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BioPatent Communications

II. Changing the Clutch

I. - Go To Removing the Transmission    III. - Go To Changing U-Joint     IV. - Go To Replacing Transmission

Gary L. Baker, Esq. Patent Attorney

You may want to change the clutch because you had to remove the transmission for other reasons and it is best to keep it fresh, because the old clutch was worn out, and/or because you wanted to upgrade to a clutch that can handle more power.  In any case, once the transmission is removed, it is no big deal to install a new clutch.

1. Go to Fly  Wheel    2. . Go to Throwout Bearing    3. Go to Installing the New Clutch    

trantorqplate.jpg (588723 bytes) Use a clutch plate centering tool to hold the clutch plate in place while you remove the clutch pressure plate (orange, left).  Turn out the six bolts that hold the pressure plate on to the fly wheel.  With the bolts removed, the pressure plate and clutch disk can be easily pulled from the face of the fly wheel. 

I always replace the pressure plate and clutch disk every time I have a transmission off.  But, in this case, I was shocked to see the parts were barely worn after 30,000 miles of hard use.  The pressure plate and disk were heavy duty units from Centerforce®.  A 240Z only weighs about 2200 pounds, so even with my upgraded engine, the clutch apparently does not work hard launching me here and there.  I am saving the used clutch plate and disk to install some other day in some other Z.   

Notice the Centerforce pressure plate has weights (squares around the edge of the inner circle in the figure) that are connected to lever fingers of the clutch diaphragm spring.  The diaphragm spring provides the pressure that squeezes the clutch disk between the pressure plate and fly wheel to start the car moving when you let out the clutch.  When the engine is spinning at high rpms, the centripetal force on the weights pushes the finger levers out, increasing the squeezing pressure on the clutch disk.  With the old stock clutch, the torque of my modified engine would often cause the clutch to slip and spin wildly when I punched it off the line.  That is, the clutch would slip and spin instead of my tires spinning (embarrassing).  I have never had this problem since I installed the Centerforce clutch.  The added benefit is that, in my small 240Z, this performance part will apparently last a lifetime.  

tranfly.jpg (423270 bytes) Fly Wheel. With the clutch disk removed, you can inspect the fly wheel (shiny disk, in the figure to the right, at the end of the engine block).  The fly wheel is the disk attached to the back of the engine crank shaft in frictional contact with the clutch disk to start a car moving.  The fly wheel is heavy and can store kinetic energy for smoother engine rotation and faster starts when the clutch is released.  So ... the fly wheel is exposed to a lot of heat and abrasion in use.   Inspect the fly wheel for cracks or warpage caused by heat stress.  Look for deep grooves caused by the clutch disk rubbing during starts.  Check to see that all the teeth are still present around the perimeter of the fly wheel.  These teeth engage the starter motor pinion gear to turn over the engine when starting the car.  If the fly wheel needs repair or placement, it is not too difficult to take out by removing the six bolts shown in the photo at right.  Typically, people will have the fly wheel "turned" (milled or resurfaced on a lathe) to provide a smooth new surface.  Some people replace the fly wheel with a lighter fly wheel so the engine "spins up" faster, for some types of road racing.  I like my heavy stock fly wheel, which releases more kinetic energy when I release the clutch.


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tranthrowout.jpg (489324 bytes) Throwout Bearing.   Stepping on the clutch ultimately causes the throwout bearing to push the diaphragm spring finger levers to release the clutch disk, thus disengaging the engine from the transmission.  That is, when you step on the clutch, hydraulic fluid (brake fluid) from the master cylinder pushes the slave cylinder, which in turn pivots the clutch fork lever, which pushes the throwout bearing into the finger levers of the diaphragm spring to release squeezing pressure on the clutch disk.  The throwout bearing rotates freely against the diaphragm spring levers, allowing the clutch fork to push the spinning diaphragm spring without grinding the parts together.  

Throwout bearings are sealed bearings, now days, and rarely ever go bad.  However they are inexpensive and should always be changed as part of a clutch job. 

The throwout bearing is pressed into a bearing sleeve that attaches the bearing to the clutch fork.  Professional shops typically use an arbor press to press the old throwout bearing out of the sleeve.  However, the photograph at left shows another way to remove the old throwout bearing from the sleeve.  Find a socket that just fits inside the internal diameter of the throwout bearing, but rests on the part of the sleeve inside the bearing.  Set the old bearing between two blocks of wood, as shown in the photo, while the sleeve hangs between the blocks without contact.   Strike the socket with a mallet to push the sleeve out of the old bearing.  A new throwout bearing can be installed by hammering it onto the sleeve carefully with the mallet.  Of course, do not strike the parts with a metal hammer or do anything that would damage the parts. 

The throwout bearing sleeve should be cleaned then the internal recess filled with grease so it can slide smoothly between engaging and disengaging positions. 

Note that if you are replacing a stock 240Z 4-speed with a 280Z 5-speed, you must use the throwout bearing sleeve from the 5-speed, or you will not be able to adjust the clutch after everything is back together. 

trandisk.jpg (655746 bytes) Installing the New Clutch.   Apply some grease to the pilot bushing in the center of the fly wheel (not too much or the main transmission shaft might not be able to fit into the pilot bushing when the transmission is installed).  Center the new clutch disk on the fly wheel and hold it centered with a centering tool .  I use the generic centering tool shown in the figures.  Cheap plastic centering tools can be purchased which specifically fit the 240Z pilot bushing and clutch disk center spline hole.  Years ago, I used to use a piece of broom handle.  The clutch manufacturers always warn you to put the clutch disk on with the springs facing out (as shown, right), and not facing the fly wheel.

tranplatein.jpg (782220 bytes) Place the new pressure plate over the clutch disk on the fly wheel.  There are alignment posts in the fly wheel that fit in corresponding alignment holes in the pressure plate, to center it perfectly.  Turn in the six pressure plate bolts loosely until the pressure plate is positioned properly in contact with the fly wheel.  Using a torque wrench, tighten each bolt to from 11 to 16 foot pounds.  Tighten in a star pattern alternating from one bolt to the next across the pressure plate. 

Remove the centering tool from the clutch disk, which is now held in place squeezed between the fly wheel and the pressure plate.  Add a tiny bit of grease to the splines in the center hole of the clutch disk, but not enough to possibly ever drip out to the clutch disk surface.    

The new clutch is now in place on the fly wheel.   



I. - Go Back To Pulling the Transmission

III. - Go To Changing U-Joint

IV. - Go To Replacing the Transmission

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