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  1. #1
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    Clutch rod clearance

    What is the general clearance on the clutch pushrod where it meets the clutch arm, mines a hydraulic clutch slave cylinder and I think I am right in thinking that some clearance is needed to stop the clutch release bearing from making contact when the clutch pedal is not depressed.
    Any advice appreciated.

  2. #2
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    That's a very complicated question to answer, but the short answer is, "Yes, you need a gap and about 1/16" is typical".

    The problem is you don't want to over-extend the pressure plate fingers when depressing the clutch. Typically the clutch disc should have about 0.040" of clearance when disengaged. But how do you measure that? Maybe through the starter motor opening.... maybe.

    An easy way to check it's enough, is to make gradual changes to the slave cylinder rod length and keep trying it until it's difficult to get the transmission to shift into reverse without clashing gears, then lengthen the rod a bit until that doesn't happen.

    After that, you should have the right clearance with the clutch engaged to keep the throw-out bearing from constantly rotating. Some argue it's OK to let it continually rotate, but in my opinion it's not. Some newer cars are designed for the TOB to continually rotate, but not knowing for sure, best to keep it from rotating.

    To keep this from happening, you ought to install a clutch fork return spring to help pull the TOB back and create clearance when the clutch is released. Not too strong a spring, but sometimes you need a strong one if the slave cylinder has an internal spring that fights your external return spring. I finally removed the internal spring from my slave cylinder.

    So,, to check your clearance, remove any external spring, push the slave cylinder rod back to depress the slave internal piston, and push the clutch fork the other way just enough to take up all clearance so the TOB is touching the pressure plate fingers. Then you can measure your clearance. For a properly designed system, it's not much... maybe around 1/16". If it's not designed all that well and you have too much clearance, you'll have too much free play at the pedal before the clutch will start to disengage. If there's isn't any free play, you may have to go to a larger master cylinder or smaller slave.

    It can be a balancing act, or it may require redesign. Every combination of parts can be different. If your whole system is the same as some factory vehicle, you may find an adjustment procedure in an appropriate repair manual.

    Hope this helps. I know I spent a lot of time on this for my car.

    p: here's a link that may help: http://www.cobraclub.com/forum/showthread.php?t=61792
    Last edited by Eggbert; 24-04-21 at 11:27 AM. Reason: added link
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  3. #3
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    Obviously I’m not fully acquainted with your system but can’t help thinking this is a little over complicated or over thought and there may also be a bit missing.

    Can’t you simply make sure the clutch fully releases before the slave piston reaches its maximum travel (presumably limited by a circlip ), as the clutch wears the slave piston will retract down the cylinder so will never need to go further?

  4. #4
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    Quote Originally Posted by GDCobra View Post
    Obviously I’m not fully acquainted with your system but can’t help thinking this is a little over complicated or over thought and there may also be a bit missing.
    Perhaps, but when you assemble something using parts never intended to work together, sometimes some extra thought is needed.

    Quote Originally Posted by GDCobra View Post
    Can’t you simply make sure the clutch fully releases before the slave piston reaches its maximum travel (presumably limited by a circlip ), as the clutch wears the slave piston will retract down the cylinder so will never need to go further?
    Yes; I plan to install a larger diameter master cylinder in mine to reduce the pedal throw (and at a slight increase in pedal effort) and as the cylinder piston will then travel too far, I'm going to install an internal stop.

    Most people use a clutch pedal stop to limit the throw, but it would be difficult in my car due to the design of the pedal assembly and the nearby structural components of the car.

    Back to the OP's question, here's a little clip of how the system should work and you can note the throw-out bearing does indeed fall back away from the P-P fingers. I took this using a boroscope insert through the clutch-fork hole.

    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  5. #5
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    My post was actually aimed at the op but your video is interesting, what causes the the bearing to continue moving once the visible fingers have stopped moving?
    Perhaps one of the fingers not in view has greater motion, perhaps as the pressure is released some deflection in the mechanism causes it or maybe there are some supernatural forces at work?
    the point is though that setting a clearance on the slave push rod clearance seems a bit pointless as the first time the clutch is used the system will sort its own clearance, this is how a hydraulic clutch is self adjusting.
    The main point I was trying to make is if a new clutch is set up with the slabs cylinder fully retracted this will result in the clearance reducing as the clutch wears followed by the release bearing being pre-loaded and eventually clutch slip.
    Setting up with the slave fully extended (or close to) will allow maximum room for the slave to retract.
    Preventing over stroking the clutch is another subject and this can only be controlled by limiting fluid displacement from the master-cylinder either by putting a physical stop on the pedal or using a master cylinder with the required displacement (it may even be possible to modify the master to reduce its stroke if you get creative).

  6. #6
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    "aimed at the OP" ... LOL. Sorry about that. I do make mistakes sometimes.

    Now, the bearing continues to move due to a return spring on the clutch fork. This makes the system "non" self-adjusting. You want this, otherwise the TOB rotates all the time, thus wearing it out prematurely.
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  7. #7
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    Yes it gets complicated, i think this is the difference between a hydraulic clutch slave cylinder and a cable operated system, in theory a cable system is easier to adjust just by allowing some slack to avoid the clutch arm from being in tension. With a hydraulic cylinder once the clutch pedal is returned there should be no hydraulic pressure being transmitted to the cylinder and if a clearance between the push rod and clutch arm exists no pressure on the clutch fingers or throw out bearing, the throw out bearing can only rotate by making contact with the pressure plate fingers, it goes without saying this happens whilst the engine is running. A small pushrod clearance is desired.

  8. #8
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    Quote Originally Posted by Eggbert View Post
    "aimed at the OP" ... LOL. Sorry about that. I do make mistakes sometimes.

    Now, the bearing continues to move due to a return spring on the clutch fork. This makes the system "non" self-adjusting. .

    Phew! At least that takes the supernatural forces off the table, I was getting worried there.

    But how common is it to have a return spring on the clutch arm? Maybe I’ve led a sheltered life but I’ve never seen this (maybe us Europeans are just too dumb to have thought of it) and one of the benefits of an hydraulic mechanism is its self adjustable-ness so seems strange to lose that.

    To the OP, does your mechanism have this return spring?


    Quote Originally Posted by Eggbert View Post
    You want this, otherwise the TOB rotates all the time, thus wearing it out prematurely.

    Really? As I say I’ve had many cars apart and never seen this “return spring” feature (both with hydraulic and cable mechanisms) and premature release bearing wear has never troubled me. My understanding is that runout in the clutch fingers (there will always be some in the real world) will tend to “kick” the bearing away and create a bit of clearance, after this the bearing will no longer turn. If this does not happen then I agree the bearing will be rotated but there will be no load on it so probably won’t be a problem (presumably this is taken into account in the design), there is also the possibility that the deformation of the seal in the slave cylinder and release thereof will also create a small clearance (same as on your brake calipers.



    BTW, just to throw something else into the mix!
    When I was investigating concentric release bearings I bought one from a production vehicle (which ultimately didn’t fit) and found it had a spring which would extend the bearing and ultimately always put a load between the release bearing and the fingers h3nce (presumably) the bearing would always rotate and under significant load which I found surprising, presumably all of the parts are designed to work in this way.
    Last edited by GDCobra; 25-04-21 at 07:00 PM.

  9. #9
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    Quote Originally Posted by methoni View Post
    Yes it gets complicated, i think this is the difference between a hydraulic clutch slave cylinder and a cable operated system, in theory a cable system is easier to adjust just by allowing some slack to avoid the clutch arm from being in tension. With a hydraulic cylinder once the clutch pedal is returned there should be no hydraulic pressure being transmitted to the cylinder and if a clearance between the push rod and clutch arm exists no pressure on the clutch fingers or throw out bearing, the throw out bearing can only rotate by making contact with the pressure plate fingers, it goes without saying this happens whilst the engine is running. A small pushrod clearance is desired.

    I think it only gets complicated if you make it so.
    If your clutch does not involve the return spring described by Eggbert then the clearance in normal use willl take care of itself, the only trouble you are likely to run into is if you are using a collection of disparate components (as we tend to do) and run out of travel, either min or max but if you take the precautions I’ve described in earlier posts you should not have any problems.
    The thing that catches some people out is that the slave piston will retract as the clutch wears (and return fluid to the reservoir - so the res’ should not be maxed out) unlike the brake system where the “slave” pistons extend and hence requires additional fluid to enter the system.


    if you think cable systems are simpler you want to spend some time with a Ford mechanism which used the self adjusting quadrant ratchet. You’ll grow to love hydraulic systems after that!

  10. #10
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    GD:

    Being a little puzzled over the two methods of throw-out bearing operation we‘ve discussed, it suddenly occurred to me there’s a third design. North American cars, or at least Ford for certain, didn’t use hydraulics at all back then. They relied on a heavy and large linkage system using an over-center spring, various levers, bell-cranks and a return spring. My 1966 Ford manual specifically states that a final check after clutch adjustment is to make sure the throw-out bearing does not rotate with the clutch engaged (pedal up) or the throw-out bearing will fail prematurely.

    There isn’t likely enough room in a Cobra for such a linkage system, so the kit designers go with hydraulics. On my car, the designer chose a BMW master and slave system. I always wondered why the slave cylinder had an internal spring as it would keep the throw-out bearing against the P-P fingers all the time unless a very heavy return spring was installed. It would likely also self adjust if used without a return spring. It must be that the BMW had a throw-out bearing designed for constant rotation. Although not certain, I have heard that late model Fords, such as the Mustang, have constantly rotating throw-out bearings.

    No doubt smaller European and British cars using hydraulics could certainly be using constantly rotating throw-out bearings. This could account the differences between what we’re used to seeing.

    As I’ve removed the slave internal spring and added a return spring, my system performs the same function as the old mechanical linkage with a fully returning throw-out bearing, but with hydraulics.

    As we both noted, the hodge-podge of assorted parts in our cars can make every installation different along with different requirements.

    Note to the OP: One thing I forgot to mention in my original post, is that pedal free-play of approximately an inch should be present. A bit of that free-play comes from the slave-cylinder rod to clutch-fork clearance, but most comes from the fact that the master cylinder must travel past the bleed hole (that goes back to the reservoir) before it seals the system and begins to generate pressure. Just check that once your adjustment is done. If your free-play is too far away from that 1”… well, that’s another bridge to cross.
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

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