[stallion112]

I work for one of the companies that design, validate, and manufacture clutch release bearings for the OEM and aftermarket so I may be able to shed some light on this subject.

The primary factor that determines whether or not a clutch release bearing is preloaded or not, is the design of the bearing.

A century ago, bronze or carbon cartridges were mounted in a cast iron or forged trunnion. This design resulted in a high level of wear.

In the early 1930’s, anti-friction ball bearings began to replace the bronze and carbon cartridges. By the mid to late 1930’s, a fixed guide tube was introduced to help stabilize the clutch release system.

Most of the improvements in the 1940’s and 50’s were a result of better greases and seal materials.

In 1967, a patent was issued for a self-centering clutch release bearing. This enabled the bearing to find its own axis of rotation with relation to the clutch diaphragm spring. This type of bearing design requires a light preload in order to maintain concentricity of the rolling elements and raceways as an assembly. With the previous designs, when the bearing assembly was unloaded as the clutch pedal was released, the relationship (concentricity) of the two raceways was lost. What this meant was that each time the pedal was pressed and the bearing was loaded, the raceways (and rolling elements) were on their own axis of rotation during the initial operation of the bearing. Eventually, concentricity of the rolling elements and the raceways would be restored but only after sliding and friction (wear and heat) had occurred.

The result of the self-centering release bearing was a reduction in bearing operating temperatures by 25%, and an increase of service life by 400% over then-current designs. This type of a bearing was first used on several high-volume European models in the late 1970’s – early 1980’s. Its introduction to the North American market was in the late 1980’s – early 1990’s.

Long gone are the days when a part can be swapped from one vehicle to another and be expected to work with a high level of confidence. For many years, OEM’s have developed each part and assembly around a very narrow set of design criteria that are based on a drivers perceptions, service life, governmental regulations, etc. In the case of clutch release bearings that have been designed to operate with preload, certain Ford applications in particular, have been used successfully for many years. There have been problems with the release mechanism itself such as the one used on the old Fox body Mustang’s but, that design has been obsolete for years.

When the Ford technicians say that the clutch release bearings on certain models are designed to operate with preload and no clearance, they are 100 percent correct. This statement obviously does not apply across the board to every clutch release bearing on the market, particularly those used in the aftermarket. Again, whether or not a bearing operates with clearance depends on the way the system is designed, more specifically, which type of bearing is being used. For example, there have been comparisons to wheel bearings in previous postings to this thread. It's a good point. Keep in mind that wheel bearings are designed to be in constant rotation. The old release bearings were not. That's why if they were in constant rotation it wasn't long before they would grenade.