Doing A C4 to AOD Transmission Upgrade

When you get the transmission try to get all of the related stuff (bolts, electrical connector, dipstick tube, converter, yoke, block plate, linkages, levers, shifter, hydraulic fittings, etc.), will make this project a lot easier.

Extension Housing - There are extension housing variations among C4's, with a short tailshaft (6 5/8 inch) used on some pickups and vans and a standard tailshaft (13 1/8 inch) used in most other applications. There are also two tailshaft lengths on AOD's. The short model is the one that is closest to the C4 in overall length. (10.1-inch)The short extension housing AOD was used in most passenger cars, 2WD F-150 trucks built after 11/81, and 83-85 E-150 vans. Don't confuse the AOD with the AOD-E, which uses electronic control.

Converter Compatibility - The AOD transmission and torque converter are a matched pair. The AOD uses a hollow two piece input shaft (one shaft inside the other) for lock-up purposes and requires a specific torque converter.

Gear Ratios - The first three gears of the AOD and C4 are similar and the AOD has a 0.67:1 overdrive ratio

the AOD is about 150 lbs. The AOD is beefier around the middle than the C4, The bosses on the AOD bellhousing are thicker than those of the C4, so longer bolts are required. The hydraulic fittings on the AOD were larger than those of the C4. Two adapter fittings (1/4 ID pipe to 5/16 OD tubing are needed to mate the fittings on the transmission lines to the AOD case. AOD's take Dexron/Mercon transmission fluid.

Transmission Crossmember and Insulator - The only bit of true fabricating is usually the transmission mount crossmember. While the AOD and C4 overall lengths are close the mounting pad for the insulator on the AOD is 2 inches farther aft, so the crossmember mounting point needs to be adjusted to fit.

Flexplate Diameter and Balance - One of the more important parts of the swap is to choose the proper flexplate for your application. The integral bellhousing AOD requires a flexplate with 164 teeth (approximately 14 1/4 diameter with an 11 7/16 torque converter bolt pattern) for proper starter placement and converter compatibility. The C4, however, uses a detachable bellhousing that is matched to one of three different flexplate sizes (148, 157, and 164 teeth). The 148 tooth flexplates are comparatively rare, having been used in cars like the V8 Mustang II. The 157 teeth flexplates were generally used in 289/302 small and mid-sized cars, while the 164 teeth flexplates were used in 289/302 full-size cars and 351W/351C applications. The C4 164 teeth flexplates and the AOD 164 teeth flexplates will physically interchange, but there are two balance weights to be concerned with. Part# OEM E0AZ-6375-A (164 teeth flexplate, 28.2 oz-in balance factor, 351W with AOD applications). If you're swapping an AOD onto an '81 or later 5.0 you'll need the AOD flexplate (164 teeth, 50.0 oz-in balance factor, for 5.0 with AOD applications) OEM part number E2AZ-6375-A.

Block Plate - A block plate (the thin stamped sheet metal plate that fits between the engine block and transmission housing) which matches the large AOD bellhousing and provides the correct starter location and engagement depth for the torque converter snout in the crank pilot (assuming you have the right flexplate).

Driveshaft and Yoke - You can use a T-5 front yoke, since it is a stronger piece to begin with, plus it does NOT have the rubber isolator. The AOD/T5 yoke is desired but you can shorten the C4 yoke to get it to fit properly. You will also need to shorten the driveshaft by the correct amount.

Speedometer Gear - The speedometer driven gear assembly from the C4, including the bolt and retaining clip, will fit into the AOD. There may be some variation with the AOD drive gear (the gear inside the tailshaft housing), so you may have to change driven gears for proper calibration.

Electrical Connections - The AOD uses a four pin electrical connector for the reverse/backup lamps and neutral sensing switch.

Throttle Valve Operation - The AOD does not use engine vacuum and a modulator valve to sense load. Instead, a throttle valve is used which moves proportional to throttle pedal travel. This is one of the most important parts of the swap to get right. When adjusting the linkage, it is safer to err on the side of hard shifts. There are a several ways of implementing the TV linkage and the linkages can be pirated from cars or trucks that came with AOD's and carbs or throttle body injection. A rod arrangement was used on AOD cars equipped with carbs and TBI while a cable was used on SEFI cars. Remember, you'll need a way to adjust the linkage to vary shift feel. Alternatively, you can purchase an aftermarket TV cable kits (available from Windsor-Fox, Total Performance, and others). I cover this important issue in another article here.

C4 and AOD Dimensions

Overall Length (bellhousing to tailshaft housing end, flat-to-flat):AOD - 30 3/4 inch
C4 - 27 3/4 inch
C6 length - 28 1/2 inch or 28 3/4 inch

Distance from Bellhousing to Insulator Mount (flat to mounting hole centerline):
AOD - 21 3/4 inch
C4 - 19 3/4 inch

Flexplate Diameters
164 teeth - 14 3/16 inch (13 inch converter)
157 teeth - 13 1/4 inch (11 inch converter)
148 teeth - ?

Transmission Weights

AOD (without converter or fluid) - 150 lbs
AOD stock torque converter - 34 lbs
C4 (without converter or fluid) - 110 lbs
C4 torque converter - ? lbs
C6 (without converter or fluid) - 140 lbs
C6 torque converter - 30 lbs small block, 31 lbs big block
FMX - 160 lbs
FMX torque converter - 32 lbs
Ford-o-Matic (pre-FMX), cast iron case - 228 lbs

C4 Variations

Large bellhousing - smooth flair case, twin bolt circles (case to housing, outer secures bellhousing to trans case, inner secures oil pump to case), 3 locating tabs, 14 3/16 flexplate, 13 converter, no dipstick hole in main case (dipstick goes into right front corner of pan), used in small-block powered large cars and light trucks.

Small bellhousing - step case, single seven bolt circle (case to housing), 13 1/4 flex plate, 11 torque converter, dipstick hole in case (seals with an O-ring), five bolt ('64 1/2 and early '65 Mustangs, etc.) and six bolt versions, used in small and medium small-block powered cars.

Main cases and bellhousings for large and small versions are not interchangeable.
5 and 6 bolt small bellhousings are interchangeable between each other (on the case side).
Entire 6 bolt (small or large bellhousings) transmissions, except for dipstick locations, are interchangeable in many cars (some smaller cars require the small bellhousing variant).
C5 bellhousing looks similar, but has a completely round hole in the front pump area and is not interchangeable with C4 bellhousing.

Standard Tailshaft - 13 1/8 inch long, most applications.
Short Tailshaft - 6 5/8 inch long, some pickups and vans.
Input Shaft Spline Count (Most 6 and 8 cylinder applications) pre-'70 - 24 splines
'70 and later - 26 splines
Note: Some 1970 input shafts have 26 splines on both ends ('71 and later have 24 splines on one end and 26 on the other), making them the most desirable for drag racing applications.
Need to add C4 valve body variations (green-dot, cruise-o-matic, etc.) and AOD shift sequence
Four and six cylinder versions are light duty (fewer clutch plates).
Some C4's (late '60s, early 70's Mavericks? need to check details) had full-manual valve bodies.

Ford Transmission Ratios Comparison
  Stock C4 Stock C6 WR C6 Stock AOD WR AOD E4OD
1st 2.46 2.46 2.72 2.40 2.84 2.72
2nd 1.46 1.46 1.54 1.47 1.55 1.53
3rd 1.00 1.00 1.00 1.00 1.00 1.00
4th ---- ---- ---- 0.67 0.70 0.71

Balance Factor Information
1981-and-up 5.0 HO - 50.0 oz
1981-and-up 302 (non-5.0 HO) - 28.2 oz
Pre-1981 289/302 all 351W and 351C-400 - 28.2 oz