Explain clutch slave cylinders to me

[abowie]

Rather than hijacking the original topic. Can anyone answer these?

To be honest I do not have a good concept in my head about what the functional differences between the different size and types of slave are. I don't for example properly understand:

what the reason behind changing from the short to long slave cylinders is
why there are different length actuating rods
what the actual function of the spring fitted to the clutch fork is
whether you need a spring with a "hydrostatic" slave or not, or indeed what it really does at all
why some slaves seem not to contain an internal spring even though the parts manuals show them in all slaves
why some slaves have a circlip that stops the plunger coming out while others don't

[christopher storey]

Andrew : these questions are all akin to those concerning the meaning of life , and as far as I know are insoluble. If someone else has the answer(s) it will be a miracle

[mgcjag]

Hi Andrew...great post, tried to understand it myself but could never quite get to grips with it, another question to add is why one was self adjusting but the other wasnt...hopefully by the end of the thread we can come to a conclusion of one type fits all

[1954Etype]

Great post......

What l have learnt is:

1. There appear to be 2 different length pushrods and are matched to the hydrostatic and non hydrostatic cylinders.
2. Fitting the correct external spring can cause the clutch seals to give up (heppenened twice). I have fitted a less stronger spring in the past.
3. The incorrect pushrod affects the clutch bite height.
4. Before messing about with the slave, if yours worked ok before, make sure you replace like with like.

[Heuer]

my learning points are:

1. you must not use a spring with the hydrostatic slave
2. the pushrod for the hydrostatic slave is a different length to the non-hydrostatic version
3. decide which one you want to use and buy the correct matching parts, don't mix 'n match. The cost is small compared to the hassle of getting things wrong
4. they both do the same job, there is not a "better one

[abowie]

Andrew : these questions are all akin to those concerning the meaning of life , and as far as I know are insoluble. If someone else has the answer(s) it will be a miracle

NOOOOOOOOOOOOOOO!!

[Gfhug]

The meaning of life, the universe and everything? Well we all know the answer is 42.

[richrv0]

Maybe this will help to explain the difference:

The spring on the forks is to take pressure off the release bearing and push the slave piston back to the top of the cylinder. The rod is adjusted to give correct movement of the release bearing and hence the clutch. This only requires a cylinder long enough for normal movement as the rod will be adjusted manually as the clutch wears. If correct adjustment is not made then either the clutch will not clear or there will be excessive pressure on the release bearing, leading to premature wear.

The later cylinders are longer and rely on the clutch itself to push the slave piston back up into the cylinder when the clutch pedal is released. When correctly adjusted, with everything new, the slave piston will be almost at the end of it's travel with the pedal fully depressed and about half way along the cylinder when the pedal is released. The light spring within the cylinder takes up any slack to ensure that the system remains correctly adjusted. As the clutch wears over time the piston simply returns to a point further towards the back of the cylinder, so further adjustment of the rod is not necessary. The clutch plate will be down to the rivets before the piston returns as far as the end of the cylinder. There is a light pressure on the release bearing, but this does not lead to premature wear and ensures continued correct adjustment, in reality the release bearing will far outlast the clutch and should be replaced anyway as part of a clutch replacement (the release bearings are different for each type of clutch, the diaphragm type being deeper).

As mentioned in previous posts, it is important not to mix and match between the two systems, hopefully the above explains why. Both systems work but it depends how much you like tinkering...

Also, in both cases it is important that the clutch pedal return spring is in good condition and effective, don't rely on the spring in the master cylinder!!

Hope this helps

Rich

[mark10337]

Very timely. I just got my car back and one of the notes from the mechanic was I am missing the spring.

Now, my S2 has a 10" clutch fitted - I won't go into the details why. How can I tell which slave I have? I thought it was only the earlier 3.8 cars that needed this spring. Now, as I don't know what the PO may have done in this area, I need to figure out which slave system I have to avoid the mix and match.

C21470 (Engine Nos 7E1001 - 7E4606) with spring C5120.
C24145 (Engine Nos > 7E4607) no spring
C29801 S2 no spring
C29801 S3 with spring C5120

[mgcjag]

Hi Mark..according the the Jaguar manual there are 3 types of clutch slave cylinder, checking a tew scources they are

1. Adjustable C21470 (superceeds part C16989) with external return spring 80mm length used up to engine No 7E4606
2. Hydrostatic Self adjusting C24145 without external spring 95mm length used from engine 7E4607
3. Adjustable Modified non Hydrostatic C29801 with external spring 95mm length used from7E18356 (ots&fhc) & from 7E5558(2+2)
So all S2 cars should be useing the 3rd type
S3 cars also use C29801..
All part numbers taken from the parts catalog in the knowledge base

Just incase i have made an error in the above it is advisable for anyone reading this to use it as a guide ....if you see anything incorrect let me know and i will edit
Just edited part no for item 2 above

[MarekH]

Don't forget the v12 Steve - I expect it is much the same as per the last of the six cylinder cars.

The clue as to how the slave operates is really in the title, but to understand the slave, you need to consider how the clutch components wear in normal operation. Friction on the clutch makes it wear and the pedal has to pressed further down to reach it. Wear also occurs on the carbon release bearing when it engages the pressure plate. This has the opposite effect on pedal travel. Pedal travel and slave pushrod movement are directly related.

To explain the slave, when pressing the pedal, it reacts to the movement of fluid being pushed into it. When the pedal is released it reacts to the super heavy clutch spring separating the flywheel and clutch.

The default situation for the clutch is that the master cylinder, its reservoir and the slave are all connected and open to each other. The first part of pressing the clutch pedal seals off the link to the reservoir at the end of the master cylinder and thus creates a closed system between the master and slave. Further pressure on the pedal now moves fluid from the master to the slave. The pushrod on the end of the slave pushes out. After a small amount of mechanical free-play, your foot separates the clutch from the flywheel. How easy this is, is a function of the diameter of the slave cylinder compared the master cylinder. If both are the same diameter, then it is 1:1 relationship, but if you have a non-standard diameter master or slave cylinder, then that isn't so. For example, if the ratio of the cylinder diameters is altered to 2:1, then you'd have to depress the pedal twice as much to shift the same amount of liquid at the slave end, i.e. release the clutch, but it's twice as easy to do:- the pedal is lighter to operate. The oem system is set up so that you don't run out of pedal travel and that it isn't too hard to press. The reasons why people consider altering the slave's relationship to the master is that the pedal may be easier to control if lighter (particularly if people have trouble with their left knee) or the clutch "bite" point" may be easier find if near the end of the pedal travel.

Going back the other way, the slave (and the rest of the sytem) is controlled by springs. The heavy diaphragm spring on the clutch pushes the pushrod back into slave cylinder. The piston in the slave is kept centred by a tiny light spring inside it and fluid is pushed back up to the master which pushes against your left foot. As the piston comes back in the master cylinder, that is kept centred by an internal spring and when mostly retracted, the small weaker spring which seals the reservoir and master lengthens back to its default position and the system becomes an open system again. There is no fluid pressure in the system at this point and here the various combinations of components differ in how they come to rest as there are now no hydraulic forces to acting.

At the pedal end, a clutch return spring on the pivot of the pedal brings the clutch pedal back up and sets up the pedal free play at the master cylinder end.

At the slave end, the external return spring slave cylinders use a spring to pull the slave pushrod back further into the slave cylinder. This maintains the 1/16" mechanical free play originally set up on the slave pushrod before it engages the clutch fork so as to keep the carbon clutch release bearing free from the clutch pressure plate. The slave's light internal spring only serves to keep the slave piston centred in its bore.

For a hydrostatic (or self adjusting) system, there is no external return spring at the slave, so the system has no way to maintain whatever pushrod free play is originally set up by reference to the head of liquid and the tiny self centring spring in the slave cylinder. In practice, once the heavy clutch spring diaphragm has pushed the carbon release bearing off of pressure plate, there nothing to actually maintain the free play, so the carbon release bearing will always sit on (or only just off) the pressure plate and will wear. This is because that light internal spring inside the slave is just enough to keep the pushrod as far out towards the clutch fork as possible but maintaining what Jaguar hoped was an insignificant pressure on the clutch fork so no appreciable wear would occur on the release bearing.

In theory, it was hoped that so long as no significant wear occurred when the clutch wasn't being used, the wear of the clutch disc and of the throwout bearing would simply mean that the pushod would return to a new but slightly different resting position in the slave as time went by. This is why the slave cylinder length was longer on these models. I suspect that the carbon release bearings were thicker on the hydrostaic clutch models also for the same reason. This self adjusting system was abandoned in favour of the slave external spring setup as that is the only way to make sure the release bearing doesn't wear when the clutch is not being used.

The light self-centring springs are necessary in the cylinders because neither pushrod moves quite in a straight line - they will want to move in an arc and it is more pronounced at the pedal end where the distance between the pedal pivot and clevis pin is quite small. This is how you end up with wear sratch marks on the internal bores of the cylinders which then compromise the seals. The other wear factor is rust:- DOT4 fluid is hygroscopic and lack of use lets the water separate out and make a nice rusty trail along the bottom of the slave cylinder.

The only other function of the internal light spring is to allow the piston to pop out when you need to change the seals one day - it is far too weak to do anything mechanically useful in normal operation.

Given the above, it should be obvious that the hydrostatic (long) slave cylinder needs to be matched a short pushrod and the extrnally sringed short slave needs a long pushrod. Slave cylinder travel will be the same in each case, but the self adjusting system needs the longer slave cylinder as the resting position of the pushrod alters with clutch/throwout bearing wear. If you are going mix and match, make sure that an external spring is always used with a long pushrod - effectively you have converted a long cylinder slave to the short slave setup that needs the 1/16" free play.

kind regards
Marek

[neil4444]

Splendid description Marek. Many thanks.
How do I identify what slave type I have fitted to my car as Hutsons fitted the slave cylinder but also sent me an external spring to fit (which I'm not sure if I need) ?

[mgcjag]

Hi Neil...have a look to see if there are any numbers on it to cross reference or contact Hutsons and ask what they fitted

[Heuer]

Superb explanation Marek! I will add it to the Forum KB.

[mgcjag]

Hi Marek great detailed explenation.....one detail to note it isnt hydro = long and non hydro = short
From the 3 types the first (non hydro) is 80mm, the second (hydro) is 95mm the third a modified non hydro was again 95mm
So you cant just assume that a long slave is self adjusting.

[MarekH]

David, if perhaps you wish to use this post a reference, append the correct slave length details. Sadly my computer doesn't always really display stuff correctly since we went to the new website, so I may or may not post many replies, because not everything displays properly.

What I wanted to get across is that all things considered, the self adjusting slave will tend to want a longer slave cylinder than the short stubby one you'll find on the s3 etype.

In the case of people setting up their clutches, I'd consider a number of things:-

1/ Is this an oem supplied manual transmission car with oem parts? If so, follow the manuals.

2/ If it is an auto to manual conversion, don't assume you have the correct parts and even if you do, remember they may be reproduction parts which have subtle but indistinguishable differences.

For example, that new clutch pedal may have its clevis pin hole slightly out and so the pushrod will make the piston scrape the master cylinder wall every time you operate the clutch.

3/ If it is a maintenance thing that you are doing, consider how part worn components may affect you.

For example, there is no reason why the pushrod length set up wrong won't now pop out of the slave cylinder when you pump it full of fluid. A small amount of wear on the clutch side of the clutch fork can be magnified greatly at the pushrod side.

4/ If you do something non-standard, document it and keep it with your copy of the Workshop and/or Parts Manual.

I have a ring binder which is full of technical diagrams and explanations off everything that I've done, even when those parts subsequently didn't get used. Come to think of it, even if you do something standard, document it. It helps because when something goes wrong. You can say that it happened between x and y dates and that narrows down how things came to be the way they are.

Also, Jaguar missed a trick with the brake fluid warning light. You can fit a brake fluid bottle in place of the clutch bottle and benefit from warning light setup. Unlike the brakes, where a typical failure at the brake end makes you lose a little fluid every time you apply the brakes, the clutch is just an open system (which is why you can graviity bleed the clutch but need to pump the brake to bleed). This means that if you get a leak, even a microscopically small one, you will fast lose your clutch as capillary action will simply drain it away in no time at all. If you see ANY change in the level of the clutch reservoir ever - you have no clutch. Don't wait to fix it. By tomorrow your paintwork will either be destroyed or your ability to easily shift gear will be gone.

kind regards
Marek

[mark10337]

Great explanation, thanks Marek!

Steve - the part numbers
C21470 (Engine Nos 7E1001 - 7E4606)
C24145 S1.5
came from the SNG catalogue. The Jaguar Spares Catalogue Series 1 4.2 also lists C24145

C21470 uses the external spring C5120 with attachment plate C5178.
This is discontinued for C24145.

S3 cars are C29801. The spring C5120 and attachment plate C5178 are re-introduced for this.

As I have the S1 clutch - and as Marek thinks, I can validate there is a difference in the height of the carbon on the release bearing, perhaps I should also be matching the S1 slave cylinder to the clutch. Need to just go and lift the car to check which slave unit is currently fitted. Now we just need a table of what should be used when.

Thermostats and Clutch Slave cylinders. Some great information KBs coming out at the moment :)

[Hugo]

I don't think your maths is quite correct; if you have a ratio of 2:1 between master and slave cylinder, the effective difference (in pressure/travel) is in fact 4:1, because the piston diameter is squared to get the piston area (Pi x r squared & all that). For example, if you increase the master cylinder diameter by 25%, it will increase pedal pressure and slave cylinder travel by 5/4 squared = 25/16 = just a bit more than half.

[mark10337]

I added the clutch spring to the slave cylinder at the weekend that isn't present (according to the jaguar spares catalogue) on the S2 models but is on the S3 with the same slave cylinder. It makes a marked difference. Not so much travel on the clutch pedal until it re-engages and a better feel.