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.