ZR51 Performance

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Transmission Oil change Procedure ('89 - '96 ZF 6 speed  equipped Corvette)

Required Tools : 17mm hex drive, load rated floor jack, (4) jack stands, oil catch pan,  2 feet of tubing and a funnel.

1) Raise and support vehicle on load rated jack stands. Place stands under front control arms and lower rear shock studs. NOTE: Be sure vehicle is level.

2) Remove Drain Plug and let drain for 5  minutes.

3) Install drain plug and tighten to 26 lbs-ft.

4) Remove fill plug on right (passenger) side of transmission. Feed the tube up the firewall and insert lower end into the transmission fill plug hole. From the engine compartment insert funnel into upper end of tube and add 2.2 liters (2.1 quarts) of  BMW Castrol TWS 10W-60 oil (P/N 07510009420) to the transmission.

5) Install fill plug and tighten to 26 lbs-ft.

6) Lower vehicle and test drive. NOTE: change transmission oil at 15K to 20K intervals.

Transmission R&R Procedure ('89 - '96 ZF 6 speed  equipped Corvette)

Required Tools (wrench and sockets for each):
21mm, 19mm, 17mm, 16mm, 15mm, 14mm, 13mm, 10mm, 8mm
A 1/4" X 12" standard screwdriver (Support beam alignment leverage bar)
Full size rolling floor jack, (4) jack stands, support stands under front
control arms and rear shock studs.
Bell housing (L98/LT1/ LT4) P/N 10126448 ~ $200 (it will be an updated -
magnesium/aluminum blend w/vents P/N 10085208)
FYI - '90-'95 Bell housing (LT5) P/N 10085201 ~ $800
O.E. Clutch Assembly (by Valeo) from Carolina Clutch (phone # 888 462-2739)
~ $300 delivered.
Note:  Clutch Assemblies by CenterForce or McLeod Industries are build upon stock Valeo Pressure Plates.

1) Remove the Shifter knob (see procedure below). Remove the lift-ring by using a 0.077" Allen wrench (if equipped), leather-boot by removing the center console and
4 egg corn nuts connecting the shift boot to the shifter.
2) Remove the exhaust system as one piece from after the catalytic converters.
3) Disconnect electrical connections from Reverse Light Switch, CAGS and Speed Sensor.
4) Support the transmission with floor jack centered at drain plug.
5) Remove drive shaft and support beam bolts.
Manipulation by varying support height with floor jack may
assist in driveline support beam bolt removal.

6) Slide support beam forward from differential mount far enough to clear the differential structure when lowered.
7) Monitor the firewall clearance while lowering the support jack allowing the engine and transmission to tilt backwards until the drive beam clears the
emergency brake cable. You will have to help the cable around the back end of the beam. The emergency brake must be completely released for this step.
8) Slide support beam rearward from the transmission and remove from vehicle.
9) Remove transmission to bell housing bolts.
10) Engage 3rd gear to allow for the shift lever to clear the tunnel when transmission is being removed.
11) Remove transmission from vehicle.


Remove the bell housing by aligning the fork on the two flats
of the release (throw out) bearing and pull the fork back from the bearing
while rotating the bell housing in a counterclockwise direction.
Note: Excessive clutch disc wear may require loosening of the
ball stud to disengage fork and bell housing.
 - The 6mm Allen socket ball stud locking screw is a
counter-clockwise turn-to-loosen thread and is tightened to 10 lbs-ft. The torque spec in the manual is incorrect.
 - The 12mm Allen socket fork ball stud is a clockwise turn-to-loosen thread  (left handed) and is tightened to 33 lbs-ft. NOTE: If the fork ball stud is not able to be loosened/removed (clockwise rotation), the allen socket portion of the threaded stud has cracks in it allowing it to expand remaining locked in position. This happens more often than one would expect likely due to the thread being the opposite direction thread type where counterclockwise rotation actually tightens the ball stud more so resulting in cracking the thread portion of the ball stud. Removal of the stud once the thread portion has cracked is best done buy CAREFULLY cutting the stud in half by using a cut-off wheel through the retainer hole of the bell housing. Once the stud is severed below the shift fork, the bell housing can be removed and a 1-1/8" socket can be used to back the stud out for replacement. When backing the stud out from the flywheel side, it is to be loosened by rotating in the counterclockwise direction.

Caution: Do not let the clutch hydraulic slave cylinder (actuator) hang from the feed line.
Be sure to secure the actuator off to the side in a horizontal position. This will reduce the likelihood of any suspended debris in the clutch hydraulic fluid from settling in the actuator at the seal which can lodge in the seal and possibly compromise the clutch hydraulic system.

***CAUTION*** Do not, under any circumstance, use any type of leverage bar between transmission and bell housing. Manipulation by varying support height with floor jack may assist in aligning the bolt holes.

R&R of the Shifter Knob:

NOTE  -  If the shifter knob is being removed for the first time, it may require a great deal of effort to pluck the locking wedge out of position. The following procedure will allow you to get into the correct position to generate enough pull force required to get the job done.

1) Remove the roof panel (you will need to stand up in the car get enough leverage to pluck the wedge from the shifter knob). It will be necessary for you to straddle the door sills in order to get your body in the correct position for max lift/pull leverage.
2) Remove the shift pattern cap with a small leverage/prying  devise. With a small set of Vise-Grip pliers, lock onto the sides of the wedge that is just under the shift pattern cover of the knob.
3) with a long screw driver slid through the pliers jaws, use both hands and pull up with as much force as you can generate. 
4) Once the wedge has been removed, unscrew the knob to access the reverse pull-up lock-out mechanism (if equipped). The reverse pull-up knob is held in place with a very small allen-head set screw.

R&R of the extension housing bushing:

NOTE  - When removing the extension housing from the transmission case, be sure that the rear main bearing shim remains in place. If the shim should be removed for whatever reason, be sure that it is back in place when the extension housing is re-installed. Failure to follow this step will result in failure of the transmission.

NOTE  -  Be sure that any deformities caused by removal of the old bushing are removed from the aluminum bore. The dimples of the original bushing tend to leave slight wakes in the Aluminum bore as the old bushing is pressed out of place. For best reinstallation of the new bushing, make sure that the high spots of these wakes are removed along with any other protrusions caused by dents in the aluminum bore. If there are a few dents or gouges remaining in the bore, it is OK. You just need to be sure that all of the high spots have been removed so that the new bushing sits perfectly true.

NOTE  -  Do not be concerned that the new bushing does not have any dimples like the old one. The replacement bushing is to be pressed into place without any lubricants or binding agents. The combination of fit between the two different types of material and their coefficient of expansion properties will provide for an exceptionally good "hold in position" of the bushing when unit is returned to operation.

  1. Remove the tail shaft seal.

  2. Remove the bushing from it's journal buy cutting the bushing (with Sawzall) along the oil channel grove so as not to damage the bushing journal.

  3. Use a scotch pad to thoroughly clean the bore surface and the bushing outside diameter surface.

  4. Carefully press new bushing and seal into place. NOTE: This bushing is the same bushing used in a T-400 automatic transmission. NOTE: be sure that the rear main bearing shim is in place prior to re-installing the extension housing.

  5. Be sure to evenly torque the extension housing bolts to 18 foot pounds of torque.

Shifter Removal:

  1. Remove the shifter bracket at the back of the transmission.

  2. Unbolt and remove the shifter control-arm Allen bolt.

  3. Cut and remove the shifter bellows lower front tie-wrap.

  4. Pull and hold back the bellows in order to gain access to the drift pin safety retainer ring.

  5. Slide the retaining ring back and push the drift pin out enough to remove the shifter.

Changing the Speedometer Gears on the Vehicle
The C4 Corvette Vehicle Speed Indication System (VSIS) uses a pulse-signal-reference generated from the Vehicle Speed Sensor (VSS). The VSS, when equipped with a 15 tooth (T) drive gear (on output shaft) and 42T driven gear turning the VSS combined with a 3.45:1 differential ration, produces 4000 pulses per mile traveled. The VSIS is calibrated to 4000 pulses equaling one mile. When the differential (final drive) ratio changes, so does the pulses generated over each mile. To correctly recalibrate the VSIS signal pulse rate back to 4K/mile either the drive and/or driven VSS gear(s) must be changed or a black box Electronic Ratio Adapter (ERA) installed. Both cost around $140.

- Raise vehicle and safely support on jack stands.

Remove or Disconnect
- Exhaust.
- Driveshaft-to-pinion strap bolts (8mm).
- Driveshaft. (little to no oil should leak out).
- Vehicle Speed Sensor 10mm bolt.
- Vehicle Speed Sensor.
   Note: Roughly 1/4 of a liter of oil will drain out of the transmission.
- Driven gear from Vehicle Speed Sensor.
- Drive gear off of output shaft.
  To do this, you will need to rotate the shaft until the speed sensor drive gear clip is visible.
  Push in on the locking tang of the clip then slide the driven gear back and off of the output shaft.
  NOTE: The speedo drive gear will clear the extension housing tail bushing and seal.
  Leave the washer in place for the new speedo drive gear.

Install or Reconnect
- Speed Sensor Drive gear and locking clip.
- Driven gear onto Vehicle Speed Sensor.
- Vehicle Speed Sensor.
- Vehicle Speed Sensor 10mm retaining bolt.
- Driveshaft.
- Driveshaft-to-pinion strap bolts (8mm).
- Oil into the transmission to replenish what was lost.
  Note - add oil through the fill plug hole until it runs back out of the hole.
- Exhaust.

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Send email to billb@zfdoc.com or call Bill Boudreau at (602) 319-6575.
Last modified: March 31, 2008