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  1. #1
    Join Date
    Dec 2008
    Location
    Newcastle upon Tyne
    Posts
    1,869

    Clutch slave cylinder spring pressure query

    Hi,

    I am planning on using the clutch slave cylinder & release mechanism that came with the E90 BMW gearbox when I install it behind the 1UZ.

    I will be removing the CDV when connecting it to the hydraulic line but am concerned by the strength of the spring in the cylinder. The push rod is extended by the spring, it can be pushed back in by hand but it requires considerable force. This means that the CRB will be held in permanent contact with the clutch release fingers. I had always believed that the correct set up required a small gap between the CRB and the release fingers - or is this old school thinking?

    It would also appear that in the E90 setup there is a return spring on the clutch pedal. I will be using a Wilwood pedal box and there is no return spring - should one be fitted?

    Any advice or suggestions, as always, much appreciated.






    Regards,
    Ian
    Pilgrim Sumo Mk3; IVA Apr. 2014; RV8 3.9 EFi ; Cosworth T5; Granada Donor
    Phase Two underway - Conversion to Japanese

  2. #2
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    840
    I'm having a similar issue. I use a return spring to get that extra clearance so the throw-out bearing doesn't touch the pressure plate fingers. The slave cylinder internal spring is quite strong, so a very strong external spring is needed to overcome it and get that clearance. What makes it bad, is that the external spring is under the least tension when the internal spring is compressed the most. The external spring can be so strong that it adds significant force to the clutch pedal.

    Since I changed clutch type, by clutch fork is no longer in the same position, so I need a new spring. The old one isn't under much preload when installed. Getting a new spring is a pain as springs don't like to be modified. They either stretch after heating them for a bend (annealed) and if you try to re-temper them, they get very brittle and not very strong.

    So.... I'm considering removing the internal spring altogether. That way, almost any external spring will work. So far, I've no conclusive information on why there is an internal spring. One theory is that it's for constant contact bearings, but another is that it slows down the clutch return much like an orifice restriction for the fluid would do. The latter may help prevent clutch damage if your foot slips off the clutch.... "they" say.

    I intend to try the system with either the internal spring removed, or replaced with a lighter spring. I will post about that when I get it done.

    As far as old school thinking goes, I like the extra clearance. Older throw-out bearings were not designed for constant rotation, although it is my understanding that a lot of newer cars use constant contact bearings. One bit of information I found, but can't confirm, is that self-aligning bearings are for constant contact and non-self-aligning bearings need the gap.
    John

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

  3. #3
    Join Date
    Dec 2008
    Location
    Newcastle upon Tyne
    Posts
    1,869
    John,

    Hi, always nice to know you are not alone!

    I too was contemplating removing the spring but am nervous as I have found over the years that things are normally there for a purpose - and sometimes you only find out, to your cost, afterwards.

    I managed to convince myself that there would be no problem resulting from the CRB being in permanent contact with the clutch release fingers assuming the force exerted by the internal spring was not excessive. There are numerous examples of bearings that are running continuously and under load without a second thought being given. Why should the CRB be a special case? - I can only assume that in the "old days" the bearing was made to exert the required force on the clutch and was possibly a plain bush rather than a roller bearing. I am sure I remember being told not to keep the clutch down at traffic lights etc because "you will wear out the release bearing". Perhaps modern engineering has now enabled a bearing design that can handle the required linear thrust but still be able to rotate without excessive wear? After all if it works in a BMW 5 series why would it not work behind a 1UZ?

    Despite all the logic I am still uncomfortable with the thought of the CRB being pressed against the clutch fingers when the clutch is released.

    Regards,
    Ian
    Pilgrim Sumo Mk3; IVA Apr. 2014; RV8 3.9 EFi ; Cosworth T5; Granada Donor
    Phase Two underway - Conversion to Japanese

  4. #4
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    440
    When I fitted my CRB a couple of years ago I checked out a number of bearings from production cars all of which had the spring you mention, I couldn't understand it either, just means the bearing is alway's being pressed against the rotating diaphragm spring.
    I think you should have a return spring on the pedal though to make sure the master piston reveals the port to the reservoir otherwise you'd never pu p the slave out

  5. #5
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    840
    Before I try modifying my slave cylinder by either removing the internal spring, or fitting a weaker one, I did some more research on the purpose of the internal spring. Still nothing conclusive, but it was common to read that the spring helped keep the piston centered.

    I made a rough (and exaggerated) sketch to show my understanding of that. However these pistons are usually quite long compared to their diameter, so I doubt that's really much of a problem. Besides, I have fairly good pushrod alignment now, so the side load is minimized. On top of that, even though the internal spring is quite strong, it's strength is minimal compared to the pressure plate fingers pushing on the rod. Of coarse, it's the side load that is more critical, and I'm not sure how that compares to the internal spring force.

    I did find two different people who posted their slave cylinder worked fine with the internal spring removed. That was in the "other"... cough, cough... Cobra forum. I tried to get in touch with them, but one had his PMs turned off, and the other never answered.

    Once I decide what to try, and then get it done, I'll report back here. Of course if I introduce advanced wear, I won't know for quite some time.

    Last edited by Eggbert; 27-06-19 at 10:18 AM.
    John

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

  6. #6
    Join Date
    Dec 2008
    Location
    Newcastle upon Tyne
    Posts
    1,869
    John,

    Interesting, I have decided to leave the clutch save as BMW (or their supplier) made it.

    From what I have read, and there does appear to be an active debate about the supposed benefits, the main advantage is to keep the "bite point" of the clutch constant with wear. I have to assume that the clutch release bearing (CRB) is made to withstand the constant rotation caused by contact with the clutch fingers - time will tell, but I am fairly confident that it will suffer less wear than it would have received in the average M5.

    Regards,
    ian
    Pilgrim Sumo Mk3; IVA Apr. 2014; RV8 3.9 EFi ; Cosworth T5; Granada Donor
    Phase Two underway - Conversion to Japanese

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