Clutch slave cylinder questions

For my 68 2+2 rebuild, I bought a new clutch slave cylinder but now I have some questions:

  1. I bought part number C29801 as this was indicated to be for all 2+2s. Can I assume that this is the “hydrostatic” type in all later cars.
  2. Why is it called “hydrostatic”? Is this because the internal spring in older units is not included?
  3. Even without the internal spring (and also without the external spring retracting the throw out bearing lever), isn’t there a little residual pressure in the cylinder when clutch pedal is not pushed?
    And, if so, does this slight pressure cause the throw out beating lever to be engaged? And can this cause wear on the bearing?

I have searched the archives and can’t find answers so please satisfy my ignorance.

Phillip,
Your searching must be about as poor as mine, as there is lots of stuff posted on this in the past. :grinning:

  1. SNG Barret does say C29802 is the long slave cylinder, which is the hydrostatic type.
  2. It is called hydrostatic because rather than manually adjusting it by adjusting the slave cylinder rod, it is automatically adjusted by having the fluid volume take up the adjustment.
  3. I believe in theory, the runout of the pressure plate would push the throw out bearing back enough to have little to no wear. But for what ever reason, be it poor design, improper adjustment, etc., most here feel it is better to convert it to the manual adjustment type, adjust to the 1/16 inch free play and not have to be concerned if the TOB is wearing prematurely. It just seems safer to most.
    Tom

Phillip,

That is hard to believe, as there are regular discussions on this topic, and several in the past few months. However, here are your answers:

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That’s a lot of good info from both of you but somewhat conflicting, just as I found with archive info So some supplemental questions;

  • Tom, you mention C29802 while I said C29801, which I bought from Moss. Did you make a typo are is there another version of the unit? In any case , the old I took out is the same size as new one I bought.
    And what do you mean by "convert it to the manual adjustment type? Is this something physical or just adjusting the free play?

  • David, my puzzle is what is the physical difference between the manually adjusted and self adjusting slave versions. I look at the before and after drawing in the Jag manual and they seem identical. So how does the later version become self adjusting? I’m any case, I will follow your advice and add a n external return spring…

It’s all in the archives, but a brief synopsis is as follows:

The first version of the slave (used on the 3.8 S1s and early 4.2s) was 3" long. It required manual adjustment on a regular basis to ensure a small amount of free play. It had both an internal and external spring, the external spring being more powerful.

At a point during the 4.2 S1 production (exact time is in one of my posts in the archives), Jaguar replaced the manual adjustment system to a self-adjusting system. The idea, presumably, was to reduce maintenance, which in all likelyhood wasn’t being done on many cars and resulting in premature wearing out of throw-out bearings. The change introduced what Jaguar referred to as the “hydrostatic” slave cylinder. It is designed to keep the throwout bearing “just” touching the pressure plate when the clutch pedal is released, but touching so lightly that wear is not excessive. Unlike the original slave which has the piston bottomed out when the pedal is released, the new slave must have the slave 3/4" or so from the bottom of the cylinder, so the length of the cylinder had to be increased to 3-3/4" to accommodate this. The external return spring was removed… The setup procedure also had to change from 1/6" free play, to 3/4" free play.

Unfortunately, the hydrostatic setup was not a success. In all likelyhood, the requirement for a different set up procedure resulted in confusion with those maintaining the cars, and as a result many new hydrostatic systems were set up with the smaller free play, which caused very rapid wear on the throwout bearing which required an engine-out service to replace. So, Jaguar reverted to the old manual adjustment mechanism with the external return spring. Though that system didn’t require the longer slave cylinder, there was no harm in having the extra length provided a longer pushrod was used to allow the piston to once again bottom out in the cylinder when the clutch pedal is not depressed. So, Jaguar kept the longer “hydrostatic” cylinder, but used it in the manual adjustment mode. This continued through the Series 2 cars, though the slave cylinder was modified in some way (new P/N C.29801) but not in a way related to the subject under discussion.

To convert a hydrostatic slave cylinder setup with 3-3/4" slave to a manually adjusted slave setup, you need to (a) fit and adjust a return spring (b) fit a longer push rod (c) adjust the setup for 1/16" free play in the manner described in the Service Manual for the 3.8 S1 cars.

-David

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Philip, go with what David says. I was mixed up, thinking when they went back to the manual adjustment, they went back to the shorter cylinder.
Tom

David, thank you for the very clear and precise description of slave cylinder history. You must have devoted some considerable effort investigating this subject.
The new cylinder I bought is 3-3/4" and I will certainly follow your recommendation exactly.

If you have not lost complete patiance with me, I have one last question. The TO bearing lever in my car has about 1/2" free play measured at the end where it connects to the cylinder push rod. Is that normal and ok?

I think perhaps you mean 1/16" (1.5mm).

It sounds like the “free play” you are describing here is: the distance you can move the end of the clutch fork lever when it is disconnected from the push rod. Did I understand correctly? If so, this movement is limited by (a) the point the TO bearing hits the pressure plate when you push the lever towards the rear of the car, and (b) the point the fork hits the back of the bellhousing when you push the lever towards the front of the car. This is not the “free play” that we need to set to adjust the clutch, but 1/2" is just fine for this measurement.

The free play adjustment that is required for the manually adjusted clutch slave (assuming you have the Bentley Service Manual) is described on page E.7 in the Routine Maintenance section to be done every 2500 miles. Be sure to push the slave cylinder piston all the way back into the cylinder (against the pressure of the internal spring) when setting the free play. Do not use the procedure on page E.X.s.2 which is only appropriate for the hydrostatic slave setup without the external return spring.

Thank you Geo, and Rob (for his PM on the same topic) for pointing out this typo. I have corrected my earlier post - it should indeed be 1/16", not 1/6".