Jim Smart
June 21, 2013

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

The simple C4 Cruise-O-Matic/SelectShift three-speed automatic can be rebuilt in your home garage using basic hand tools, though it is not recommended because it takes more than tools to do the job; it also requires knowledge and experience. Seasoned transmission technicians know specifics your Ford Shop Manual will not cover, such as what techniques work well in the real world.

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.

The C6

First introduced for ’66 as the C6 “Dual Range” Cruise-O-Matic, the C6 is a simple heavy-duty three-speed automatic transmission. There are four different case types for four engine families. Some cases are finned for strength. The C6 has Simpson planetaries and one friction band for the forward drum along with a low-reverse clutch package.

When Ford was developing the C4 light-duty three-speed automatic, powertrain engineers were also working on an FX/MX replacement for heavy-duty applications. Although the C6 is more heavy-duty than a C4, function is basically the same. Unlike the C4, a C6 bellhousing and main case are one large casting. Expect to see at least four different casting types for four different engine families. Castings across these engine types will vary a lot. Some cases will be finned while others will be smooth.

FE big-blocks: Round six-bolt bellhousing
385-series 429/460/351M/400: Angular six-bolt bellhousing with closed bottom
6.9L/7.3L Power Stroke Diesels: Angular six-bolt bellhousing with open bottom
302/351W/351C: Six-bolt small-block bell

Ford's C6 consists of a torque converter and front pump like the C4 with Simpson compound planetaries like a C4 controlled by a single friction band, three multi-disc clutches, a one-way roller clutch, and a low-reverse clutch pack. The C4's second friction band controls the low-reverse drum function. The C6 uses friction clutches splined into the case instead of a band and drum, which are more durable.

Like the C4, the C6 was a "Dual Range" green dot transmission prior to 1967. Because the Mustang was never fitted with a Dual Range C6, there isn't much point in discussing them at length here. However, if you're shopping for a core, it is wise to make sure you're not buying a Dual Range for your'69 428 Cobra Jet. The best means of identification are Ford casting numbers in the case and valve body. Anything prior to 1967 (C7OP or C7AP) is a Dual Range.

The C6 didn't change much over its long production life. Even after C6 transmissions were no longer available in new Ford vehicles, they were popular with heavy equipment vehicle manufacturers. Ultimately, the C6 was replaced with the E4OD, which was little more than C6 internals with overdrive in a new and rugged case. In fact, you can improve your C6 with E4OD planetaries, Torrington bearings, and hard parts for less friction and improved acceleration with components from TCI Automotive. When you build a C6 using close-ratio E4OD parts, you're building a more durable C6 offering improved acceleration and longer life.

The larger "R" intermediate servo or TCI 2.465-inch servo will increase pressure apply area by 25-30 percent for incredible drum gripping power. Important to remember is which intermediate servo apply lever to use. Use the "E" or "F" apply lever with the "R" size servos, never anything more aggressive. As with the C4, you want as many intermediate drum frictions and steels as possible. You also want the widest intermediate band available.

Because the C6 is a rugged transmission from the factory, very little needs to be done to make it stronger. For street and occasional strip, all the C6 needs is the larger intermediate servo and band. Where the C6 falls short is internal friction. When you upgrade to E4OD parts from TCI, you reduce friction with Torrington roller bearings. You also improve gear ratio.

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FMX

Ford’s venerable cast-iron FMX three-speed automatic was a ’69-’73 Mustang mainstay consisting of the FX main case and MX internals with a Ravigneaux compound planetary gear package, two friction bands, two clutch packs, and a one-way clutch. The FMX’s value is rugged dependability. It is not a performance transmission.

If you have a '69-'73 Mustang with the cast-iron FMX transmission, you have durability in a very rugged three-speed automatic box. However, the FMX is old transmission technology with little, if anything, available to improve performance and efficiency. At best, you can purchase clutches, bands, and some hard parts. The FMX was in no way a fresh design when Ford began installing it behind 351W and 351C engines. Prior to 1966, Ford used the MX Ford-O-Matic three-speed transmission in most heavy-duty applications. The FMX is basically MX components in the lighter-duty FX case. Like the C4 and C6, the FMX is hydromechnical with a vacuum control throttle valve and kick-down linkage. A tailshaft governor contributes to shift control.

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Vacuum Modulator

The C4, C6, and FMX transmissions employed throttle valves, or vacuum modulators, to control shift points. Based on intake manifold vacuum, throttle valves adjust transmission control pressure as you drive. In light throttle driving, manifold vacuum is high, which causes control pressure to be low for a soft upshift. By the same token, downshifts as you slow down are soft so you can barely feel them. At wide open throttle, vacuum is low and control pressure is high, causing a hard upshift and firm clutch/band engagement.

There are two basic types of throttle valves—screw-in with a gasket or press-in with an O-ring. C4, C6, and FMX had screw-in prior to '72. From '72-up is a press-in valve with a stamped steel clamp.

Some throttle valves are adjustable. For those that are adjustable, turn the adjustment screw clockwise to increase control pressure (firmer upshift) or counterclockwise to reduce control pressure (softer upshift). A firm upshift means longer clutch and band life. Make your adjustments carefully with one-quarter and one-half turns. One full turn increases or decreases control pressure by 2-3 psi. Ideally, you will have a pressure gauge connected to the transmission's pressure port for an accurate indication and proper adjustment. If you don't have a pressure gauge, try this. Make your adjustment clockwise one-quarter or one-half turn and take a drive. At wide-open throttle, observe upshifts. You want firm, but not hard. Firm means solid clutch and band engagement. Mushy upshifts mean burned clutches and bands if you don't make proper adjustments. Bone crushing upshifts mean too much control pressure and potential transmission failure. You want firm upshifts and shouldn't be able to feel downshifts coming to a stop.