Mustang MonthlyHow To Drivetrain
How To: Understand and Identify Automatic Transmissions
Vintage Mustangs used a variety of C4, C6, and FMX automatic transmissions
Few of us understand how automatic transmissions work nor do we know what it means when they suddenly don't. We slip the selector into Drive, feel the tug of torque, press the accelerator, and get on our way if all works as it should. We're perplexed when that doesn't happen.
Automatic transmissions, be they vintage C4s or more contemporary Automatic Overdrives (AOD), work basically the same way via carefully programmed and routed hydraulic pressure, which activates a series of friction clutches and bands to channel engine power through planetary gears and shafts. When all of the upshifting is over, power travels straight through the torque converter and transmission's input and output shafts to the rear axle in a 1-to-1 ratio. Automatic Overdrives shift one more time to achieve overdrive gear reduction void of torque converter involvement to lower engine rpm for reduced wear and improved fuel economy.
A variety of automatic transmissions have been employed in the Mustang's 49-year history, including late-model AODs, but we're more concerned here with the vintage automatics. The C4 Cruise-O-Matic ('65-'66) and Select Shift ('67-up) was a light-duty three-speed engineered for six-cylinder and small V-8 engines. The C6 heavy-duty three-speed automatic was developed for big-block V-8s and medium-duty 351W and 351C V-8s. The FMX was an old-school Borg-Warner cast-iron three-speed automatic used with the 351W and 351C engines when there weren't enough C6 transmissions to meet production demands, which is why some Mustangs have FMXs and others have C6s. There was also a C3, a light-duty, French-produced three-speed used in the Mustang II. It looked similar to the C4.
C4 Cruise-O-Matic & Select Shift
Prior to 1964, Ford fitted most of its vehicles with Borg-Warner designed two- and three-speed automatic transmissions. The C4 was Ford's first company designed and built automatic transmission assembled at Ford's Sharonville, Ohio, transmission plant. The C4 is so simple it can be rebuilt in your home garage using basic hand tools. Where the C4 gets tricky is knowledge. It is one thing to knock a transmission apart and quite another to assemble one that functions the way it should.
Though the C4 is a light-duty three-speed automatic transmission, it is rugged and can be built to withstand more than 1,000 horsepower using aftermarket parts. A C4 can be built for many years of reliable operation when you use the best combination of available parts. And when the C4 does require service, removal and overhaul are easy so you're back on the road quickly. The C4's architecture begins with a conventional torque converter and engine-driven front pump leading to a Simpson compound planetary gear train controlled by two friction bands, two multi-disc clutches, and a one-way roller clutch. It functions via a hydraulic control system that gets its pressure from a cast iron gear pump fitted into the front of the case. The hydraulic control system is calibrated for vehicle and engine type where shift points happen based on throttle position and load. Shift points are determined by both a vacuum-controlled throttle valve and kick-down linkage coupled with a flyweight tailshaft governor.
What makes the C4 confusing is engineering changes over its '64-'82 production life. An extra added point of confusion is the C5 produced in the 1980s, which was a C4 with a locking torque converter. The C5 has a C4 main case with improved hydraulic system circuitry and a different valve body. It has a wider bellhousing which will not clear the tunnel in a Mustang. However, the C5's main case is interchangeable.
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Running changes in the C4 include:
'64-'66 Dual Range "Green Dot" Cruise-O-Matic. Small dot is second gear start-out for driving on snow and ice. Valve body and shift unique to '64-'66.
'67-up is the P-R-N-D-2-1 pattern which means different valve body and shifter.
'64-'69 has 24/24-spline .788-inch input shaft and forward clutch.
'70 only has 26/26-spline .839-inch input shaft and forward clutch.
'71-up has 26/24-spline .839-inch input shaft and forward clutch.
'70-up has different main case and valve body bolt pattern—nine-bolt versus eight-bolt.
Case-fill versus pan-fill—Mustang is case-fill only.
Case-fill uses a stepped bellhousing.
Mustang II gets a case-fill with unique bellhousing and 148-tooth flexplate.
Unless you're concerned with originality and matching numbers, opt for a '71-up C4 core for your project, which is the best evolution of the C4 with compatibility through '82. One other consideration is the five-bolt bellhousing common with 260 and 289 prior to the '65 model year. If you're working with a '64½ with the original 260/289, you will need the five-bolt bellhousing.
To build a rugged, reliable C4, you want the best friction materials—clutches and bands—and the largest intermediate servo available. The Ford "C" intermediate servo used with the 289 High Performance V-8 is popular. However, the "H" servo for full-size Fords offers the same apply area and there are millions of them out there. Scott Drake reproduces the Hi-Po "C" servo. B&M, TCI Automotive, and Performance Automatic all offer high-performance intermediate servo and band kits for the C4. Opt for the widest intermediate band available for durability. You want as many forward clutch frictions and steels as you can get in the drum while still maintaining proper clearances.