Servos, Accumulator and Bands
Most late A-727s have a controlled-load servo kickdown servo piston assembly. It applies the kickdown band at the correct time and cushions the 3–2 downshift. The earlier basic servo assembly uses no internal spring/piston assembly and was found in pre-1971 transmissions and some later applications where soft down shifts at higher torque loads would be detrimental. The A-904s also uses the two styles of servo piston assemblies.
A Very Important Steel Ball
A steel check ball is used in the rear clutch apply piston and front clutch retainer. When the retainers are spinning but not hydraulically operating, residual fluid can force the apply piston against the pressure plate in the front clutch or the Belleville piston spring in the rear. This fluid may force the friction plates and driving discs together creating “clutch drag.” The steel ball acts as a check valve and lets fluid (under pressure due to the clutch assembly spinning) out of the retainer before the piston inadvertently clamps clutches together. ■
A-727 and A-904 Rear Clutch
The A-904 rear clutch assembly is smaller but operates like the A-727. A difference is that the A-727 assembly is two separate pieces but the A-904 is one. ■
Similar in operation, the A-727 (top) and A-904 (bottom) rear clutch retainers differ in size.
These are A-727 servos, bands, accumulators, and associated springs and levers. The kickdown band apply lever, above the C-shaped flexible kickdown band, comes in different ratios, depending on transmission usage. The low-reverse band and its hardware are on the right.
Higher-performance transmissions used apply levers with 3.8:1, 4.2:1, or a 5.0:1 ratios. The higher ratio levers multiply the apply force more to tighten the kickdown band harder around the front clutch retainer but may take longer to do so, creating timing issues if used incorrectly.
Throughout its production, A-727s had different styles of kickdown bands. Some were cast and maintained their circular shape; others are flexible and don’t retain their shape. These are known as “Flex” bands and most late TorqueFlites had them.
The Hemi and 440 6-barrel TorqueFlites had wider kickdown bands to stop the heavier and wider five-friction disc front clutch retainer during the 1–2 shift. The bands fit the five-friction disc A-727 retainers but are too wide for the standard retainers. With equal line pressure, the wider kickdown band, with the 5.0:1 ratio apply lever, produced greater holding power than the standard-width stock cast or flex kickdown band combination.
The accumulator piston and spring combination cushions the application of the kickdown band during the 1–2 upshift and the application of the rear clutch when the gearshift is placed in any forward driving range. When placed in a forward gear, the application of the rear clutch is not completed until the accumulator seats against the valve body’s transfer plate. The 1–2 upshift is not completed until the accumulator piston has moved its maximum distance in the opposite direction. This is why the type of spring (or lack of it) in the accumulator circuit affects rear clutch engagement and 1–2 upshift quality.
The low-reverse band and servo are similar between the two families of TorqueFlites, but the A-727 has only one width of low-reverse band. It applies to clamp the low-reverse drum in Reverse and in Manual Low; the low-reverse band works with the overrunning clutch to hold the low-reverse drum stationary. The A-904 and A-727 depend on the low-reverse band’s additional holding power to prevent breakage of the overrunning clutch race during hard acceleration.
Kickdown and Low-Reverse Bands
The TorqueFlite uses different widths and styles of kickdown bands to stop the front clutch retainer’s rotation. The A-904 kickdown band is smaller than the A-727’s. Most A-904s are the same width, but some A-998 and A-999s have a five-friction disc front clutch retainer and use a wider kickdown band. The A-904 started with cast, round bands but later switched to “flex” bands.
The low-reverse bands are similar but the A-904 uses two different types of bands. One is a double wrap band, used to give greater low-reverse drum holding capability for V-8 vehicles. The other is a single wrap band, which has less torque-holding capability for use with 4- and 6-cylinder engines.
Later 1970s TorqueFlites had flexible or “flex” kickdown bands. The larger A-727 band is above; the A-904 band is below.
The A-727(left) uses a single-wrap low-reverse band; the A-904 (right) uses a double-wrap band for V-8 applications.
Planetaries, Output Shaft and Governor
Three separate assemblies, the planetary gear assemblies (planetaries), the output shaft and governor, and the output shaft support bearing transfer and time torque changes. The planetaries and sun gear provide gear reduction and rotation reversal to create three forward gears and Reverse. The output shaft transfers engine torque from transmission to driveshaft. The governor controls the shifts and a bearing supports the shaft.
The front annulus gear splines to the rear clutch retainer friction discs, and the gear surrounds and drives the front planet carrier. Depending on which gear the transmission is in, the sun gear–driving shell assembly is driven by the front or rear clutch packs and may transmit torque to the rear planetary assembly. Thrust washers prevent wear between the various rotating parts and control the overall endplay.
The output shaft has a thrust washer between it and the input shaft that is fiber or bronze, depending on the transmission. Later model A-727s and A-904s have a hardened steel washer between a bronze washer and the output shaft. The output shaft has four sets of splines; small ones fit in the front planet carrier, a second set holds the rear annulus gear, a third holds the governor-park assembly, and the longest set is for the drive shaft yoke. The shaft also has worm gear teeth to drive the speedometer gear.
On the front of A-727 planetary assemblies (from left to right), a snap ring holds the planetaries on, a front planetary gear, thrust washer, front annulus gear, thrust washer, sun gear-driving shell assembly, thrust washer, rear planetary gear, thin steel thrust washer, and rear annulus gear. The last three parts are contained “inside” the low-reverse drum. The govenor and bearing also mount on the shell.
The governor assembly and output shaft support bearing are held on the output shaft by snap rings. The governor creates a hydraulic signal, which works with the throttle pressure circuit to control shift points according to the vehicle speed and torque requirements. Two sealing rings, the governor body-park pawl assembly, and the governor valve-weight-spring assembly create and deliver the governor signal to the valve body. A thin snap ring holds the governor body-park pawl assembly to the output shaft. The output shaft bearing supports the shaft and internal components and is held on by one (A-904) or two (A-727) thick snap rings.
A Planetary