Jim Smart
February 29, 2012

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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