The servo assembly and 2-4 band are located in the front part of the transmission and are applied in second and fourth
gears. In third gear range, the servo assembly releases the band and acts as an accumulator for the 3-4 clutch apply
circuit. The band is held stationary to the transmission case by the band anchor pin and wraps around the reverse input
housing. When applied by the servo assembly, the 2-4 band holds the reverse input housing preventing it's rotation.
To apply the 2-4 band in second gear, 2nd clutch fluid is routed to the apply side of the second apply piston. Second
clutch fluid pressure moves the piston against the servo cushion and servo return spring forces. These spring forces
help cushion the 2-4 band apply in second gear. The second apply piston moves the apply pin to compress the band around
the reverse input housing.
In third gear, 3rd accumulator fluid is routed to the release side of the second apply piston. The release side of the
2nd apply piston is the side facing the transmission when installed. The surface area on the release side of the second
apply piston and servo cushion plate is greater than the surface area that 2nd clutch fluid pressure covers on the apply
side of the piston. Therefore, the force from the 3rd accumulator fluid pressure, in addition to servo return spring
force, overcomes the force of 2nd clutch fluid pressure. The 2nd apply piston then moves the apply pin away from the
2-4 band to release the band from the input housing.
Third accumulator fluid is fed by 3-4 clutch fluid which is used to apply the 3-4 clutch. The movement of the 2nd apply
piston against 2nd clutch fluid pressure acts as an accumulator to absorb initial 3-4 clutch apply fluid. This action
helps cushion the 3-4 clutch apply, as well as the 2-4 band release.
In fourth gear, 4th fluid is routed through the center of the apply pin and acts on the apply side of the 4th apply
piston. The apply side of the 4th piston is the side facing the servo cover when installed. Fourth fluid pressure
moves the 4th apply piston and apply pin to apply the band. The 4th apply piston moves against the 4th apply spring to
help cushion the band apply in fourth gear.
As you can see from the operation of the servo the size of the apply area is very important to the apply and release of
the 2-4 band. Each time the band is applied or released in 2nd thru 4th gear it is because the larger apply area
overcame the fluid pressure of a smaller apply area. In hydraulics the larger the apply area the greater the holding
force. In a high performance transmission we want larger apply servos and more holding force on our clutches and
There are (3) three popular servos for high performance applications. There is the factory style corvette servo, the
Superior billet 2nd and 4th apply servos and the Sonnax 2nd and 4th apply servos. In the 2nd apply servo category the
corvette servo has the smallest apply area of the three. The Sonnax comes in next with an approximate increase of 18
percent over the corvette servo and then the Superior 2nd apply servo is the largest with an increase of approximately
35 percent over the corvette servo. There is one drawback to a larger 2nd apply piston, if you remember, the 3rd
accumulator fluid has to overcome the 2nd apply pressure to release the band. That means, with the Superior billet
servo the 3rd accumulator fluid has a more difficult time overcoming the 2nd apply pressure because the apply areas
are similar in size. The superior servo comes with a heavier return spring to assist in releasing the band. In the 4th
servo category the Sonnax 4th servo has the largest apply area, because of its dual piston design, with a 40 percent
increase over the factory piston. The Superior 4th servo has a 10 percent increase in apply area.
In a transmission that will be driven daily and with an engine that does not exceed 450hp the corvette servo would be
my choice. In a daily driver, this servo's 2nd band apply will be more comfortable and the holding power will be
sufficient. If you want more holding power in your daily driver go with the Sonnax 2nd servo. The Sonnax "super hold"
4th servo would be my choice with either of these 2nd apply servos for a strong 4th gear (especially in trucks used
for towing). For racing or in applications with engines over 500hp my choice would be the Superior billet 2nd and
There are several bands available for these transmissions. The band friction material is the single most important factor
in choosing a band for your high performance transmission. There are (3) three different friction materials to consider
for high performance, the high energy friction band, the Alto Red Eagle lining and the carbon lined band. There is also
the choice of band width, Alto products has developed the Alto power band which increases band width by 25 percent. also,
Alto increased anchor thickness on these bands to prevent premature failure. The Borg Warner standard width high energy
band is an excellent choice for your stock or mildly modified rebuild. For your high performance applications I recommend
the Alto Red Eagle or Alto carbon lined bands. If you plan to use the extra wide Alto bands I strongly recommend replacing
the reverse input drum to give the wide band a new surface to grab. If replacing the drum is not in your plans I do stock
the standard width Alto red eagle and carbon lined bands. If you have any questions about servos or bands please contact
All Sonnax, Superior, Alto and Borg Warner servos and bands are available in my online store.