of a Clutch Job
The normal clutch job starts
when your vehicle stops. If it was still working, you would not be working on
it. Most people drive their car until it won't go anymore. That is a pretty good
indication that something needs some attention. I'll start with the basics. A
clutch is a wear item - just like tires - they wear out. They have to. As you
let out your clutch, you are rubbing the clutch disk surfaces between the flywheel
and the pressure plate until the car starts to move, and then locks up when you
let the clutch out completely. A tremendous amount of heat is generated in the
clutch, pressure plate, and
flywheel. This happens each time you start moving and to a lesser degree,
when you shift. As time goes by, you will eventually wear the friction surface
thinner, and it will get to a point where it is too thin to transmit enough torque
to move the car normally. The clutch is now slipping and probably smells - really
bad! Your neighbors think you are burning your trash in your trunk, your wife
won't let you park in the garage (That's not a possibility at my house. You couldn't
park a go-cart in my garage), and your dog runs away when you drive up. It's time
for a clutch job. I'll assume you can remove the transmission without any instruction.
If you can't, it would be best to leave the clutch job to a professional. This
is not rocket science, but you do need some basic mechanical knowledge. Click
here to see a typical clutch setup.
After you have removed the
transmission, clutch, pressure plate, flywheel, and throwout bearing, each part
should be inspected. A careful examination of each part will usually tell you
what caused your problem.
-- The surface of the flywheel should be free of cracks, scored areas, and hot
spots. Lay an accurate straight edge across the wear surface of the flywheel.
There should not be any air gap under the straight edge. When flywheels get excessively
hot, they warp. The usual warp is is concave. About .006" warp on a 12" flywheel
is the maximum allowable. This is less on a smaller diameter flywheel. If you
have a recessed flywheel, you may have to cut a straight edge to fit down into
the recess. Some flywheels (usually imports) have a raised wear surface. Take
this into consideration when checking for warpage. Hot spots will appear as a
discolored area - usually blue. This will be a hard crystallized area created
by a lot of heat. Scoring is self explanatory. If any of these conditions are
found, the flywheel
MUST be resurfaced. Many clutch kit suppliers will not even warranty a clutch
if the flywheel has not been resurfaced. *** IMPORTANT *** DO NOT resurface the
flywheel on a lathe. It MUST be ground on a flywheel grinder! A lathe cannot remove
the crystallized areas and cannot reference the face of the flywheel to the back
of the crankshaft mounting surface to eliminate runnout. A flywheel grinder can
do both of these.
- CLUTCH -- The clutch will
probably be replaced anyway, but go ahead and inspect it. It should be free of
oil and grease. If you find any, you have more work to do. I won't go into fixing
leaks here, but the clutch cannot tolerate ANY grease or oil. The most common
sources of oil are the rear main seal, leakage around the bellhousing, torn dust
boot, missing inspection cover, leaking seal behind the transmission
front bearing retainer, and over lubricating the input shaft or bearing retainer.
The total thickness of clutches varies with applications, but most are between
.300" and .325" thick when new. Broken springs in the disk are usually caused
by overloading a clutch (hard starts or heavy trailer loads). Driving habits are
normally the cause of broken
springs. A loose or broken center hub is usually caused by a bad pilot bearing
or misalignment between the transmission input shaft and the crankshaft. A severe
misalignment problem will break
off the clutch tabs. A misalignment problem can be very difficult to correct.
Finding the cause of the misalignment can be even more difficult. Loose transmission
or bellhousing bolts, a bent crankshaft, flywheel runnout, bent input shaft, worn
bellhousing pilot holes, or worn transmission front bearing can cause this. Don't
forget to clean out the bellhousing and inspect it for cracks if you find a loose
center hub or broken tabs. The crack can open up under high torque loads. The
stop rivets hold the clutch top and backplate together. If they are worn or broken,
the clutch has been overloaded. The most common cause of worn stop rivets is downshifting
at excessive speeds. This can load the clutch more than acceleration. The stop
rivets on most clutches do not have as much clearance toward the downshift side
as they do on the acceleration side; so it can be just as easy to destroy a clutch
by downshifting as by overloading.
- PRESSURE PLATE -- Check
the pressure plate for hot
spots and warpage, just like the flywheel. One other thing to consider here
is that heat created on the pressure plate will be transmitted to the springs
diaphragm spring. This will weaken the clamping pressure of the pressure plate
and cause more slipping which creates more heat and so on, and so on. The tips
of the levers on diaphragm is where the bearing runs. Some wear here will be normal.
Excessive wear indicates a bad throwout bearing, improper adjustment, worn front
bearing retainer, worn linkage, or riding the clutch.
- BEARINGS -- Just replace
them. They are cheap items. Some bearings do not come with the small retainer
clips that hold them to the fork on some applications. Don't lose the old ones.
Now that you have all your
parts and your flywheel is resurfaced, it's time to put it back together. GO WASH
YOUR HANDS! The last thing you need on a new clutch disk is grease. Most brands
of facings will absorb the grease and you will not be able to get it out, wipe
it off, or wash it out. Before you install the clutch disk - spin it! I know it's
new or rebuilt, but spin it anyway. Put it on the input shaft and check it for
runnout. Try to get it as straight as you can. If you can't get less than .060"
runnout, replace it. Disks will bend in shipping and while sitting on the shelf.
We straighten each clutch when we build it and then again when it is delivered
to a customer. Clutch disks are easy to bend by hand. Just imagine how much you
can bend one when you let the transmission hang on it. Use an alignment tool!
Bolt the pressure plate down using a crisscross pattern and then check that the
levers or diaphragm fingers are at even heights. If they are uneven, check that
the bolts are tight and the pressure plate is pulled up against the flywheel.
If everything checks out and they are still uneven, do not install it. Get another
pressure plate or have a clutch shop check the lever height. Put a VERY light
film of anti-sieze coumpound on the transmission input shaft. If you wipe it off,
you will leave a flim on the shaft. It's better to leave it dry than to have too
much on it. DO NOT force the transmission into the clutch spline. Many disks are
bent during installation. Be gentle. Most disks install with the springs toward
the pressure plate and have a slight chamfer on the rear of the spline to help
stab the transmission. If you have a
Borg & Beck pressure plate, do not try to move the levers. If you do,
the strut inside can slip out of position and will not allow the clutch to release.
STUFF -- The clutch fork, pivot ball, front
bearing retainer, cable, slave cylinder, master cylinder, bell crank &
bushings, and EVERY piece of linkage from the pedal to the throwout bearing should
be checked for wear. If it's worn, replace it. It's a lot easier to do it now
than after you finish the job and it won't work.
After all of the parts and pieces have been put back together, your clutch should
work like new. Realize that it will feel different from the old clutch. The most
difficult time to get a clutch to work is when it is new. ALL of the associated
parts that operate the clutch must be in good condition for the clutch to work.
If you have problems check out the TIPS
section for some specific common problems to check. Most of the tips are specific
to Ford and GM products, but can be applied to any other make vehicle.