Jim Smart
February 12, 2014

Automotive clutches have been in use for more than a century, including the Mustang for 50 years. When the Mustang was introduced in 1964, manual three-speed and four-speed models used crude three-finger Long Style clutches with six stiff return springs, which meant a hard clutch pedal, excessive wear and tear, and driver fatigue. Over the years, clutch technology has improved significantly to yield a kinder, gentler pedal. New Mustangs are easier to drive thanks to improved clutch design including hydraulic release. For classic Mustangs, options from the aftermarket, including cable and hydraulic release, have made them a lot more enjoyable to drive.

For Mustang driving enjoyment, whether classic or late model, you need to know how to choose the right clutch and release system. We met with Bruce Couture of Modern Driveline for a look at Mustang clutches and release systems.

Clutch Basics

A clutch is designed to engage and disengage the engine and transmission for start off, shifting, and stopping. The pressure plate applies a powerful spring-loaded clamping force on a friction drive disc that connects the engine's crankshaft and flywheel to the transmission's input shaft and, ultimately, to the driveshaft and drive axle.

Bruce tells us there are basically two types of clutch pressure plate designs—three finger with coil springs (also known as Long Style) and diaphragm. Although the term "Borg & Beck" has also been used through the years, it is little more than a brand name for a Long Style clutch.

Long Style clutches offer excellent clamping pressure, but they are old technology and require a lot of leg muscle, which can make them unpleasant for daily driving. They're also hard on the clutch release mechanism, engine thrust bearings, and crankshaft. Vintage Mustang Z-bar clutch linkages will bend or break if you use a 3,000-pound Long Style racing clutch.

Diaphragm clutches are more common in '74-up Mustangs. Consisting of a Belleville spring diaphragm to load the clutch disc instead of coil springs, a diaphragm clutch offers greatly reduced pedal effort, making it user friendly for the street. Centerforce Clutches are diaphragm-style with flyweights attached to the fingers for comfortable pressure during normal driving but increased pressure at high rpm.

Mustang clutch release mechanisms were cable from '74-'04, then hydraulic release from '05 to current.

Not all clutch pressure plates are created equal, Bruce tells is. When you're shopping for a clutch, you want a ductile iron pressure plate, which is stronger and safer than traditional cast iron. It's what you want if you're going to spin your engine to 7,000 rpm.

Clutch Disc Facts

There's more to a clutch disc than simple engagement and power transfer. A clutch disc is engineered to slip as the driveline and vehicle catch up to engine speed. Examine a clutch disc and you will see how this happens. A clutch disc consists of a hub, a drive plate with friction material on both sides, torsion springs, and drive limit pins. Torsion springs act as shock absorbers during clutch engagement where the hub and drive plate interact with one another with an elastic rebound. If dampening springs are weak, you can expect limited pin contact and clutch chatter.

Another clutch disc dynamic is called Marcel, or "Marceling," which is the amount of disc compression that occurs when the clutch is engaged. Disc compression is the result of the wafer-like Marcel spring action that occurs between the clutch frictions. There is a greater amount of Marcel in an organic clutch disc than in a Kevlar or metallic clutch disc. Choice depends on how you want your clutch to engage. Smooth engagement calls for organic or Kevlar. Quick engagement requires metallic, which performs better in high performance use. Kevlar engages smoothly but applies a firm grip for performance driving. Street clutches with organic or Kevlar friction materials engage more smoothly than those with metallic frictions on both sides.

The type of friction material you use depends on your type of driving and how you want a clutch to hook up. Organic clutch frictions slip and compress more liberally than Kevlar and metallic. One popular concept employs two types of friction material (metal and organic) in segmented pucks on the flywheel side of the clutch disc. Because the pressure plate side of the disc engages first, this is the side more significant to the friction material used.

Metallic clutch disc frictions tolerate heat better than organic and Kevlar. However, metallic clutch frictions connect the flywheel and transmission input shaft more aggressively, which works well in racing but will jar your teeth in street use. Kevlar is a good compromise between organic and metallic because it stands up to heat while yielding the slippage for the street. Kevlar is also good for the weekend racer.

1 Classic Mustangs through ’73 were equipped with three-finger Long Style clutches. Many enthusiasts also use the term “Borg & Beck,” which is a brand name for the Long Style clutch with wider fingers.
2 The three-finger Long Style clutch pressure plate has a reputation for firm engagement without slippage, which makes it excellent for drag racing. However, stiff coil springs and eccentric-style disengagement fingers makes for extreme clutch pedal effort.
3 This is the Superior diaphragm clutch from Modern Driveline with a metal and Kevlar disc combination for high-performance street and strip use.
4 A diaphragm clutch employs a Belleville spring, which is supported by pins and fulcrum rings. There are no coil springs in a diaphragm clutch because the diaphragm serves as the spring. The release bearing is a constant duty part that rides against the diaphragm and spins all the time.
5 This is the clutch disc side of a Superior diaphragm clutch with an organic disc for street use. Five torsion springs around the hub act as shock absorbers for smooth engagement.
6 Examine this organic clutch disc and it becomes clear how a clutch disc functions. Friction material is riveted to the drive plate, which is suspended around the hub. The springs act as dampeners, or shock absorbers, during engagement to prevent clutch chatter and abrupt engagement. Organic clutch friction material is best for street use. Daily commuters that are raced on weekends need Kevlar frictions.

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Clutch Installation

Bruce tells us that a successful clutch installation calls for selection of the right parts and a methodical approach. Take clutch installation step by step and make sure you have everything you're going to need going in.

When replacing a clutch and flywheel, take time to inspect for leaks and other irregularities. Also, inspect the engine’s rear main seal and pan gasket for leaks and correct before assembly. And remember, you cannot patch up a leak from the outside. If the pan gasket or rear main seal are leaking, you must disassemble and replace.
Flywheel to crankshaft bolts must receive a thread sealer because these bolt holes go right into the crankcase. Always use new bolts because old bolts stretch and should not be reused. While you’re at it, install a new clutch pilot bearing.
When installing the clutch, use a clutch pilot installation tool to center the disc on the crankshaft and pressure plate. Leave the pilot tool in place until the clutch mounting bolts have been torqued to specification.
Before installing the transmission, check clutch fork installation and alignment. The release (throwout) bearing and pivot ball should be lightly lubricated at the pilot bearing and on the input shaft bearing sleeve. Make sure it is secure on the clutch fork.

Offset Balance

Because small-block Fords are externally balanced, you need to know which generation of small-block Ford you are working with when ordering a flywheel. Early small-block Fords (221/260/289/302) prior to '82 used a 28-ounce offset balance flywheel. When Ford gave the 5.0L small-block more connecting rod and a one-piece rear main seal for the '82 HO, it had to add more offset weight—totaling 50-ounces—and calling for a 50-ounce offset flywheel and harmonic balancer. If you get this backwards with a 28-ounce flywheel on a 50-ounce engine, or vice-versa, you can count on vibration that will jar the disc jockey's teeth out of your FM stereo.

This is a 28-ounce offset balance flywheel for 221/260/289/302ci engines built prior to ’82.
Here’s a 50-ounce offset flywheel with a wider 50-ounce offset weight. Notice how much larger this offset is compared to the 28-ounce.

Clutch Release Systems

Choosing the right clutch release system is as important as choosing the right clutch. There are three basic clutch release choices—mechanical, cable, and hydraulic. Each has its benefits.

Classic Mustangs were factory equipped with a Z-bar style mechanical clutch release, which never worked very well because they were not engineered well for the job. They are hard to operate, especially with a high-performance Long Style clutch with steep spring pressures. If you're going to go with the factory Z-bar system, opt for the Muscle Z-Bar from Modern Driveline, which is engineered to take on the most powerful Long Style clutch out there and operate smoothly thanks to the use of Heim and ball/socket joint technology.

7 Here are three types of clutch friction discs, from left to right — metallic segmented, Kevlar, and organic.
8 The McLeod RST Twin-Disc clutch from Modern Driveline is designed for high-performance street use and employs organic clutches for smooth yet rock solid engagement. You get twice the friction without the harshness.
9 This is a good example of “Marcel” or disc compression. Marcel is the amount of clutch disc compression that occurs when you release the clutch pedal. Organic clutch frictions compress more than Kevlar or metallic, which makes them more suitable for street use.
10 Modern Driveline offers a variety of steel, aluminum, and steel/aluminum combo flywheels for all Mustang generations and applications. Replace your flywheel if scoring is excessive and thickness goes beyond factory specifications. Closely examine the starter ring gear teeth for damage and replace if necessary.
11 Bruce strongly recommends bellhousing alignment using a dial indicator to get your bellhousing centered on the crankshaft’s centerline. This makes for easier transmission installation, enhances shifting, and eliminates damaging side loads on the transmission input shaft and clutch pilot.
12 The Mustang’s original Z-bar clutch release system was cheap and effective, but wore out quickly. Binding points and poor mechanical advantage make it a poor choice for your clutch release system.
13 Here are three different clutch equalizer bars, also known as Z-bars due to their shape. From left to right are ’65-’66 V-8 original, reproduction, and new-old-stock. None are up to the task of a powerful high-performance Long Style clutch because they will bend and fold over under the load.
14 Not only is the factory Z-bar clutch linkage not up to the task of a stiff high-performance clutch, it is not as adjustable as it should be.
15 There are two basic types of clutch forks for classic Mustangs—wire style (left) and slide on (right). Slide on is the better of the two, which first saw use in 1967.
16 This is the Muscle Z-Bar fully adjustable clutch linkage for classic Mustangs from Modern Driveline. It will not fold over under the high stress of a stiff Long Style clutch. The beauty of the Muscle Z-Bar is adjustable Heim and spherical joints throughout for a minimum of frictional losses.
17 Modern Driveline’s clutch cable kits for ’65-’04 Mustangs are fully adjustable and require virtually no maintenance throughout the life of the cable.
18 This is Modern Driveline’s ‘71-‘73 Mustang Clutch Cable Kit, which is fully adjustable and offers buttery smooth operation. When you combine the clutch cable package with a Superior diaphragm clutch and flywheel, clutch operation will never feel the same again.
19 Easily the most dramatic change in vintage Mustang clutch operation comes from Modern Driveline’s Hydraulic Clutch Conversion Kits, which use the power of fluid under pressure to make light work of clutch operation. This is a bolt-on swap you can accomplish in your garage over a weekend.
20 This is the clutch pedal helper spring, which reduces clutch pedal effort. When you install a cable or hydraulic clutch release system, remove this spring.
21 Solid clutch and brake pedal support and function come from Scott Drake’s Pedal Support Roller Bearing kit #C5ZZ-2478-RB, which can be installed in an afternoon. This kit gets rid of the factory’s pin and bushing approach. Once you’ve installed the Scott Drake roller bushing kit, you will never have to worry about sloppy pedal function again.