How to Adjust the Clutch Brake on a Mack Truck

Most all Mack truck models have the same model of manual transmissions. The transmission inside of the Mack truck requires a clutch for the transmission to be able to operate. The purpose of the clutch is to disengage the transmission and engage the transmission so that the gear shifter may be shifted. Once the clutch is pressed in, the friction from the clutch plate against the flywheel will stop the flywheel from turning long enough to shift the gear levers.

Things You'll Need:

• Block of wood
• Flashlight
• Large flat head screwdriver
• 1/2-inch drive ratchet
• 9/16 socket
• 9/16 clutch adjustment tool

1. Drive the Mack truck onto a level surface and shut off the engine

Drive the Mack truck onto a level surface and shut off the engine. Apply the air brakes to keep the truck from rolling.

2. Push the clutch pedal completely to the floor and lodge a block of wood between it and the base of the driver's seat

Push the clutch pedal completely to the floor and lodge a block of wood between it and the base of the driver's seat. This keeps the clutch compressed while it is being adjusted. The block of wood should be at least three feet long to hold the clutch pedal to the floor.

3. Slide under the Mack truck near the door on the driver's side

Slide under the Mack truck near the door on the driver's side. Locate the bell housing on the bottom of the transmission. The bell housing will have an access hole on the bottom. Shine the flashlight through the access hole and locate the 9/16 bolt head that is attached to a small locking fork. The bolt head and locking fork needs to be at the 6 o'clock position (straight down) to be able to adjust the clutch.

4. Insert the tip of the flat-head screwdriver through the access hole

Insert the tip of the flat-head screwdriver through the access hole and turn the flywheel to get the 9/16 bolt and the locking fork to the 6 o'clock position. Turn the flywheel by positioning the tip of the screwdriver between the gears and turning.

5. Unscrew and remove the 9/16 bolt from the clutch with the ratchet and 9/16 socket

Unscrew and remove the 9/16 bolt from the clutch with the ratchet and 9/16 socket. Then pull the bolt out along with the locking fork. The locking for keeps the clutch from turning out of adjustment.

6. Position the bolt side of the clutch adjustment tool up to the clutch

Position the bolt side of the clutch adjustment tool up to the clutch. Screw the bolt into the bolt hole that the 9/16 bolt came out of until it is tight. Then, finish tightening the bolt with the ratchet and socket.

7. Hold the handle of the clutch adjustment tool back toward the rear of the bell housing

Hold the handle of the clutch adjustment tool back toward the rear of the bell housing. Engage the tip of the tool between the bolt head and the notch on the clutch plate. Turn the clutch clockwise to adjust the clutch. Make one complete turn of the clutch with the tool and then check the free travel of the clutch pedal.

8. Remove the block of wood and check the free travel of the clutch

Remove the block of wood and check the free travel of the clutch by pushing on it with your hand. There should be at least two inches of free travel in the pedal before the clutch engages. If not, continue to turn the clutch adjustment tool clockwise until you have two inches of free travel in the pedal.

9. Unscrew the tool from the clutch and screw the 9/16 bolt

Unscrew the tool from the clutch and screw the 9/16 bolt with the locking fork back into the clutch plate. Make sure that the locking fork is positioned between the two notches on both sides of the bolt head. Tighten the bolt down tight with the ratchet and socket.

10. Remove the block of wood and crank the engine

Remove the block of wood and crank the engine. Push the clutch in and out to ensure that the clutch pedal remains at two inches of free travel. Turn the engine off.

Tips and Warnings

The clutch has to be compressed to the floor with the block of wood each time the tool is used to turn the clutch plate. If the clutch is not compressed to the floor, the clutch will not turn at all. The clutch adjustment tool can be bought at most any auto parts stores or from any parts store that sells parts and supplies for diesel engines.

Make sure that the clutch adjustment tool is completely tight inside of the bolt hole. If there is too much slack in the bolt that tightens the tool to the clutch, the bolt can break off in the clutch while turning the clutch plate.

Video: How to adjust the newer style "quick adjust" clutch on a big truck

Helpful comments on this video:

• Thank you so much Gordon! This is like the Holy Grail. I have asked so many mechanics about this and they just refused to say. Marking the crank? Genius!!! Thanks again!
• Dude marking the Crankshaft is BOTH Genius and Obvious. Every owner operator should does this. I prefer to use a Double Ended Swivel Head spanner so I can force the socket head on the adjusting bolt by levering off the window. Also I used a 3/8 extension round section to measure the 1/2 gap.