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Clutches: Getting a Grip


12/15/2008
By Larry Carley

 

When a customer needs a replacement clutch, what kind of clutch should you recommend? One that fits, works and feels the same as the original? Or one that can handle more torque and provide better all-round performance than the original?

The worst thing you can do is recommend a cheap replacement clutch, or one that’s too weak for what is being demanded of it. We’ve seen replacement clutches that don’t fit right, that make noise, that bind, that fail to release when the clutch pedal is depressed, or that slip when the pedal is let out, and that fail at low mileage because of cheap materials and workmanship. The biggest expense in replacing a clutch is labor, not the parts. So why scrimp on the quality of the parts? Recommend the best from a quality supplier you can trust.

Replacement clutch options include both new and remanufactured units as well as stock, heavy duty and performance clutches. There are different clutches for different purposes and budgets. So before you recommend a particular clutch to a customer, talk to them about how they drive their vehicle and what kind of life and performance they want in a replacement clutch.

New or reman?

In recent years, the price difference between new and remanufactured clutches has narrowed considerably, especially on passenger car clutches. The design of many late model diaphragm clutches makes them difficult and expensive to rebuild. Many clutch suppliers have found it is easier and more economical for them to offer new clutches for popular applications than to rebuild old clutches.

What you may not know is that all new clutches are not the same. Some new clutches that are imported from low-cost offshore manufacturers may not have the same quality or durability as the OEM clutches they replace. A cheap replacement clutch that seems like a good bargain may turn out to be no bargain at all.

Stock or better?

Some customers may want to upgrade their clutch if the original clutch needs to be replaced. If a customer only uses his vehicle for normal driving and daily transportation, a stock replacement clutch should work just fine. But if a vehicle has been modified for more power, is driven aggressively, or is used for towing, off-roading or racing, then a stronger, more durable clutch would be a good upgrade for the drivetrain.

Selecting a performance clutch isn’t as easy as it sounds because there are a lot of alternatives from which to choose. Most clutch suppliers offer a range of upgrade options including larger diameter clutches, higher clamp load ratings and better friction materials. Such products may be labeled as heavy-duty clutches, performance clutches or racing clutches. Some are designed for very specific types of uses while others may be suitable for a broader range of possible driving applications. In other words, you don’t want to recommend a racing clutch for a street application. The key here is to read the product descriptions and follow the clutch supplier’s recommendations — to the letter.

For example, if a clutch supplier says a particular clutch is for off-road or racing only, heed their advice. There’s no need to install a performance clutch in a vehicle that doesn’t really need the extra gripping power. Higher clamp load combined with more aggressive friction materials may increase the harshness with which the clutch engages. The more aggressive the clutch linings, the harsher the engagement. That’s not an issue on a race track, but for everyday driving it can get really tiresome in stop-and-go city traffic. The extra shock loading on the drivetrain also increases the stress on motor and transmission mounts, U-joints, CV joints and driveshafts.

On the other hand, if a customer has a car with a turbocharged or supercharged engine, nitrous oxide or other performance upgrades, and the extra power was too much for the stock clutch, a performance clutch is a must.
Some performance clutches have a larger diameter disc and pressure plate. Increasing the surface area of the clutch increases the friction area and the ability to dissipate heat. But bigger is not necessarily better. Some clutch manufacturers take a different approach and use a stock sized clutch with superior friction materials and other design enhancements to increase the torque capacity of the clutch. Or, they may use a smaller diameter clutch for performance applications because less mass allows the engine to rev and decelerate more quickly. Another approach is to use a multi-disc clutch that stacks several clutch discs sandwich-style to handle increased loads.
The key is to find a performance clutch that provides the best combination of durability, longevity and performance — and also fits your customer’s budget.

Clutch inspection and replacement

Reading the old clutch should help you choose the right replacement clutch. If the original clutch has simply worn out from the accumulated effects of mileage, a stock replacement from a reputable supplier should be the right replacement. But if the original clutch shows signs of abuse or severe overheating, upgrading to a “premium” replacement clutch or a street performance clutch may provide better durability and longevity.

Most of the wear that occurs in the clutch assembly occurs in the friction linings that are mounted on the clutch disc. So if the linings are worn, damaged or contaminated with oil or grease, the disc must be replaced.
Pay close attention to the condition of the springs in the center hub of the clutch disc. The springs provide torsional vibration dampening. If the springs are loose or broken, or the hub splines are damaged, the disc must be replaced. Also, inspect the input shaft splines for damage or wear.

An important point to keep in mind here is that oil leaks that reach the clutch can ruin a new clutch disc. If the rear main oil seal is leaking, the new clutch should not be installed until the oil leak has been fixed.

The pressure plate is another component that must also be replaced to restore like-new clutch performance. Most late model vehicles have diaphragm clutches that use a single diaphragm spring to push the pressure plate against the clutch disc and flywheel.

The clamp load on a diaphragm clutch actually increases for the first half of the clutch system’s life, then gradually decreases to its original level during the second half of its life. The spring itself usually doesn’t wear out but the area where it contacts the release bearing can wear. And if the spring is cracked or damaged, it can’t apply normal pressure to the clutch disc. Also, if the face of the pressure plate is worn, scored, cracked or warped, it should be replaced.

A new release bearing should also be installed when changing the other clutch components. The release bearing pushes against the spring fingers on the pressure plate to release the clutch.

On some vehicles, it pulls against the spring. Either way, if the bearing is worn, it can make noise every time the clutch pedal is depressed. If the old release bearing is reused and fails in a few thousand miles, your customer will kick himself for not changing it when he installed the clutch. That’s a lot of labor to have to do over — which gets very expensive if the installation was done by a shop that charges $75 to $100 or more per hour for labor.
Another item that must be replaced is the pilot bushing (if the vehicle has one). This is a small bushing in the end of the crankshaft or flywheel that supports the transmission input shaft and aligns the clutch disc to the flywheel. It’s used mostly on rear-wheel drive vehicles. If the pilot bushing is worn, the vehicle can experience rapid clutch and throw-out bearing wear as well as clutch engagement/disengagement problems.

The condition of the flywheel is also important, but is often overlooked. The flywheel does not have to be replaced when a clutch is changed, but it must be inspected for wear or damage that could have an adverse effect on the new clutch.

The surface of the flywheel must be smooth, flat and free from scoring or cracks. Most clutch suppliers recommend resurfacing the flywheel any time a new clutch is installed. If the flywheel is cracked, it is unsafe and must be replaced. Many new vehicles are equipped from the factory with a flywheel that has no machining allowance. So if the flywheel is worn, it must be replaced. Resurfacing is out as a repair option.

Some late model engines have a “dual mass” flywheel which is like two flywheels in one. A dual-mass flywheel is designed to dampen noise, vibration and harshness. If a dual-mass flywheel is cracked, damaged or the internal springs have failed, it needs to be replaced. If only the ring gear is damaged, that can often be replaced separately. Some dual-mass flywheels (Ford) can be resurfaced, but others (GM, BMW and Porsche) should only be replaced. Dual-mass flywheels are very expensive.

One alternative is to replace them with a conventional one-piece aftermarket flywheel. These are available for Ford and GM, but they require a different clutch set than the OEM dual-mass flywheel. Also note that a solid flywheel won’t “feel” the same as the OEM dual mass flywheel when the clutch is engaged.

The flywheel is also used to start the engine. Gear teeth around the outside of the flywheel are engaged by the starter to crank the engine. Missing or damaged teeth can interfere with reliable starting. If the gear is damaged, the flywheel should be replaced.

When removing a flywheel, the flywheel’s index position on the crankshaft must be marked so it can be reinstalled in the same position as before. This is necessary to maintain proper balance on some engines.


Release system

Installing the right clutch won’t make much difference if the vehicle has a problem with the clutch linkage. Most vehicles today have a hydraulic clutch release system with a master and slave cylinder rather than a cable or mechanical clutch linkage. A hydraulic system requires less pedal effort, can be snaked more easily through a tight engine compartment, and eliminates the need for bulky return springs and troublesome cable adjustment mechanisms.

When the clutch pedal is depressed, a rod connected to the pedal pushes against a piston in the clutch master cylinder. This pushes fluid through a hose to the slave cylinder mounted on or in the bellhousing. On most vehicles, the slave cylinder piston moves the release fork to disengage the clutch. But on some such as late model Ford pickup trucks, the concentric slave cylinder piston telescopes outward with the release bearing to disengage the clutch.

There’s no recommended change interval for the hydraulic fluid, but that doesn’t mean the system will last forever. The constant scuffing of the piston seals against their bores combined with the inevitable corrosion that affects all metal parts eventually causes one or both seals to leak. Loss of fluid or a seal failure in either the master or slave cylinder will prevent the clutch from releasing.

Diagnosis is the key to replacing the correct part. If the fluid level in the master clutch cylinder is low, it usually means there’s a leak somewhere in the system. If air has gotten into either cylinder, the system must be bled to remove the air.

If no leaks are found but depressing the clutch pedal fails to release the clutch, the piston seals in the master or slave cylinder might be leaking — or there is a mechanical problem with the clutch itself (broken release yoke, broken clutch components. etc.), or there is another problem such as firewall flex, a pilot bushing that is hanging up, an improperly adjusted master cylinder or a flywheel that has been resurfaced and is too thin.

If the vehicle has an external hydraulic linkage, and the release fork moves when the clutch pedal is depressed, but the clutch fails to release, the problem is not the hydraulic linkage but the clutch itself, a thin flywheel, a pilot bushing that is hanging up or a misadjusted master cylinder. No movement of the release fork would indicate a failure to build or hold pressure.

Isolating the components by connecting a hydraulic gauge to the master clutch cylinder outlet may help pinpoint which component (master or slave cylinder) is not holding pressure. If the pressure reading on the gauge rises and holds steady when the clutch pedal is depressed, the problem is a leaky slave cylinder seal. If the gauge shows little or no rise in pressure, or the reading drops as pedal pressure is maintained, the master cylinder piston seal is leaking.

On high-mileage vehicles, most experts recommend replacing both the master and slave cylinders if either unit has failed. This will minimize the risk of a repeat failure in the near future. On some vehicles, both must be replaced anyway because they come as a complete assembly.

On applications that have a telescoping internal concentric slave cylinder, the cylinder may have a plastic strap to hold the components together. This strap must not be removed. The strap is designed to break free the first time the clutch is applied.

When replacing a master or slave cylinder (or both), fresh brake fluid or the type of hydraulic fluid specified by the vehicle manufacturer should be used to refill the system (the fluid type is usually specified on the master cylinder reservoir lid). The system must also be bled to remove any trapped air. This can be tricky on some applications, and may require the use of a special bleeder tool to get a firm clutch pedal.

  Previous Comments
avatar   Marshall   star   3/2/2009   4:08 PM

Seriously guys, GET A GRIP! lol

















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