Understand Automatic Transmissions, Mustang C4, C6, and FMX
The ins and outs of the simple transmissions
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 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.
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.
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.
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.
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.