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Many ’60s and ’70s Ford owners can benefit from the use of the C-6 automatic transmission. The C-6 is a very strong transmission and was originally used in many V-8 Ford passenger cars, as well as many pickup truck applications. The C-6 is also a good transplant candidate for non-Ford applications. Aftermarket suppliers offer C-6 conversion parts that will allow you to install this transmission behind most GM and Chrysler performance engines.
Ford began using the C-6 as a heavy-duty replacement for the C-4 in 1966. It was originally designed for Ford big-block engines. You will find them in Ford full- and mid-sized cars up until 1980 and in many light-duty trucks and Broncos until the late 1990s.
You can locate a C-6 transmission by looking for one of these production passenger cars: 1977-’79 Lincoln Mark IV and Mark V and Ford, LTD II; 1966-’80 Ford and Mercury full-size cars; 1966-’69 Comet; 1967-’78 Cougar; 1968-’76 Cyclone and Montego; 1967-’79 Thunderbird; 1966-’70 and 1972 Mustang; 1966-’79 Elite, Fairlane, Ranchero and LTD; 1968-’76 Torino; and 1971 Falcon. And in these trucks: 1975-’93 Econoline vans; 1973-’83 F100; 1975-’91 F150; 1968-’91 F250; 1968-’96 F350; 1992-’96 F450; and 1978-’91 Bronco.
Most donor cars will have an engine size over 351 cubic inches, up to 460 cubic inches; however, you will find some C-6 transmissions behind small-block Fords, especially in truck applications.
Because of the C-6 transmission’s strength, you will find many of these units in four-wheel-drive applications. Because they are attached to a transfer case, they lack the necessary tail shaft for rear-wheel drive applications. The C-6 can work well with high horsepower engines without major modifications, however they are very heavy, at over 200 pounds. You can visually identify the C-6 by its 17-bolt oil pan. The pan is mostly square in shape but with a pronounced jog in the rear of the pan on the passenger side. A vacuum modulator mounts in the valve body just above this jog. The oil screen is metal and brass and can be replaced or cleaned and reused when changing the fluid. The dipstick is on the passenger side just behind the taper of the bellhousing. The C-6 also has a very large fluid capacity at 24 pints, including the fluid in the torque converter. Ford used type FA transmission fluid, which was specific to Ford transmissions until mid-1977 when C-6 transmissions were converted to Dexron/Mercon late in that model year. The easiest way to tell what fluid you should be using is to check the dipstick. The stick will reference either the FA specified fluid or an M2C138-CJ specification, which is Ford’s designation for Dexron/Mercon.
The ratios of the C-6–First 2.46:1; Second 1.46:1; Third 1:1; Reverse-2.18:1–are virtually the same ratios that Ford used on the later AOD transmission (with the exception of the .67:1 overdrive gear).
The transmission is 33-1/2 inches long including tail shaft (on 2WD applications), which makes it three inches longer than the Ford C-4 and 2-3/4 inches longer than the newer AOD or AOD-E transmissions. The transmission mount location is 22-1/16 inches from the front of the bellhousing, which makes it a good candidate for replacing a Ford AOD overdrive transmission for racing applications. This conversion would require minimal adaptation of the crossmember. When swapping into some Ford C-4 applications, it will be necessary to move the crossmember back about two inches. The C-6 has a bulky case, and it may be necessary to modify the shifter tunnel on C-4 equipped cars to accommodate the C-6 shifter linkage. A bellhousing adapter plate is necessary for small-block V-8 engines, and the driveshaft will have to be shortened. You will also need the 31-spline C-6 slip yoke.
Using the proper flexplate is important. Ford engines used two 164-tooth flexplates due to a design change. Ford 302 engines built between 1969 and 1980 or 351 engines built from ’69-’81 with the C-6 transmission, used a 28.2-ounce imbalanced flexplate which is 117/16 inches in diameter. Ford engines built after ’81 used a 50-ounce imbalanced flexplate. When installing a C-6 behind an early FE motor (352, 390 and 427), you need to find an aftermarket 164-tooth flexplate for internally balanced engines to replace the original 184-tooth unit. For engine conversions where you are mating a C-6 to a 460, you need an imbalanced unit, Ford number D9TZ-6375A, which fits all of the 460 engines from ’79 and up. Performance SFI-rated flexplates are available for all three applications from aftermarket parts suppliers. Replacement crossmembers are available to help you with the installation of the C-6 into 1955-’62 full-size Ford V-8s as well. The C-6 is a good transmission choice to use for installing a 429 or 460 engine into earlier Ford models, too. The 31-spline output slip yoke required to hook your driveshaft to the C-6 is Ford part number C7SZ-4841A. This yoke accommodates either a Spicer 1330 series u-joint.
Because millions of these transmissions were produced, a good used unit can be found easily and inexpensively. Checking with a local salvage yard, we found many available for prices ranging between $125 and $400, depending on the amount of mileage on the transmission and if a torque converter was included.
The Ford C-6 can be comfortable in almost any application you want to throw at it. Whether it is a daily driver, work truck, street performance machine or weekend drag racer, the Ford C-6 can handle heavy-duty use or horsepower.
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