Clutch and Release Bearing

Clutch

 Courtesy of Gary Morrell

OK, its time for some clutch education here.

First, some SHO clutch history: When the SHO was about to be introduced in late 1988, the powertrain engineers went to upper management and expressed concern that the 9.25" clutch was a weak spot and would likely become a warranty headache. Management said don't worry about it. By mid 1990, the warranty pipeline was full of slipping SHO clutches and Ford ponied up a bunch of N.R.E. money to Valeo to make a better clutch for the application, the only caveats being that the bell housing could not be enlarged and that retrofit to earlier SHO's would be possible.

The result was the 9.75" clutch that debuted in the 1991 SHO's. Here's what was changed: 

  1. The diameter of the disk and pressure plate was increased by 0.5", increasing the total friction material area by about 22%, accounting for both sides. 
  2. The stamped steel cover was replaced by a much stiffer ( and heavier ) cast iron cover. A stiffer cover is better at keeping clamp loads consistant. 
  3. The clamp load on the friction materials was increased by about 40%. 
  4. The cover was located on the flywheel by 3 steel dowel pins, in addition to 6 bolts, making for a much stronger assembly, this necessitated changing the flywheel. 
This clutch is an orphan: it has NO other applications, and shares parts with NO other clutches in the Valeo line, which BTW, is quite extensive, comprising large OEM markets in Europe and North America. Consequently, the only place to buy parts for this clutch is from Valeo, its distributors, or from Ford. If you buy it from Ford, the box says 'Made in France', which is where most of Valeo's manufacturing is located. In fact, Valeo does not sell parts, only a complete assembly, consisting of cover, pressure plate, and diaphram, along with a separate disk assembly. Fitchel and Sachs, a major German clutch manufacturer, sells a clutch for the SHO, their applications people freely admitted to me that they just buy the Valeo clutch and rebox it.

There are several ways to improve on the current clutch: 

  1. Change the friction materials from organic to some other material, such as semi-metallic ceramic (Steve Hazard's Grizzly Paw), or Kevlar (SHO Shop, Grizzly OEM Replacement). Both of these materials have higher coefficients of friction than the stock material and will therefore be able to transmit more HP without slipping. A minor downside of these materials is that they can be quite 'grabby', making smooth engagement and takeoff quite a learning experience. They will also wear the pressure plate and flywheel surfaces faster. Additionally, high tech friction materials are quite a bit more expensive. There is no free lunch. 
  2. Increase the clamp load on the friction materials, this also will increase the HP handling ability of the clutch, with a slight increase in pedal effort. There are several ways to do this: 
    1. changing the diaphragm pivot points on the pressure plate (kinda' hard, requires re-machining the pressure plate), 
    2. changing to a stronger diaphram spring; to my knowledge, and Valeo's, one is not available, or, 
    3. installing a shim between the pressure plate and the diaphram, This is OK to do as long as the 'drop' measurement of the diaphram fingers doesn't go too deep, if it does, then the release bearing can't stroke the diaphram far enough to fully disengage the pressure plate and release the clutch. 
  3. Increase the strength of the torque straps and rivets, this is where most of us with high HP cars are tearing things up. The torque straps are what transmit engine torque from the pressure plate to the clutch cover, they allow the pressure plate to move in and out to engage/disengage the clutch, and they locate and center the pressure plate within the cover. High RPM shifts can de-head the soft steel rivets that secure the straps or pull the rivets right out of the pressure plate iron. Aggressive friction materials also create problems because they drastically increase the shock loads on the straps and rivets. Huge straps are not the answer either because the strap stiffness and spring constant must be properly matched to the diaphram strength and pressure plate mass, otherwise the clutch could chatter on engagement or the straps would fight the diaphram, reducing the clamp load. Steve Hazard's fabricator has changed to a very strong, stainless steel rivet, which seems to have fixed the rivet strength problem. 
Hopefully, this has not bored you but has increased your appreciation of the pain and development expense that Vadim and Steve have gone through to improve the stock SHO clutch. Either of their offerings will certainly last longer and tolerate more HP than the Ford replacement.
 

Release Bearing

 There are essentially two release bearings (aka throw-out bearings) available for the SHO: the OEM Ford bearing and the NESHOC Grizzly Paw bearing.

Ford Bearing

The original stock bearing used in the earliest SHOs rode on the aluminum quill shaft in the transmission case. This steel bearing would gall the shaft, causing the bearing to bind up and ruin the clutch assembly.

In 1993, Ford updated the bearing to include a steel sleeve that slides over the aluminum quill shaft, providing a steel surface for the steel bearing to ride against. Unfortunately, it appears that under some use conditions this bearing can become clogged with clutch dust and binds up, causing the bearing to bind up and ruin the clutch assembly. This revised bearing is a dry lubricated ball bearing with a plastic separator for the balls. Extended high RPM operation will cook this bearing, it gets hard to turn, and buzz-saws its way thru the diaphragm fingers, ruining the clutch assembly.

NESHOC Grizzly Paw Bearing

This bearing is alleged to solve all of the problems, the earlier models had some failures, but the current version of it is highly touted by the vendors.  It is a high-speed sealed, greased ball bearing without ball separators.