DeTomaso Mailing List: July 2000, Message #169

Herr Peter wrote:

>Does anybody know who sells a conversion kit for the=20
Clutch Slavecylinder,to make it a Hydraulic Throwout
Bearing.
I've seen it somewhere but don't remember where.

>>>Pantera Performance Center, in Colorado.  (303) 660-9897.

>It gets rid of the slave Cylinder and puts a Combination
Throwout bearing--probably from a Saab inside the 
Bellhousing.

>>>Actually, McCloud and Tilton both make these types of units; I don't know 
which brand Dennis Quella utilizes.

Since I tore my spare parts room to shreds (yes, I have an entire bedroom 
devoted to spare Pantera parts) looking for the heater valves a minute ago, I 
figured I'd dig out an old hydraulic throwout bearing and ZF actuating lever 
for purposes of illustration:

http://members.aol.com/mikeldrew/throwout.jpg

This bearing is a Brand X unit from my 427 Cobra, but the general physical 
characteristics are similar enough to the PPC unit for purposes of our 
discussion.

First, a brief description of the stock system:  When you step on your clutch 
pedal, this depresses a rod into your clutch master cylinder, which is merely 
a hydraulic piston with a fluid reservoir above it.  The rod moves the 
piston, and fluid is displaced by the piston into the single line which leads 
to the back of the car.

Back there, another hydraulic piston resides in a bracket bolted to the 
bellhousing; this is the slave cylinder.  The hydraulic pressure works 
backwards; instead of the piston moving the fluid, now the fluid moves the 
piston.  As the piston extends from the cylinder, it presses on an arm, which 
is connected to a shaft that disappears into the bellhousing.

>From there, it's all magic since most people don't have the benefit of a 
torn-apart gearbox to see what goes on.  

In fact, attached to that shaft on the inside of the bellhousing is a 
two-pronged arm (right side of the photo.)  That arm presses against the 
throwout bearing (basically a round steel donut) which in turn presses 
against the spinning fingers of the clutch, compressing the springs and 
releasing the clutch to enable you to smoothly shift gears.

This type of system was common automotive practice when the Pantera was being 
designed, but in more recent years, for reasons unknown (simplicity?  
effectiveness?) auto designers have shifted to an internal hydraulic throwout 
bearing.

Now, the hydraulic pressure from the master that had previously acted upon a 
slave cylinder, instead acts upon this rather complex hydraulic device, which 
is essentially a large-diameter hollowed-out circular piston with a throwout 
bearing grafted onto the end.  You step on the pedal, the fluid flows into 
the side of the device, then flows around the circumference and acts on the 
piston which pushes the throwout bearing against the clutch.

The purported advantages are smoother pedal feel, increased throw for a given 
amount of fluid movement, and the ability to point to your gearbox and brag 
to your Pantera buddies, "Look, I've got an internal hydraulic throwout 
bearing!"

(And yes, I've got one on my car.)

When these things work, they work great.  But when they don't work, it's a 
major catastrophe.  When a stock hydraulic slave cylinder blows up (and they 
do), you merely unbolt it, and either rebuild or replace it and then 
re-install it.  But when an internal hydraulic throwout bearing blows up (and 
they do), you have to pull the entire gearbox out of the car just to get to 
the damn thing!

These things are extremely sensitive to proper adjustment, too.  It's quite 
easy to mis-adjust them so the bearing rides against the clutch all the 
time--this leads to premature failure.  You can also mis-adjust them so that 
it's too far from the clutch--this requires the piston to move a great 
distance before it contacts the clutch; over-extension leads to seal failure, 
and after awhile the thing just blows itself apart (this is what happened on 
my Cobra, due to a fundamental mismatch of components by the guy who built 
it--it lasted about 3500 miles and four years before it folded.)

Not all of these bearings are created equal.  The brand X unit (not marked, 
unknown manufacture?) on my Cobra was designed for a multi-plate, on/off 
clutch as used by the roundy-round racers, and it had a total of about 1/4 
inch throw.  The clutch on a street car needs about 3/4 inch of movement, so 
every time I stepped on the pedal, one of the two internal seals was exposed 
to the outside world.  Eventually it just failed, with no warning--the pedal 
simply flopped to the floor when I went to shift gears.

Jack DeRyke, who is both very talented and slightly deranged, managed to 
adapt a generic Tilton unit to his car by machining up a bunch of necessary 
brackets and widgets, although as I recall he had to struggle once or twice 
with blowups, etc. before it finally worked, but it's been working fine for 
years.  

I've had one of Dennis Quella's on my car ever since it came back together, 
and it's managed to hold up under severe conditions--i.e. not being used!  
(As an aside, there are few things as damaging to a hydraulic system as 
stasis, so therefore when the car sat for years without moving under its own 
power, I'd make a point to jump in it every few weeks and pump the clutch and 
brakes a few times, just to keep the seals fresh.)

My advice:  If you want one of these things, order it from Pantera 
Performance Center and follow their installation instructions TO THE LETTER.  
It should give a lifetime of service, and they're guaranteed for life (they 
have gone through at least three design changes since Dennis introduced them; 
early units were prone to failure and he always did (and still does) replace 
them with new, improved units at no charge.)

Mike (completely geeking out on his new digital camera, but damn, my spare 
parts room is a disaster right now!)