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Can this throw-out bearing question be answered once and for all??

5K views 10 replies 4 participants last post by  P49Y-CY 
#1 ·
Is this really too much to ask?? lol Here is a quote from the 'official twin disc clutch installation' on the TTOC:

"So, we used an adjustable pivot stud. First we tried -2mm (lower than stock height). This gave us some play, but not enough. At -4mm lower than stock height, it was perfect. The stock height of the pivot ball for us was 37.5mm. We lowered that height to 33.5mm. With this adjustment, we got .125 thousanths (1/8") of play in the clutch fork. This way the throw out bearing doesn't ride on the pressure plate fingers."

Clutch choice notwithstanding, I have read people swear the TOB should not contact the fingers of the PP as is stated here. Others vehemently disagree claiming ours is a constant velocity style TOB and should just rest on the fingers of the PP when the clutch pedal is up and the clutch disk(s) are seating against the flywheel with the input shaft engaged. I have chosen to side with Helomech and others who follow this maxim and say yes, it is a CV style bearing and should rest on the PP fingers with the clutch engaged. He went so far as to say clutch freeplay mods are worthless; stay away from them. With mine set up the way Helomech describes is correct, I've never had a single issue but I don't guess that automatically makes it 'right'. I have the trans outta my car for unrelated issues but thought I would try and get some final resolve for when I get it back together.

What is the (hopefully) final verdict on this??
 
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#2 ·
I am on the constant velocity wagon for an OEM TOB. The factory self adjusting mechanism does not pull the TOB off of the pressure plate fingers (at least in my experience), so I have to assume that it was designed to function that way.

As for aftermarket TOB's, I cannot say. It would be worth reaching out to the manufacturer and get their input. I have had good luck with OEM TOB's and Ram TOBs. I had an intermitant squeek with a mcleoud one, and the centerforce one I used was junk within 200 miles. I have always adjusted my clutch cable (Fiore Quadrant/Adjuster) to set the pedal height. This does put the TOB in contact with the PP, but there is not a lot of pressure there.
 
#4 · (Edited)
Jeff, the CV style is OEM and just works. Where guys get into trouble with the CV approach is when they do not leave room for clutch disc wear. As the discs wear the cutch fingers will rise. There needs to be adequate room to accommodate the finger rise attributable to the inevitable disc wear.

The problem arises when the TOB is adjusted to contact the clutch fingers and there is no provision made for disc wear. As the discs wear, the fingers will rise. If the TOB does not have sufficient room to also move (backward), the fingers will begin to apply a continuous pressure to the TOB. While designed to operate in what you describe as a CV state, TOB's are not designed to operate that way and be continuously under pressure from the clutch's pressure plate release fingers. This continuous pressure from the pressure plate release fingers will kill the TOB in short order. It will also smoke a clutch because the pressure plate load is now less than spec (because the fingers are not fully released) and the clutch will slip and burn up.

So how to set it up correctly? You should do the following: find a pivot ball height that will position the TOB face parallel to your clutch fingers approximately half way through the clutch finger releasing motion. You don't have to be exact, this is a good place for Tennessee Elevation and Kentucky Windage — your eye balls will work just fine.

If your TOB continually adjusts it's parallelism throughout its travel (as virtually all OEM units do) then all you have to do is make sure it can accommodate the full range of motion it will have to go through from fully engaged to fully released without binding. With the clutch fully engaged (released) adjust the clutch cable to allow the TOB to float ~1/8 of an inch above the clutch fingers in this fully released position. Install your clutch return spring and adjust it to pull the TOB arm forward bringing the TOB into contact with the clutch release fingers.

You are basically done. If you want to get fancy you can jack up the rear of the car and have a friend help you to locate the clutch pedal position at which the clutch releases and the tire turns. This is the same approach as the HYD TOB guys use. When you find that spot you install a mechanical clutch stop on the clutch pedal to allow full release without pushing the TOB over center and damaging the pressure plate's diaphragm spring. Once you find the release point, it is good to put one to two more turns of travel on the stop just to make certain the clutch reaches full disengagement in service. If it does not then the transmission will be difficult to shift.


Ed
 
#7 ·
Don't forget to leave room for disc wear ...

Ed
What do you mean when you say clutch return spring? The only way I am able to get the slight bit of pre-load on the PP fingers with the pedal up is with my firewall adjuster. This may not leave room for disc wear but I am pretty anal about checking it regularly. I've never had a problem in the past.
 
#8 ·
The clutch return spring is normally built into OEM linkage, Jeff. On a cable clutch it may be a spring on the cable somewhere. I use a hydraulic TOB in an early model Cobra so I don't have an 03/04 example to look at. Most parts stores sell replacement springs. The only purpose of the spring is to gently bring the TOB back up against the clutch release fingers when your foot is off the clutch pedal.

The firewall adjuster could likely also do that except it would not provide the freedom of movement you need as the discs wear. That is OK though as long as you periodically reset the firewall adjuster. The easier was is leave the 1/8" clearance for disc wear and put a return spring on the throw out arm to pulll it back against the fingers when you take your foot off the clutch pedal.

You are not looking for valve spring class return springs you're looking for something like one of the old throttle return springs. That's all that is required. They are readily available at your local parts stores.


Ed
 
#9 ·
The spring/pawl set-up like what is factory...is that what you are talking about? I've had an aftermarket aluminum clutch quadrant on my car forever so that mechanism is long gone. And i don't recall ever seeing a spring on the cable itself. Hmmmmm....oh well, not a sticking point. No biggie. The firewall adjuster does the trick. Just gotta keep an eye on it ;)
 
#10 ·
The firewall adjuster will work just fine, Jeff but you must keep an eye on it. When the discs begin to wear and the fingers rise, if the firewall adjuster does not allow the TOB to move back to compensate for the wear you will quickly kill the TOB and potentially shorten the clutch life. A throttle return spring at the parts store to pull the clutch fork forward is about $2. Just saying ...


Ed
 
#11 ·
this is a somewhat old thread but i think that ed did a terrific job in explaining the mechanics of it.

http://www.modularfords.com/threads/218641-McLeod-RXT-adjustment?highlight=adjustable+pivot+ball

i have referred back to it on several occasions recently, as i was a person who did not realize the importance of the fork placement, and thought i could get away with the stock pivot ball with my aftermarket twin disc clutches. however, i burned through a few clutches between both my cars in short order because of it, just like ed described above. but i guess these are the types of mistakes an amateur hobbyist like myself will invariably make and tries to learn from them. but man-o-man do i have alot to learn haha

i have since put adjustable pivot balls in both cars and it is true that you can really see the difference as far as having enough room for the fingers to rise.

the aftermarket firewall adjuster is used to just keep a slight tension on the fingers (very slight - similar to the stock setup), and you have to turn it in clockwise to loosen the cable as time goes on and the fingers rise. again, just as ed describes.

so to answer the op, count me in the camp that there should be a very slight tension on the tob (cv). however it is important there should also be enough gap available - which is a "looseness" that you should be able feel at the top of the pedal.

fwiw, mcleod pivot ball #16912 for the cobra t-56 is the part that seemed to do the trick for both of my cars. it is the exact same overall length as the the stock ball, however, since it is threaded all the way it can go into the transmission face deeper (the stock ball leaves about 10mm of unused thread in the blind hole).
 
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