/fluid leak between the transmission- case opening and the servo piston. Leaks breed inefficiency and eventual problems.  To prevent wear and leaks,...  To prevent wear and leaks, the Sonnax Industries billet-aluminum servo piston has a Teflon sleeve around the piston.  To improve clamping power...  To improve clamping power by 25 percent in high gear (drive or Third gear), WTS machined the pressure plate to make room for an additional high-gear clutch plate, bringing the clutch pack total from four to five.  With good reason, TransGo...  With good reason, TransGo Performance patented the term Shift Kit. WTS utilized TransGo Performance's Shift Kit to renew the C4's valve body (shown to the right of the Shift Kit installation manual).  By installing the heavy-duty...  By installing the heavy-duty pressure-regulator spring, included in the TransGo Shift Kit for rebuilding the valve body, mainline hydraulic pressure was increased by 15 pounds.  Since the stock three-pinion...  Since the stock three-pinion cast iron with steel pinions planetary gear was in great shape, WTS cleaned, inspected, and reemployed it.  Until mid-1972, vacuum modulators...  Until mid-1972, vacuum modulators were screw-in type, as shown on the left. The vacuum modulator on the right was used in C4s from mid-1972 through 1987. The push-in type is preferable, since the component has two diaphragms available for obtaining vacuum. Obviously, engines with radical cams work better with the push-in-style vacuum modulator. Quicker shifts result. The C4 in Steve's '70 Bronco is a late-'72 unit (thankfully).  With the worn oil-pump gears...  With the worn oil-pump gears in the background, Joe pointed out the worn surface on the C4's original oil pump that the gears rotated within.  From the factory, C4 oil pumps...  From the factory, C4 oil pumps have Babbit bushings, as shown on the left. WTS utilized a new Ford oil-pump housing and Clevite 77 Bronze bushings, as shown on the right. These Clevites are harder in composition, and therefore more durable.  As built from WTS, the oil...  As built from WTS, the oil pump is brand new, with gears from Transtar Industries, Clevite 77 Bronze bushings, and a new Ford oil pump body.  All the cleaning, refurbishing,...  All the cleaning, refurbishing, and replacing complete, WTS was ready to assemble the much-improved C4.  For assembly, Mike nested...  For assembly, Mike nested the forward clutch drum into the direct drum.  He next installed the front...  He next installed the front ring gear in the forward clutch drum.  So long as you don't start...  So long as you don't start singing "The hip bone's connected to the leg bone," we'll indicate that he installed the front planetary gear into the front ring gear.  Mike liberally applied Lucas...  Mike liberally applied Lucas Oil Transmission Fix (LOTF) to all metal surfaces. WTS uses LOTF as an assembly lube--it works really well.
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