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The Basics 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.

  • FLYWHEEL -- 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 or 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.
  • OTHER 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.
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.
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.

 
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