Up To Standards
- Subject: Installation of manual-transmission clutches
- Essential Reading: Shop Owner, Center Manager, Diagnostician, R&R
- Author: Mike Weinberg, Rockland Standard Gear, Contributing Editor
Manual-transmission availability in late-model cars has been declining steadily as automatic transmissions have become the manufacturers’ favorite option. This is a byproduct of several issues.
The first is that the federal government, through the EPA and other agencies, doesn’t like sticks. The automatic transmission shifts at open throttle; the manual trans shifts at closed throttle. The automatic makes controlling emissions a lot simpler, as the driver has little control.
The second issue is production volume. The automatic usually is manufactured by either the carmaker (Ford, Chrysler or GM) or, in the case of imports, Aisin, ZF etc. In either event the huge volume of production provides economy of scale, bringing costs down considerably. This means that most domestic vehicles with manual transmissions are either sport models or work vehicles; the proportion of import vehicles with manual transmissions is larger, as the rest of the world still knows how to drive a stick shift. This will change as the newer models begin to be equipped with dual-clutch models shifted by computer without a clutch pedal. This design gets rid of the torque converter and reduces the weight borne by the crankshaft.
The clutch-driven vehicle will be here for the foreseeable future. The good news is that driver skills have declined, which makes clutch replacement a bigger share of your business.
Clutch inspection
Every time you remove a manual transmission for repair, it makes good technical sense and good business practice to remove the clutch for inspection. A large percentage of manual-transmission failure and damage is created by clutch slippage or release problems. The few extra minutes you spend removing the clutch for inspection will be offset by the larger number of clutches you sell and the drop in comebacks on manual-transmission repairs.
Another often-overlooked item is the flywheel and hydraulic or cable system that applies and releases the clutch. Many vehicles today are equipped with dual-mass flywheels, which always need to be inspected. A dual-mass flywheel has primary and secondary plates. The inner plate, which mates with the clutch disc, is spring loaded to help absorb harmonic vibrations resulting from the engine’s firing pulses. The flywheel is precisely matched to the damper mechanism on the clutch disc to filter out the engine harmonics to create a smooth clutch engagement and eliminate gear rattle and neutral-rollover noise in the transmission.
It is very important to move the inner plate against the outer plate on the dual-mass flywheel and note the amount of travel. Excessive travel and low turning resistance indicate a spring failure or wear on the dual mass and the need for replacement. Most of the major manufacturers supply a solid-steel flywheel and matching clutch set to replace a dual-mass setup, lowering the cost of repairs. The clutch disc has a much-increased damper section to compensate for the solid flywheel and must be used as a matched set.
Other components that need to be inspected carefully include the hydraulic release system, which consists of master and slave cylinders, clutch cables, fork, release (throw-out) bearing, fork pivot ball, and pilot bushing or bearing. A worn pivot ball or clutch fork will create all kinds of noise when the clutch is released and occasional noise or clicking when the clutch is engaged. The pilot bushing or bearing aligns the input shaft into the centerline of the crank. If the bushing has excessive wear the input shaft will have increased runout, which usually shows up in 4th-gear jump-out.
Most of the pilot bushings are made of oilite bronze material. The bronze is heated during manufacture and placed in oil. As it cools, the bronze absorbs the oil. Never lubricate a bronze bushing. To do so will cause the bushing to carbonize the lube and create a birdlike noise when the clutch pedal is depressed, or the input shaft will seize to the bushing and cause transmission damage, as the input will not stop turning during the clutch release. This beats the synchronizers to death in short order, as they are designed to slow the gears or speed them up to synchronize the shift but do not have the capacity to fight the engine torque. Pilot bearings should always be lubricated so that the rollers can turn freely with minimal friction.
Transmission inspection
Doing a first-class clutch job is really simple if you follow the rules. When the transmission is removed inspect the input splines for rust, straightness, and burrs or dings. Any missed items here will prevent proper movement of the disc on the input and poor clutch release or engagement. Pay attention to the quill tube, or input-bearing retainer, upon which the release bearing rides. Any wear or damage here will create release and engagement problems. On certain transmissions the quill tube is part of the case and not replaceable separately. In some instances repair kits are available to sleeve the tube, and it may be possible to cut the tube to a smaller diameter and add your own sleeve, restoring it to the correct internal bore for the release bearing. Most newer transmissions have the quill tube as part of the hydraulic slave cylinder, so this is no longer an issue.
If the ring gear is good on the flywheel, always resurface the flywheel on a grinder. Never use a brake lathe for this, as the hot spots on the flywheel will cause the lathe bit to skip and you will not get a good finish. The flywheel must be either resurfaced to be flat and concentric and parallel to the crank flange, or replaced.
Another quick thing to check is endplay on the crank. Do this with a pry bar on the crank flange to make sure the thrust bearing on the engine is good and the crank has no endplay. Be very careful in handling a clutch disc. Do not get your fingers on the friction material. The oil from your fingertips is enough to take some life off the disc. Handle the disc by holding the outside edges or the damper assembly. Always slide the disc onto the input shaft to assure free movement along the splines and to make sure the splines match. LIGHTLY lubricate the splines with some Neverseize to prevent rust and aid the movement of the disc. Lightly means just a little, as too much will be thrown off by centrifugal force and wind up on the disc.
Make sure that there are no oil leaks on the transmission input or front case. Oil is the instant killer of clutch friction material and a rapid failure that the manufacturer will not warrant. Always use an input shaft or clutch-alignment tool when installing the pressure plate. Tighten the pressure-plate bolts in a star pattern a few turns at a time to draw the pressure plate down on the flywheel. Failure to do so will bend the pressure plate. This is not a place for air guns. When the bolts are fully tightened the fingers of the diaphragm should be level and parallel to the flywheel.
Installation precautions
Use a jack when installing the transmission into the disc splines and pilot bearing. Never hang a unit on the disc, as you will bend the Marcel plate and have a crooked disc that will never release properly. Make sure the alignment dowels are in the engine block and are in good condition. A light sanding and some light lube will make the bellhousing easier to draw down flush to the block.
Misalignment problems create some hard-to-solve issues and a lot of wasted time. Look at the old disc you are replacing. If it is covered with a fine red dust, this unit has an alignment problem. Don’t ask me where the red dust comes from, but take it to the bank that if the clutch has a fine red dust on it the unit was misaligned.
Make sure no wires or clips get trapped between the bell and the block. If you have an alignment issue you will need to mount a dial indicator on the crank flange and check runout on the bellhousing, where the trans comes through. This is difficult or impossible on many front-wheel-drive units or transmissions that have an integral bellhousing, but you need to make sure that the trans is aligned properly.
Carefully adjust the clutch cable or fully bleed the hydraulics. On the hydraulics the best way to do this is with a vacuum bleeder hooked up to the slave-cylinder bleed plug. A number of designs are very hard to bleed because of positioning of the master cylinder. A good rule of thumb is to check under the hood and see whether the master cylinder is parallel to the floor. If it is not, jack up one end of the vehicle to make it so, and the bleed process will go quickly. Remember that 3 or 4 inches of clutch travel at the pedal will result in about 0.050 inch of movement at the disc.
Advanced clutch diagnostics
Some people can just swap parts and never be curious as how the originals failed; others need to know and learn. By looking carefully at the components that came off the vehicle you can tell a lot about how and why they failed.
The disc material may be burnt, glazed or oil soaked. The disc’s Marcel cushion plate may be bent or flat; it should have a wave in it to cushion application. Damper springs can be loose, broken or missing. Examine the damper-assembly stop pins for wear or damage. These pins limit the travel of the damper. If they are worn or bent the damper is over-traveling. This is caused by someone who likes to side-step the clutch at high rpm or downshift from 5th to 2nd without applying the brakes.
See figures 1-4 for problems and causes. After you have completed your repairs, take the customer on a test drive and let them drive. In some instances it will open your eyes. Some gentle instruction may be the difference between a good customer relationship and a comeback to be argued over. Remember, the driver controls the life of the clutch. Most older drivers will not be a problem; look out for the 18-year-old who is impressing his girlfriend.