by Brent Kiser, BSME, Purdue University, 1980
NOTE: This article can also be found on the Minnesota Triumphs web page.
Our wonderful TR-6s have a notorious reputation of troublesome clutches, which is not without merit but may not be deserved.
The symptoms of a troublesome clutch are an overly firm force required to push the clutch pedal to the floor, release bearing failure, shearing of the clutch operating fork tapered pin, seal failure in the master and slave cylinders, bursting of the hydraulic line and excess wear of the clevis pins. Also, the car can be difficult to shift due to the clutch not fully releasing. But the most common problem is that the clutch pedal sticks and will not smoothly move from the floor to the clutch engaged position.
I have unfortunately experienced most of these problems. I have replaced the clutch release bearing at least four times, the clutch master cylinder twice, rebuilt the clutch slave cylinder twice, replaced the clutch disk twice, replaced the release bearing pilot twice and installed four pressure plates, two of which were brand new aftermarket Borg & Becks. On the bright side, I can now replace a clutch in only four to five hours!
Not wanting to improve my record setting time for clutch replacement, I decided to get to the bottom of these problems. Being an analytical nerdy type, I started the investigation by observing who was or wasn't having these problems -- whether the racers who abuse these clutches were having problems or whether the Sunday drivers were having them too. Was it happening in early or late 6s or all of them? Most importantly, what common problem could cause all of the ills?
Having had the pleasure of working on several club members' TR-6s, I noticed the cars with the original clutches never experienced any problems. In fact, Jo Ann Broom, Jim Larson and Dave Swanson, who are the original owners of their cars, knew the clutch was original. All had wonderfully operating and reliable clutches. A coincidence? I think not!
Then, I observed us TR-6 owners who were having problems: Ed Wirtz, Wayne Morris and, of course, me. What did we have in common? None of us had original clutches. We all had aftermarket Borg & Beck replacements. I also consulted a Web site called Listquest.com, which is now defunct but was an electronic gathering place for car nuts. Of about 1,500 Triumph listings, about one-third to half of these were related to clutch problems with one common thread: all had replacement aftermarket Borg & Beck plates.
I knew the only way to get more information was to get an original Laycock pressure plate and compare it with an aftermarket Borg & Beck. So out to my garage and collection of antiques (my wife lovingly calls junk) to get a 30-year-old original pressure plate made by Laycock and an aftermarket Borg & Beck.
Original Laycock Cover Side Aftermarket Borg & Beck Cover Side
Original Laycock Plate Side Aftermarket Borg & Beck Plate Side
Looking at them, I noticed the two biggest differences were the spring diaphragm and the diaphragm pivot point.
The original Laycock has a solid, one-piece plate for all of the fingers of the spring diaphragm to pivot about when the clutch was released. Plus, the pivot point was outside of the point where the fingers of the spring diaphragm joined. This design creates a solid, single plain for all of the spring diaphragm fingers to rotate about.
Original Laycock Pressure Plate
The aftermarket Borg & Beck has two round rings held in place by several bent fingers on the cover plate as the fulcrum point for the diaphragm spring fingers. Plus, the fingers joined outside of the round ring.
Aftermarket Borg & Beck
Bottom line: The aftermarket Borg & Beck design would be almost impossible to mass produce with the exacting tolerances required to assure all of the fingers pivoted about the same plain. Having multiple pivot points at different plains would cause the binding and sticking seen on engagement of the pressure plate.
The other observation was the force required to push the spring diaphragm. In other words, release the clutch. By standing and bouncing on fingers of the Laycock, I could get them to move. But on the aftermarket Borg & Beck, even by jumping on the fingers, I could not get them to move. If I would have to guess, I think the Laycock clutch probably has a release force of about 225 to 250 pounds, where the aftermarket Borg & Beck is probably over 400 pounds. No, the Laycock pressure plate is not worn out. It was in my car and worked great. Plus, my engine is modified to produce about 120 to 140 hp. Since then, I experimented with a "Magic Clutch Kit" Sachs pressure plate. More on that later.
Even after all this research, I still was not ready to conclude the problem was the aftermarket Borg & Beck pressure plate. So, back to the World Wide Web.
I discovered the aftermarket Borg & Beck pressure plate is made by a firm in England called AP Automotive Supply and that they had an office in the U.S. Time to let my fingers do the walking at (248) 377-6999. I reached a fine chap named Paul who was interested in my problem, especially when I explained there were more than 92,000 TR-6s produced and I had cause to believe his product was a major pain for us owners. Well, he patiently listened to my observations and wrote down the Borg & Beck part number (HE5132Q) from the pressure plate, which I recently purchased from The Roadster Factory (TRF P/N 214321).
Paul did not have the technical resources for these foreign cars, so he had to call England to search for answers. As promised, he called back a few days later and confirmed my suspicions. The pressure plate I had was recommended ONLY FOR TR-6s WITH ENGINES WITH INCREASED HOURSEPOWER. Personally, I think this is a bunch of crap. This pressure plate was designed for a Saab, not a TR-6 and should not be used for any Triumph. My guess is some guy thought a higher spring force is always better, but did not think about the rest of the clutch system (i.e. release bearing, taper pin, hydraulics, etc.).
My belief that a higher spring force is not required is confirmed by the Triumph Competition Preparation Manual. The manual states:
"The majority of our testing was accomplished with the standard clutch and flywheel units. The standard clutch is of the diaphragm type and completely adequate for the job of racing with increased power."
From Triumph Competition Preparation Manual , Jaguar Rover Triumph, Inc., 2 nd Edition, 1980, page 17.
The pressure plate AP Automotive had listed for the TR-6 was a P/N HE4787Q, which has a lower spring force than the plate I had.
There was only one thing left to do to close the book on this mystery. Form a hypothesis for each problem and see if the problems can be related back to the pressure plate.
Yes, an excessive spring force on the pressure plate will cause the clutch pedal to be difficult to push.
From driving several original, unmolested TR-6s, I believe the clutch pedal force should be around 15 to 25 pounds. This force is the about the same as my wife's 1991 Ford Probe or any other modern car. The aftermarket Borg & Beck required at least two to maybe even three times that force.
Yes, if a bearing is overloaded it will fail. Boy, real rocket science here.
Again, not real difficult to see the correlation with the increased pressure plate force to the failure of the tapered pin. If more force is required to release the clutch, more shearing forces will be placed on the pin as the operating fork rotates.
Seals are designed for specific operating pressures by varying the thickness and fit of the seal support. The higher the pressure the more solid the seal support. Also, smaller clearances between the piston and cylinder are required to prevent the seal from being squeezed into the clearance. If the operating pressure is doubled or more and the seal is not designed for it, it will fail.
Also, the length of the piston must be considered in higher pressure applications, since high side loads will increase piston to cylinder bearing stresses. If the bearing stresses reach the yield strength of the piston and cylinder, excessive and rapid wear will occur. This is the cause of the brake fluid turning black when the aftermarket Borg & Beck pressure plate is used (aluminum oxide from the master cylinder).
Duh?? You mean a plastic line will burst if excessive pressure is placed upon it? Enough said.
Let me see, the shear or bending forces on the pin double or even triple. Nobody ever greases these things like the maintenance manual requires and then they fail.
This is the most difficult problem for us car nuts to understand.
The increased spring force of the pressure plate requires a greater effort on the clutch pedal and subsequently increased hydraulic pressure. Well, the only way to get greater hydraulic pressure is to push on the clutch pedal.
When the pedal is initially moved, the hydraulic pressure is zero. Then, as the pedal continues to move, the pressure increases until it equals the force required to overcome the clutch disk spring.
On an original Laycock clutch pressure plate, the pedal does not have to move very far to create the pressure required to release the clutch. Subsequently, the clutch releases farther.
But on an aftermarket Borg & Beck, the clutch pedal must be moved significantly further to create the pressure required to start releasing the pressure plate. By the time the clutch starts to release, the clutch pedal is only about 1/2" away from the firewall and cannot be moved any more to fully release the clutch.
Since the clutch cannot be fully released, the transmission is difficult to shift.
The sticky engagement of the clutch, I believe, is due to the pivot point of the spring diaphragm not being along the same plain.
My recommendations to any TR-6 owner who has an original clutch assembly and is considering replacing their clutch is to replace only the release bearing and clutch disk. Keep the original Laycock pressure plate.
Update, Summer 2001
Original Borg & Beck Pressure Plates
Since writing this article a year ago I have had several people tell me the Borg & Beck pressure plate was an original piece of equipment and theirs works wonderfully. Well, this was a mystery to me so I did some more investigation.
I got a hold of an original Borg & Beck Pressure Plate and observed the following.
Original Borg & Beck Cover Side Original Borg & Beck Plate Side
It became obvious at first glance that the original and aftermarket Borg & Beck pressure plates are not even close to the same design. The original Borg & Beck is what I call a "Pin Design" pivot point versus a "Bent Finger" pivot point.
Original Borg & Beck Aftermarket Borg & Beck
As you can see in the pictures, there is a great difference between the original and aftermarket units. It is no wonder that people have been telling me their original Borg & Beck work great and I have claimed the Borg & Beck does not work. We were talking about two different designs!!
Since discovering this difference, I have installed over six rebuilt original Borg & Beck pressure plates and have not had any problems. I strongly recommend the rebuilt original Borg & Beck pressure plate.
The "Magic Clutch Kit"
One day I decided to replace my good old original Laycock pressure plate since it was 30 years old. Besides, there is not much to do during the winters here in Minnesota and I wanted to do something with my car. What a big mistake!
I tried one of the "Magic Clutch Kits" with a Sachs pressure plate, Borg & Beck disk and a Toyota release bearing. It had the same problems as the aftermarket Borg & Beck!!! It had an overly stiff feel and when engaging the clutch, the pedal sticking returned. In addition to the pressure plate problem, a new problem occurred. The Toyota "heavy duty" bearing "squealed" during the initial engagement of the clutch. The car was driven this way until I could find a NOS Laycock clutch assembly and bearing.
In addition to my own problems with the Sachs pressure plate, I have replaced a Sachs pressure plate in another car with an original rebuilt Borg & Beck pressure plate due to excessive pedal force and the car could not be shifted.
When you look at the Sachs pressure plate you will see it is like the original Borg & Beck design with the pin type pivot point.
"The Magic Clutch"
Sachs Cover Side Sachs Plate Side
Sachs "Pin Type" Pivot
The design of the Sachs is the better than the aftermarket Borg & Beck it still has too strong of spring. I cannot recommend the Sachs unit.
When the Toyota bearing was removed from my car the cause of a squealing which occurred when the clutch was initially engaged became obvious. It was caused by the contact between the bearing and pressure plate. Both parts showed excessive wear for having only a couple of thousand miles. I noticed the original bearing was a SHIELDED bearing the Toyota bearing was a SEALED unit. The additional friction drag of the seal did not allow the bearing to turn with the pressure plate when the clutch was engaged. Therefore, when disengaging the clutch, the bearing had to be sped up to match the speed of the pressure plate. This differential speed caused the squeal and excessive wear.
By pure luck, I found a NOS Laycock clutch assembly and it is now in my car and has well over 10m miles on it. I drove the car to Colorado from Minnesota for the 2001 VTR convention with the Laycock assembly. It still works beautifully, is smooth with a light pedal feel and has NEVER slipped even in the mountains of Colorado.
Since writing this article I have had the pleasure of replacing at least six other TR6 clutchs. ALL, every one of then, were experiencing the problems described here and ALL problems have been solved on EVERY car. All cars had the aftermarket Borg & Beck pressure plates for the Saab.
I can strongly recommend using the following to fix your clutch. They are listed in order of preference.
IMPORTANT!! If you do find a NOS Laycock assembly the pressure plate must be disassembled and all moving surfaces greased with silicone brake grease. I've seen clutch chatter problems due to old grease not allowing the pivot surfaces to move.
Please keep the feedback coming on this article. Without the feedback I would not have discovered the differences with the original and aftermarket Borg & beck. Thanks to all who took the time to call or e-mail.
Phone: (651) 490-0923