Ford C-6 transmissions – Jim O’Clair @Hemmings

Ford C-6 transmissions – Jim O’Clair @Hemmings


Strength and heft: Good candidates for racing and hauling

In situations where your Ford, or any other engine has had performance improvements, it makes sense to also upgrade the transmission’s performance. For years, the Ford C-4 automatic was a reliable three-speed automatic transmission for many Ford products, and is fine for most 289, 302 or 351 engines that have received modest modifications. A later-model AOD overdrive unit that has been tweaked is one alternative, and certainly a Toploader manual transmission is another. However, for engine modifications, in which increases of more than 400 or 450 horsepower have been achieved, or for racing purposes, you should consider an automatic and changing from your C-4 or AOD to the stronger Ford C-6.

In case you were curious: Cutaway view of the Ford C-6 and typical Ford transmission identification tag

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 1990. Because of the C-6 transmission’s strength, you will find many of these units in four-wheel drive applications, however, because they are attached to a transfer case, they lack the necessary tailshaft for two-wheel drive applications. Still, these are viable conversion candidates, if you can locate a tailshaft assembly to mate to them. The C-6 is also used behind many 429 or 460 engines swaps.

The C-6 has a 17-bolt oil pan shaped similar to the state of New Mexico; mostly square, with a pronounced jog in the pan at the passenger-side rear. Units built after 1975 have a deeper pan than earlier transmissions. A vacuum modulator is mounted just above this jog in the valve body. The oil screen is metal and brass, and can be washed out and re-used when a fluid change or other service is performed.

Four-wheel drive versions use a stepped fluid pan, and the filter has an extension tube on it to lower the filter further into the pan. The bellhousing is integral to the transmission, and the dipstick is located on the passenger side just behind the taper of the bellhousing. The thickness of the bellhousing is different on some engines. You will find a narrow and shorter bellhousing on a 351W or 351C, which will be compatible with all small-block Ford, engine displacements. The 351M and 400 engines, as well as the 429 and 460 have a wider and taller bellhousing, which is compatible with the big block engines. First gear ratio is 2.46:1 and second gear is 1.46:1. Third gear is 1:1 and reverse gear is 2.18:1. These are practically the same ratios that Ford used on the later AOD overdrive transmission (with the exception of the 0.67:1 overdrive gear).

The transmission is 33-1/2 in. long, including tailshaft (on 2WD applications), which makes it 3 in. longer in overall length than the C-4 and 2-3/4 in. longer than an AOD or AOD-E. The transmission mount location is 22-1/2 in. from the front of the bellhousing, which makes it a good candidate for replacing a Chrysler 904 or a big block 727 automatic as well as the Turbo Hydra-Matic 700R-4 GM or Ford AOD overdrive transmission with minimal adaptation of the crossmember. When swapping into Turbo Hydra-Matic 350, Powerglide or some Ford C-4 applications it will be necessary to move the crossmember back about 2 inches, in addition to using a bellhousing adapter plate, shortening the driveshaft and changing the slip yoke. The C-6 is a bulky unit, measuring over 16 inches tall and 20 inches wide, so it may be necessary to modify the shifter tunnel on the driver’s side to accommodate the shifter linkage and complete this conversion. All of the pertinent transmission comparison dimensions are listed in the parts locator section of our online website,

The C-6 can handle more than 500 hp without major modifications. It is also very heavy–204 pounds, so have a transmission jack or plenty of help handy. The C-6 holds 24 pints, including the fluid in the torque converter. C-6 transmissions used type FA hydraulic fluid, which was designed specifically for Ford transmissions, until mid-1977, and then converted to Dexron/Mercon late in the 1977 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.

You can locate a C-6 transmission by looking for one of these production vehicles:

Donor cars you are looking for will have an engine size between 351 and 460 cubic inches; however, you will have to find units from a 351W to bolt directly to small-block applications. These are most commonly found in trucks. Larger engines sizes will have the bigger bellhousing and will not bolt up to small-block applications without an adapter plate.

When looking for one of these transmissions at your local junkyard, we recommend you also buy the flexplate, shifter and torque converter. You’ll want to grab the kickdown rod or cable as well, if it is not still attached to the transmission. Using the proper flexplate is important. As with the AOD transmission we featured a few months ago, Ford V-8 engines used two 164-tooth flexplates. An engine balance design change was made by Ford in the 1980s and will determine if the flywheel on your donor transmission is the correct one for your application or not. Ford engines built from 1969-’81 with the C-6 transmission used a 28.2-ounce, externally balanced flexplate which is 117/16 inches in diameter. The Ford part number is E0AZ-6375A. Ford engines built after 1981 used a flexplate that was a 50-ounce, externally balanced flexplate with the same 117/16 inch diameter. The Ford part number is E2AZ-6375A. When making this conversion, you can use the original 1969-’81 C-6 flexplate found on 302s and 351s built before 1981, but a 164-tooth, 50-ounce flexplate would be required for any small-block Fords that are newer than 1982.

When installing a C-6 into an early FE engine (352, 390 and 427), which was internally balanced, you need to find an aftermarket 164-tooth flexplate with no weights to replace the original 184-tooth unit. For engine conversions where you are mating a C-6 to a 460, you will need a different externally balanced flexplate, Ford number D9TZ-6375A. This fits all of the 460 engines newer than 1979. All three of the above-listed Ford part numbers are available from any of a number of suppliers for around $50. The 31-spline output slip yoke required to hook your driveshaft to the C-6 is also available new. The Ford part number is C7SZ-4841A. This yoke accommodates either a Spicer 1330 series U-joint or a Cleveland S55-series joint. Replacement adapter joints are made by many manufacturers to mate your existing driveshaft to the C-6 slip yoke. The starter from a C-4 vehicle will work fine, however, the AOD starters have threaded holes and you will have to drill out the threads or get yourself a C-4/C-6 starter. These are available from most part stores for less than $50.

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