C4 Transmission

The “Medium Duty” automatic transmission was installed from 1964 to 1981. The housing is made entirely of aluminum and the bell housing, unlike the C6, can be removed. The middle part of the FMX gearbox is made entirely of cast iron, so it can be easily distinguished from the rest.

Even with the C4 transmission, there are some differences, not all gears fit together. The early models from 64 to 69 have an input shaft with 24 teeth (spline) and a diameter of 0.788 inches. The 1970s with the case number D0 had a certain special position. It has a 26-inch input shaft with a diameter of 0.839 inches and 26 teeth on the clutch hub. In 1971 this was changed again into a clutch hub with 24 teeth (until the end of production).

Then there is a difference in the oil ruler, the so-called dipstick. With some C4’s the dipstick leads directly into the housing, with others laterally into the oil sump. Please take care when ordering oil sumps!

There are also 2 (actually 3) different transmission bells. The small bell with the flying wheel with 157 teeth and the 10.5 inch converter. The big bell with the flying wheel with 164 teeth. This offers space for the 11 7/16 inch converter. The inch specification at the converter is not the diameter but the distance between the bolts. The third gear bell for some Mustang II and Pintos was even smaller and was for the 143-teeth Flying Wheel. But that was very rare and for 4- and some 6-cylinder.

Worth mentioning is the balancing of the flex plate (imbalance). The 302 HO (HO stands for High Output) engines are balanced with 50oz. The normal 302 and 351W are balanced with 28oz.

The gear ratio is:

• first gear: 2,46:1
• second gear: 1,46:1
• third gear: 1,00:1
• reverse gear: 2,18:1