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.
1 This is the C4 three-speed...
1 This is the C4 three-speed automatic known as a "Dual-Range" Cruise-O-Matic from '64-'66 and Select-Shift from '67-'82. In '64-1/2 Mustangs, C4s had a five-bolt V-8 bellhousing. From model year '65 and later, it had a six-bolt V-8 bellhousing.
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.
2 This cutaway shows the...
2 This cutaway shows the C4's internals with its intermediate (front) and low-reverse (rear) bands, two multi-disc clutches, and two planetary sets. The C4 is a simple automatic transmission with separate bolt-on bellhousing for six and small V-8 applications. There’s also a big-block C4 for 385-series and 351/400M engines.
3 C4 bellhousings differ...
3 C4 bellhousings differ quite a bit. Shown here are the mid-'65-'82 six-bolt V-8 bellhousing with two-bolt starter (left) and the smaller six-cylinder bellhousing with a three-bolt starter (right).
There's also a big-block bellhousing...
There's also a big-block bellhousing (not shown) that's extremely rare for the 429/460 and 351/400M.
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.
4 Shown here are two types...
4 Shown here are two types of 28-ounce offset balance flex plates for C4 applications—157-tooth (left) and 164-tooth (right). There's also a smaller 148-tooth flex plate for '74-'78 Mustang II and '71-'80 Pinto. If you're running a '82-up 5.0L engine with a C4, you're going to need the 157-tooth with a 50-ounce offset balance.
5 C4 identification is simple...
5 C4 identification is simple and easy. This is the stepped-case/case-fill C4 for 157-tooth and 148-tooth flex plates. The 148-tooth flex plate and bellhousing were designed for the smaller Mustang II. The stepped-case six-bolt bellhousing bolts to the front pump.
6 This is the blended-bell/pan-fill...
6 This is the blended-bell/pan-fill C4 for the 164-tooth flex plate and bellhousing. The blended bell was developed for full-size and truck applications and will not fit your compact or intermediate Ford. The pan-fill dipstick tube will not clear the frame rail. Blended seven-bolt bellhousings bolt to the main case instead of the front pump.
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.
7 Identification comes from...
7 Identification comes from casting and part numbers. This C4AP indicates a first year main case casting, which will work with nine-bolt valve bodies from '67-'69.
8 Power flows through a C4...
8 Power flows through a C4 transmission via the input shaft, which is splined into the forward clutch and torque converter turbine to drive the clutch and gear train package.
9 What makes the C4 and FMX...
9 What makes the C4 and FMX different than a C6 is how power gets from the input to the output shaft. Both the C4 and FMX have an intermediate band (left) and low-reverse band (right). C6 transmissions use clutches for the low-reverse function instead of a band, hence only one band in 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.
10 This is the governor,...
10 This is the governor, a two-valve assembly mounted on the tailshaft, which operates based on centrifugal force and vehicle speed. At 10 mph, the governor begins to operate with activation of the primary valve, which opens first. From 10 mph up, the governor contributes to shift points based on vehicle speed (secondary valve). Shift points depend on secondary valve positioning and vehicle speed. Governor calibration is based on vehicle application.
11 C4 input shafts and forward...
11 C4 input shafts and forward clutch hubs changed three times. From '64-'69, a .788-inch 24/24-spline input shaft and forward clutch hub was used. In '70, Ford used a .839-inch 26/26-spline input shaft and forward clutch hub. From '71-'82, a .839-inch 26/24-spline input shaft and forward clutch hub was used. The 26/24-spline shaft is 26-spline at the torque converter and 24-spline at the forward clutch hub. The shaft on the right looks more like a C5 input shaft with a hydraulic passage for a locking torque converter. Keep this in mind when you are shopping for an input shaft. You want a C4 input shaft, not C5.
12 The C4's intermediate...
12 The C4's intermediate band works best when it’s the widest you can find. Although servo size is important, band and drum width are more important.
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.
13 There's lots of chatter...
13 There's lots of chatter about intermediate servo size with the C4 but here's the real deal. What you want is the largest servo size available—the "H" servo common to full-size Fords and Mercurys. There are plenty of "H" servos out there. Although the Hi-Po "C" servo gets a lot of attention, it isn't the largest servo, nor does it come cheap when you find one. However, it is available as a reproduction. A large intermediate servo, coupled with the widest band width, is what you want for your C4.
14 Choose a C4 valve body...
14 Choose a C4 valve body carefully, because it gets involved. There's the C4 "Dual Range" valve body—that's not what you want unless you're restoring a '64-1/2-'66 Mustang.
There's also the '67-'69 nine-bolt...
There's also the '67-'69 nine-bolt valve body and main case, and the eight-bolt valve body and case from '70-up. And it gets more finite, such as the Pinto valve body, which is not what you want for your Mustang. Watch for details, including casting numbers and an internal valve body detent or external, which locks the selector in each gear range.
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.
15 There are two types of...
15 There are two types of C4 venting--main case with a vent tube or tailshaft housing mushroom vent. If you have a main case vent, shown here, you don’t want the tailshaft mushroom vent.
16 If you have an unvented...
16 If you have an unvented main case, you want the mushroom vent tailshaft (left).
17 This is a B&M small-block...
17 This is a B&M small-block C6 automatic designed for 351W and 351C engines. Although it will bolt up to any 289/302/351Wci engine with a six-bolt bellhousing, this is not the transmission you want because it is too heavy and inefficient for a small-block. Where the C6 differs from the C4 is its one-piece design with the bellhousing and main case cast together. There are four C6 bellhousing bolt patterns: six-bolt small-block, round FE bolt pattern, 385-series and 351/400M pattern, and the diesel bolt pattern, which is identical to 385/M-series except for torque converter opening at six o'clock with a dust cover.
18 A look inside the C6 shows...
18 A look inside the C6 shows intermediate band (right) with a low-reverse clutch package (left).
19 Here, Brian Fortune of...
19 Here, Brian Fortune of Tom's Transmissions loads low-reverse clutches into a C6. Clutch steels are tied to the case. The clutch hub is tied to the friction discs.
20 Intermediate servo selection...
20 Intermediate servo selection for the C6 is simple—opt for the widest band and drum. Use the "R" servo/piston from the 428 Cobra Jet.
21 What makes the FMX different...
21 What makes the FMX different is its Ravineaux planetary gear set incorporating fore and aft planetaries. Otherwise, operation is basically the same as C4 with two bands and clutch drums.
22 Here's a typical FMX transmission...
22 Here's a typical FMX transmission with cast-iron main case and tailshaft housing with an aluminum bellhousing. The FMX came with both the 351W and 351C in ’69-’73 Mustangs.
23 This mechanic's eye view...
23 This mechanic's eye view of the FMX shows its wide, deep pan and cast-iron case. The FMX's gear train continued in the '80-up AOD, AODE, and 4R70W.
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
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.
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.
This is the C6 case for 390...
This is the C6 case for 390 and 428 with a round seven-bolt bellhousing. The smooth case is early and the finned case later.
Here's the 429/460 fat-block...
Here's the 429/460 fat-block case with a large six-bolt bellhousing. Also fits 351M and 400M Clevelands. Diesel is same pattern with bottom cutout and dust cover.