Jim Smart
February 29, 2012

You probably see automatic transmissions as a convoluted maze of gears, clutches, bands, and complicated hydraulics. However, it may surprise you to know that most automatic transmissions work via a very simple principle known as hydromechanics—planetary gear sets, multi-disc clutch packs, bands and drums, and a hydraulic control system.

Most transmission builders agree that Ford's C4, C6, and FMX transmissions are not only rugged and dependable, they're also simple to identify, rebuild, and service. Our goal here isn't to show you how to rebuild these vintage Ford automatics. Instead, we want to provide you with information about how to identify them and understand how they work.

Because the C4 and C6 were developed at the same time, they're very similar; the only real differences are size and low-reverse function. The FMX is more old-school with a Ravineaux twin planetary system instead of the independent Simpson-type common to C4 and C6. The C4 has a low-reverse band as does the FMX. The C6 has low-reverse clutches instead of a band and drum.

The C4

The C4 Cruise-O-Matic, assembled at Ford's Sharonville, Ohio, transmission plant, entered service in 1964 as a Ford designed and built three-speed automatic to replace the dated MX and FX Ford-O-Matics. Early on, the C4 was known as the "Dual Range" Cruise-O-Matic, also called the "Green Dot" due to its rather unconventional dual shift pattern. As owners of '65-'66 automatic Mustangs can attest, the shifter had a large "green dot" indicator in the second position for normal 1-2-3 shifts, along with a smaller dot indicator in the first position for starting out in second gear on snow and ice.

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For '67, the C4 became the "Select-Shift" with a conventional P-R-N-D-2-1 shift pattern and a new valve body. This is the way the C4 remained until production ended in 1982. At that point, Ford began production of the C5 Select-Shift, which was little more than a C4 with locking torque converter and revised hydraulics for fuel efficiency. Because the C5 has a wider bellhousing to accommodate a locking torque converter, it will not fit in the tunnel of your classic Mustang.

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Over the years, there were important changes to the C4. From '64-'69, the only change of note was the '64-'66 "Dual Range" valve body, which is different than '67-'69. If you want to upgrade your Dual Range C4 to the conventional '67-'69 pattern, all you have to do is swap the valve body. For '70, Ford changed the C4's main case and valve body to an eight-bolt pattern.

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The C6

Ford introduced the C6 for '66 to replace the outdated Borg-Warner MX cast-iron transmission, which was never available in the Mustang. An all-new design, the lightweight C6 employed a lot of the same features as the C4. The C6 remained in production through '96 because it was used in a variety of non-car and truck applications.

To improve a C6's durability, go with a wider intermediate band and "R" servo (428 Cobra Jet) for a more solid hook-up during 1-2 upshift. Outside of the intermediate band issues just mentioned, the C6 was engineered for durability from the get-go. It is a fiercely dependable transmission.

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The '66 C6 valve body is a standalone with the "Dual Range" Green Dot/Small Dot feature. Don't make the mistake of picking up a "Dual Range" valve body for your C6. Shifter detent is another issue to watch for. Does your C6 have a valve body detent or transmission case detent? And finally, is your throttle valve (vacuum modulator) screw-in (before '72) or press-in ('72-up)?

From '64-'69, C4 transmissions had a .788-inch 24/24-spline input shaft and forward clutch hub. In '70 only, Ford went to a .839-inch 26/26-spline input shaft and forward clutch hub. Another change came in '71 with a .839-inch 26/24-spline input shaft, which was used through the end of production. If you're running a lot of power, the 26/26 is your best option, available from TCI Automotive.

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The FMX

The FMX is a standalone cast-iron three-speed automatic that was in production in various forms beginning in 1950. There was the large-case MX Ford-O-Matic and the small-case FX transmissions prior to '66. When Ford introduced the C6 in '66, these old ironsides were dropped in favor of the newer, lighter C4 and C6 transmissions.

When C4 and C6 production shortfalls created challenges in the late 1960s, Ford turned to the tried and proven FMX with beefy MX internals inside the smaller FX case for the 351 engines that arrived in '69. It was a rugged and dependable three-speed automatic. Some companies, such as Trans Go, still offer rebuild and performance kits for the FMX today.

What makes the FMX different than the C4 and C6 is its Ravineaux twin-planetary gearset design, which was also employed in the later AOD, AODE, and 4R70W automatic overdrives. The Ravineaux planetary design consisted of fore and aft planetary gear sets in a single package, which is what makes it different than the separate Simpson planetaries used in the C4 and C6. Aside from this difference, the FMX operates much the same way as a C4. Band adjustment is similar, with a setscrew and locknut like C4 and C6. FMX intermediate and low-reverse servos are located inside the case.

Throttle Valve/Vacuum Modulator

The C4, C6, and FMX operate off the same basic principle of throttle valve operation. The throttle valve, also known as the vacuum modulator, gets its signal from intake manifold vacuum, which is controlled by engine load and throttle position. With the accelerator pressed, manifold vacuum is low. With your foot off the throttle, manifold vacuum becomes high. Low manifold vacuum (open throttle) causes the throttle valve to modulate high transmission control pressure. High manifold vacuum (closed throttle) causes low control pressure.

Pressing the accelerator pedal causes the engine to rev and manifold vacuum to drop. This is when the transmission's clutches and bands need high application pressure for better power transfer. Nailing the accelerator does two things in your transmission—high control pressure and delayed upshift. Let up on the accelerator and manifold vacuum increases, which causes reduced control pressure and a smoother, more immediate upshift. By the same token, when you're slowing down, you want reduced control pressure, which causes smoother downshifts.

As a rule, throttle valves are factory adjusted and don't require adjustment. In fact, original factory-installed throttle valves cannot be adjusted. However, service replacements are adjustable. Just follow your Ford Shop Manual instructions. One full turn clockwise increases control pressure 2-3 psi. One full turn counterclockwise does the opposite. Make your adjustments in baby steps (quarter and half turns) and take a test drive.

Think of your transmission's hydraulic control system like a finely tuned watch. We want aggressive upshifts during wide-open throttle, yet smooth and unnoticed during deceleration to a stop. Throttle valve operation works hand in hand with the governor and kick-down system to get shift points close to where they need to be. It takes patience and a feel for shift points when you're adjusting the throttle valve. We suggest the use of a pressure gauge for best results.

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C4 vs. C6 vs. FMX

C4
Three castings: bellhousing, main case, tailshaft housing
Two bands: intermediate and low-reverse
Dual Range "Green Dot" Cruise-O-Matic for '64-1/2-'66 Mustang
Select-Shift "P-R-N-D-2-1" for '67-'82
Five-bolt V-8 bellhousing for '64-1/2 Mustang
Six-bolt V-8 bellhousing from '65-'82
Screw-in vacuum modulator for '64-1/2-'71
Press-in vacuum modulator with bracket and O-ring, '72-'82
Vent tube in main case through early '70s
Mushroom vent in tailshaft housing from early '70s-up
157-tooth and 148-tooth flex plate with stepped case (case fill)
164-tooth flex plate with blended case (pan fill)—does not fit Mustang
Nine-bolt valve body and main case, '64-1/2-'69
Eight-bolt valve body and main case, '70-'82
24/24-spline input shaft and forward clutch, '64-1/2-'69
26/26-spline input shaft and forward clutch, '70 only
26/24-spline input shaft and forward clutch, '71-'82
"C" and "H" intermediate servos are the largest, with "H" being the largest and most plentiful
Many casting variations depending upon application and year

C6
Two castings: bell/main case and tailshaft housing
One band (Intermediate), one clutch pack (Low-Reverse)
Dual Range "Green Dot" Cruise-O-Matic in '66 only
Select-Shift "P-R-N-D-2-1" for '67-'96
Four bellhousing patterns: six-bolt small-block, rounded six-bolt for FE big-block, six-bolt for 385/M-series engines, and six-bolt for diesel with bottom dust cover
Screw-in vacuum modulator, '66-'71
Press-in vacuum modulator, '72-'96
Mushroom vent in main case
"R" intermediate servo is the largest and most desirable
Many casting variations depending upon application and year. Some are smooth while others are finned

FMX
Cast-iron main case and tailshaft housing with aluminum bellhousing
Borg-Warner designed and manufactured from '68-79 for 351W and 351C engines. Used in Mustang from '69-'73
Used the heavy-duty MX gear train in the smaller FX case for durability
Ravineaux planetary gearset (a single, twin planetary unit), which was continued in the Automatic Overdrive (AOD) introduced in '80

C6 Quick Identification

390/428 FE: Round seven-bolt bellhousing with three-bolt starter boss; smooth or finned back
429/460 and 351M/400M: Large six-bolt spread with two-bolt starter boss
Diesel V-8: Same as 429/460 except opening at bottom for torque converter access. Do not use with 429/460/351M/400M
351W/351C: Six-bolt small-block pattern. Will also fit 289/302.

Stop Leaks

Old automatics don't have to be leakers. Leon's Transmission in Reseda, California, suggests a thin film of Permatex Form-A-Gasket on the pan gasket as well as the outer seal perimeter to prevent leaks. Check all contact surfaces for scratches and nicks, which will cause leaks. Make sure seals have lip springs once installation is complete. Lip springs often pop out unnoticed, which will cause leakage.
Band adjustment locknuts should be replaced at every adjustment. Manual shift shaft seals are also potential leak sources. Examine shaft surfaces for scoring during a rebuild. Resurface the shaft or replace if scoring is found.

The Fluid

You don't have to use Type F fluid in your vintage Ford automatic transmission anymore unless it is already serviced with Type F. If you're pressing a freshly rebuilt C4, C6, or FMX transmission into service, you are free to use Dexron III or Mercon IV fluid. Type F was originally specified due to its friction enhancers and seal compatibility, but this rule doesn't apply with rebuilt transmissions with new seals and frictions.
During C4, C6, or FMX installation, fill the torque converter with a minimum of one quart of transmission fluid to both prime the pump and ensure lubrication on start-up. Allow the engine to idle for a minute prior to placing the selector in gear. Run the shifter through all gears with the brake applied to bleed the hydraulic control system. Check the fluid level hot.

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