How To Build A Better C4 Transmission
Dial Reliability And Performance Into Your Classic Mustang's Automatic Transmission
Ford's C4 Cruise-O-Matic transmission is the most reliable automatic Ford has ever produced. It is simple by design. In fact, it is so simple you can tear it down and rebuild it in your home garage using the Ford Shop Manual because no special tools are required. However, transmission building is a finite science, so we suggest turning yours over to a professional with experience.
We visited Ron Hazelton at Transmission Rebuilding Company (TRC) in Chatsworth, California, which has been in business since 1974. Suffice it to say Ron knows a thing or two about automatic transmissions and has the challenge of keeping up with the latest technology from Detroit, Europe, and Asia.
For this article, we asked Ron to rebuild Gary Schweitzer's C4 Dual-Range Cruise-O-Matic transmission from 5F08T383386, one of 12 convertibles built for the 1965 portion of the New York World's Fair Magic Skyway. Because Gary's transmission had not been operational since 1978, it suffered from the elements in storage. Internally, it was perfect from just 35,000 miles of use.
Jaime Sanchez, a certified transmission tech at TRC, unloaded our C4 and placed it on his bench. Gary's C4 was typical, even with its low mileage. Burned, rancid fluid poured onto the bench, which isn't always as bad as it may seem despite its nasty smell and color. Discolored automatic transmission fluid (brown to black in color) has been hot, laced with friction material from clutches and bands. This means Gary's C4 did its share of slipping in 35,000 miles, burning clutches and bands as it made its way through clutch packs and planetaries. Fortunately, Gary's hard parts survived nicely, ready for a return to service with little more than inspection and resurfacing.
As we knocked down Gary's C4, the wonderful simplicity of this transmission rang true. It is a simple, old-fashioned, three-speed automatic sporting an interchangeable bolt-on aluminum bellhousing, removable tailshaft housing, and main case. The main case didn't change much over this transmission's 22-year production life (including C5 production from '82-'86). Bellhousings and tailshaft housings mainly depended on vehicle application, with Mustangs employing the majority of C4s built.
'64-'66 Known as the C4 Dual-Range Cruise-O-Matic with a very specific transmission selector with a small dot and a large dot (hence the name "Dual-Range"). There has always been a lot of confusion about which dot to use. Use the large dot for normal driving. The small dot keeps it in second gear for driving on icy roads or when a gentle start is required. This version has a 24-spline input shaft (.788-inch) and 24-spline output shaft.
'67-'69 Select-Shift with standard PRND21 selector pattern. Has a 24-spline input shaft (.788-inch) and 24-spline output shaft.
'70 Only Select-Shift with PRND21 pattern with 26-spline input shaft (.839-inch) and 26-spline output shaft. If you're going with the larger 26-spline input shaft in an earlier C4 (prior to 1970), you will need to change the reverse/high clutch drum (forward most clutch), forward clutch cylinder, #3 thrust washer, and front pump.
'71-'81 Select-Shift with PRND21 pattern with 26-spline input shaft (.839-inch) and back to 24-spline output shaft.
'82-'86 Select-Shift C5 is an improved C4 with locking torque converter for better fuel economy. The C5 case is easily identified by casting numbers that begin D2, D3, D4, D5, and D6. The C5 was replaced by the Automatic Overdrive (AOD).
There were two bellhousing sizes for V-8 C4 transmissions - 157-tooth flex plate and 164-tooth flex plate.
There was also a pan-fill C4 designed for trucks with a 164-tooth flex plate. Stay away from these for Mustang applications.
Pinto ('71-'80) and Mustang II ('74-'78) got their own C4 with a smaller bellhousing and 143-tooth flex plate to clear smaller transmission tunnels. They also had a unique valve body.