clutch slave supposed to pre-load throw out?

[Cascade Carl]

It's a 98 4.0 external slave. I fixed leaky clutch line, then bled the system out of the vehicle with the slave rod clamped in retracted position. When I took the clamp off to reinstall the slave into the bellhousing I was surprised that the rod wanted to spring out pretty far. So much so that any free play between the TO bearing and pressure plate would be gone and the TO bearing would always be riding on the PP. I am old school and used to mechanical clutch linkage where there is a gap until you press on the pedal. So is what I described above normal for hydraulic clutch? Please help 'cause I am afraid to put everything back together and fry the TO bearing two days later. Thanks.

 

[ngrover]

If it helps any, I recently replaced my master and slave cylinder (bought them as a pair and pre-bled). The rod did not contact the arm when retracted... therefore there is no contact with the throw-out bearing when my pedal is released.

I cut my old one apart if you are interested in what is inside the slave cylinder.

 

[ngrover]

Also, I bought my master/slave on ebay from "clutchcityonline" to the tune of $165 for both cylinders in a pre-bled package. It pains me that I had to go that route when only my slave cylinder was bad... but I had tried using a slave cylinder from the junk yard and it put up a good fight (couldn't get a good pedal feel out of it and reverse would grind).

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=350078011336

 

[ChicksDigWagons]

It's normal for hydraulic clutches for the TO bearing to constantly be in contact with the pressure plate fingers, yes. Cable/Mechanical clutches typically have some pull-back, which does save the life of the TO bearing, but there is a reason it's called a Throwout bearing

At least you guys don't have to replace the whole slave just to replace the TO bearing! (or remove the transmission to replace the slave...)

 

[ngrover]

Not trying to be stand-off'ish but as I mentioned, my rod did not contact the release arm when the clutch was engaged (pedal out). I checked.

If indeed the throw out bearing in an ax-15 (external slave) is in contact with the fingers all the time, then it's not because the rod is always touching the arm, at least not on mine!

While the slave cylinder can be replaced outside the trans, the throw out bearing can't.

 

[ngrover]

I've tried to make my own slave before using a generic cylinder. See this guys writeup on how it's done:

http://www.universityofjeep.ca/Tech/Clutch/Clutch_Slave_Repair.htm

Mine home-brew cylinder didn't last very long so I don't recommend you try it.... however, there is one paragraph in there that directly addresses what you are asking (for more context, read the content linked above. Remember this is take from a site whereby you make your own slave):

"individual Jeeps and 4cyl/6cyl will have a slightly varying amount of play in the fork before engagement. NOTE; if you have any doubt about the length, then error on the side of making it too short. If the cylinder doesn’t extend quite far enough you can shift gears no problem, but if you’ve got the rod too long the shift work won’t be allowed to come all the way back and you’re clutch won’t be fully engaged when you let the pedal out resulting in a slipping clutch. (and that’s bad!)Once you have cut the striker back a bit, and done a re-test fit, you are ready for final (hopefully forever!) install."

 

[ChicksDigWagons]

Not trying to be stand-off'ish but as I mentioned, my rod did not contact the release arm when the clutch was disengaged. I checked.

If indeed the throw out bearing in an ax-15 (external slave) is in contact with the fingers all the time, then it's not because the rod is always touching the arm, at least not on mine!

While the slave cylinder can be replaced outside the trans, the throw out bearing can't.

I read that, but I will say that it's not necessarily the norm - in fact I almost don't believe you! I can't imagine having that big fat spring inside the slave behind the plunger and not have it contact the arm. If that spring can't push the rod into the clutch arm, then how in the world is the hydraulic pressure going to do it?


While I've never put much thought into it, thinking back, most if not all slave cylinders I've replaced or removed for a clutch job on were spring loaded and put a preload on the clutch arm. Cars and Trucks alike.

The early XJs have an internal slave that is also most definitely spring loaded.

This is pretty much essential. Otherwise, the slave could retract creating a dead spot at the top of the pedal travel.

 

[Cascade Carl]

Ngrover, you say the rod did not contact, was that when the slave was first installed with plastic straps things still intact? Maybe it's just that spring pushing the rod out- I thought maybe I did not bleed it well enough and the trapped air was making the cylinder act like a gas spring.

I see you also cut off the bleeder boss. Is that an o-ring at the bottom where the screw seats?

 

[ngrover]

The big white cap in the centre will limit how far the rod will extend. The length it extends should be less than the distance from the arm to the slave cylinder mounting surface... therefore the rod does sit in the clutch arm cup but not apply pressure to it.

 

[ngrover]

Ngrover, you say the rod did not contact, was that when the slave was first installed with plastic straps things still intact? Maybe it's just that spring pushing the rod out- I thought maybe I did not bleed it well enough and the trapped air was making the cylinder act like a gas spring.

I see you also cut off the bleeder boss. Is that an o-ring at the bottom where the screw seats?

I snapped the bleeder boss off while cutting it open. Strangely, there was an o-ring embedded in the plastic right where it broke.

I measured the length that the rod extends some time after initial install of the slave... meaning, the plastic strap was broken.

Just to be sure I don't confuse anyone, the end of the rod has to sit in the cup on the arm so that it doesn't fall out etc... but by my measurements, it would not have been extended long enough, when retracted, to apply PRESSURE on the arm. It's a tight tolerance I realize, probably only a few millimeters.

 

[ngrover]

Here is a pic of my home-brew slave (it worked for month or so but because the rod doesn't float inside the cylinder, it gets side-loaded and it blows out.. different topic).

notice though, that there is a black plastic piece that gets clipped onto the surface that mates with the transmission (I used the original one in the pic below). Make sure you re-use that.

 

[Cascade Carl]

I measured the length that the rod extends some time after initial install of the slave... meaning, the plastic strap was broken.

Just to be sure I don't confuse anyone, the end of the rod has to sit in the cup on the arm so that it doesn't fall out etc... but by my measurements, it would not have been extended long enough, when retracted, to apply PRESSURE on the arm. It's a tight tolerance I realize, probably only a few millimeters.

If you happen to have the dimensions handy it would be very useful to me. I used a non-OEM throwout bearing which might be affecting the distance from slave mounting face to cup in arm.

 

[Cascade Carl]

A side note for anyone else dealing with leak in hydraulic clutch line: I found the Ford repair kit has exact same o-rings and roll pins as Jeep. Ford part number is 2C3Z-7C522-AA for a kit of two o-rings and two roll pins. Not cheap at $17 US but I don't know anywhere else you can get these super-special o-rings unless you are buying new cylinders.

 

[ChicksDigWagons]

The big white cap in the centre will limit how far the rod will extend. The length it extends should be less than the distance from the arm to the slave cylinder mounting surface... therefore the rod does sit in the clutch arm cup but not apply pressure to it.

Think about what you're saying.

If that spring causes the plunger to bottom out limiting the amount the rod sticks out, what happens when you force hydraulic fluid into the slave?

Some brand new slaves have limiting straps that don't pop until the first time you press the clutch pedal, but I personally haven't seen any external style slaves like this, only internal. But I don't doubt external slaves have this feature.

I'm telling you, you are flat out wrong. That home-made unit is not the same because it's NOT spring loaded and the plunger sits in the fully bottomed position. Over time, this type of setup will develop play as the clutch fingers relax and pivot bushings and bearings wear. In this case, you have to do the lash adjustment manually but that does not mean it is normal. If it was, all external slaves would be adjustable. The spring makes the adjustments automatic.

 

[RCP Phx]

You should NEVER have constant contact with the TO bearing/pressure plate!

 

[ChicksDigWagons]

You should NEVER have constant contact with the TO bearing/pressure plate!

You might want to look into that because as a blanket statement it's absolutely not true. Most modern TO bearings are designed to spin all the time. This may not have been true 40 years ago but it's definitely the case now.

 

[ngrover]

Think about what you're saying.

If that spring causes the plunger to bottom out limiting the amount the rod sticks out, what happens when you force hydraulic fluid into the slave?

Some brand new slaves have limiting straps that don't pop until the first time you press the clutch pedal, but I personally haven't seen any external style slaves like this, only internal. But I don't doubt external slaves have this feature.

I'm telling you, you are flat out wrong. That home-made unit is not the same because it's NOT spring loaded and the plunger sits in the fully bottomed position. Over time, this type of setup will develop play as the clutch fingers relax and pivot bushings and bearings wear. In this case, you have to do the lash adjustment manually but that does not mean it is normal. If it was, all external slaves would be adjustable. The spring makes the adjustments automatic.

I won't argue with you other than to add that the rod is moved up and down by hydraulics, despite the fact that there is a spring or not. The spring doesn't over power the hydraulics or something crazy like that. Hence I was able to build a custom cylinder that has no spring at all. You need to take the whole system into account. There is likely a spring under the clutch pedal etc. I can't recall off the top of my head how strong that spring was and quite frankly I didn't analyze the purpose of the spring or it's net effect to much extent. All I know is, my rod does not put force on the arm when clutch is engaged and no force is on the pedal. Period.

 

[Cascade Carl]

I pulled my slave back out and did some measuring. The slave rod is 3.09" beyond the rubber biscuit thing in it's free state. The bottom of the pocket in the TO lever is almost 1.5" inside the surface of the bellhousing where the slave mounts. Can someone verify the 1.5" distance is normal?

 

[ChicksDigWagons]

I won't argue with you other than to add that the rod is moved up and down by hydraulics, despite the fact that there is a spring or not. The spring doesn't over power the hydraulics or something crazy like that. Hence I was able to build a custom cylinder that has no spring at all. You need to take the whole system into account. There is likely a spring under the clutch pedal etc. I can't recall off the top of my head how strong that spring was and quite frankly I didn't analyze the purpose of the spring or it's net effect to much extent. All I know is, my rod does not put force on the arm when clutch is engaged and no force is on the pedal. Period.

The rod is only moved one way by hydraulics and is returned by the pressure plate fingers. Seriously, draw a diagram of what you proposed:

The big white cap in the centre will limit how far the rod will extend. The length it extends should be less than the distance from the arm to the slave cylinder mounting surface... therefore the rod does sit in the clutch arm cup but not apply pressure to it.

If the white cap limits how far the rod extends as you said, then it can't move out any further, right? If this is true, then how does it push on the clutch arm when you step on the pedal? The answer is 'It can't work that way'. Thus, the spring in the OEM slave cylinder MUST apply some pressure on the clutch arm at rest, a couple pounds worth by my finger-estimate. I know for a fact this is how the internal-slave works as well. Then when you press on the clutch pedal, the fluid pushes the rod with more force, disengaging the clutch. When you release the pedal, the pressure plate pushes the rod back in and returns the fluid to the master cylinder.

Now you don't NEED the spring for the system to work, but it is there to provide a zero-clearance for the throwout bearing. This helps reduce clutch pedal travel and provides consistent pedal feel. It also keeps the bearing moving because in theory it makes more sense to keep the bearing moving than to accelerate it from 0rpm to a couple thousand RPM every time you cycle the clutch. And it also extends the life of the clutch because it ensures a full release every time.

 

[ChicksDigWagons]

I pulled my slave back out and did some measuring. The slave rod is 3.09" beyond the rubber biscuit thing in it's free state. The bottom of the pocket in the TO lever is almost 1.5" inside the surface of the bellhousing where the slave mounts. Can someone verify the 1.5" distance is normal?

Without having one in my hand to measure, I can say that anecdotally those measurements are in line with all of clutch jobs I've personally done on modern hydraulic-release systems. The springing action is completely normal for the slave cylinders. You are worried about nothing. Put it together, make sure you have full release of the clutch and drive off into the sunset.