BMW M5 Forum and M6 Forums banner

1 - 20 of 28 Posts

·
Registered
Joined
·
60 Posts
Discussion Starter #1
After installing the UUC V12 clutch I have found that the 850 slave cylinder is way too light and the M5 slave cylinder is too stiff. The 850 slave was so light I thought I was having problems bleeding the thing. I have also noticed that the engagement point of the clutch is almost all the way to the floor with the 850 slave, but at the beginning of the pedal travel for the M5 slave. I had to completely remove my clutch stop to even disengage the clutch with the 850 slave.

So it seems my choices are an extremely light pedal with almost zero feel that you have to push all the way to the floor (850 slave), or a stiff pedal that you barely have to push to use (M5 slave). I am still trying to decide which of these I like better but I think I am going to go with the original M5 one. I really like not having to push the pedal that far every time and there is more feel to it.

My question then is are there any other slave cylinders that would fit our car that would fall in the middle. I have searched all over and have only seen references to the 850 slave. What about the F10 M5, or how would one obtain information about the various slave cylinders specs without trial and error.

Thanks for the help.
 

·
Registered
Joined
·
60 Posts
Discussion Starter #2
One other thing I have seen in the discussions of slave cylinders that I think might be wrong is the choice of which pin to use. Wouldn't a longer or shorter pin simply change the location of the piston inside the slave cylinder and have zero effect on the feel or performance of the clutch. Since the spring in the clutch is far stronger than the spring in the slave cylinder wouldn't extending the pin just compress the slave cylinder by the same amount. There would be less overall fluid in the lines and the pedal would still be in the same position, you are just replacing fluid with metal. Obviously too long or short of a pin could overextend the travel of the slave cylinder.

I would think that it would be important to use the pin that matches the fork. For example the tip of the M5 pin that was in my car was only about 7mm in diameter and had a nylon or some thing cover on it. The tip of the 850 pin that I received was about 10mm in diameter and was bare metal. I am pretty sure either would have worked but the dent in my fork perfectly sat on the M5 pin.
 

·
Registered
Joined
·
8,722 Posts
The 850 slave does provide a very light feel. It's something I grew to love, but I can see how opinions could differ.

With the longer pin, the pedal should be engaging the clutch earlier, not towards the floor. That was another aspect of the 850 slave and pin setup I had to get used to, viz., nearly immediate engagement. You may need to bleed some more because your engagement should not be where it is with the 850 slave and pin. Or at least, so my experience indicates.

--Peter
 

·
Registered
Joined
·
3,968 Posts
how are you bleeding the slave? you have to remove it from the tranny and rig it so the pin is secured and can't pop out when you put pressure on the system. hold the slave so the end with the bleeder is angled up, crack the screw and make sure you get alll he air out.
 

·
Registered
Joined
·
60 Posts
Discussion Starter #5 (Edited)
The 850 slave does provide a very light feel. It's something I grew to love, but I can see how opinions could differ.

With the longer pin, the pedal should be engaging the clutch earlier, not towards the floor. That was another aspect of the 850 slave and pin setup I had to get used to, viz., nearly immediate engagement. You may need to bleed some more because your engagement should not be where it is with the 850 slave and pin. Or at least, so my experience indicates.

--Peter
It would seem to me the only way a longer pin would engage the clutch earlier would be if it took up some of the slack of the fork and I don't believe this is the case. The strength of the springs in the clutch force the fork to the same position regardless of the pin. The longer pin would just make the slave cylinder piston to be more compressed and the engagement point of the clutch and feel would be exactly the same. I read one thread on m5board where a guy tried different pins and his shop even welded some on the tip of one and it did not change anything. In order for a pin to take up some of the slack of the fork would mean that the slave cylinder is fully compressed and the length of the pin is causing the fork not to be able to release entirely. I would think this would be bad as the springs on the pressure plate would be putting constant pressure on the piston and housing of the slave cylinder.

I may try the 850 slave one more time but I bled the thing 10 times and 8 different ways all with the same result. I think the engagement point would have to be lower than the m5 slave since it is so much easier to push. Since the 850 slave is easier to push means it is causing the fork to move less as compared to the m5 slave with the same amount of pedal travel. This would mean you have to push the pedal farther with the 850 slave to accomplish the same fork travel, and since the same amount of fork travel is required with both slaves to engage and disengage the clutch, then the engagement point would have to be farther down.
 

·
Registered
Joined
·
60 Posts
Discussion Starter #6 (Edited)
how are you bleeding the slave? you have to remove it from the tranny and rig it so the pin is secured and can't pop out when you put pressure on the system. hold the slave so the end with the bleeder is angled up, crack the screw and make sure you get alll he air out.
I have tried a pressure bleeder, pumping 10 times and holding, and a combination of the two. In the end I think my favorite way is to hook up the pressure bleeder, open the bleed screw and let it pour out and while it is doing that I pump the clutch pedal with my foot several times, quickly at first and then a few with constant fairly slow pressure on the clutch pedal. Then I quickly crawl back under and tighten the bleed screw. I do have the slave disconnected while doing this and propped in a way to have the bleed screw on top. I definitely knew the bleed screw was aiming up as the pumping of the clutch pedal with my foot spit fluid all over the bottom of the car. When I switched from the 850 to m5 slave I used this method and my clutch was perfect in one shot. Again I think most of my problem was that I was assuming there was air in the lines since the 850 slave is so soft. I think it was bled properly each time, I was just assuming something was wrong because it was so soft and due to my clutch stop would not travel far enough to disengage the clutch. At first I wasn't even sure the fork was moving.

Also, the pin on the slave cylinder just pops in and out of a rubber boot on the end of the piston inside the slave cylinder. It does not maintain any of the pressure of the hydraulic system. It will not pop out due to pressure on the system, it may fall out due to a bad boot. I have bled systems with the pin completely removed from the slave and not had any problems.
 

·
Registered
Joined
·
8,722 Posts
All I can tell you is that when I changed to the 850 slave and pin, my clutch was super light and engaged very early. The shop that did the install had some experience with this phenomenon and said they had swapped pins to accommodate a different engagement point. Theorize all you want. My advice is borne of experience. You need to bleed the slave some more or your damaged the slave when you installed it, which is not hard to do.

BTW, you absolutely can pop the pin out when bleeding with pedal pressure and that can absolutely damage the diaphragm. As me how I know!!

--Peter
 

·
Registered
Joined
·
3,968 Posts
All I can tell you is that when I changed to the 850 slave and pin, my clutch was super light and engaged very early. The shop that did the install had some experience with this phenomenon and said they had swapped pins to accommodate a different engagement point. Theorize all you want. My advice is borne of experience. You need to bleed the slave some more or your damaged the slave when you installed it, which is not hard to do.

BTW, you absolutely can pop the pin out when bleeding with pedal pressure and that can absolutely damage the diaphragm. As me how I know!!

--Peter
yep, you cannot put pressure on the slave without it rigged so the pin can't fly out.
 

·
Registered
Joined
·
1,060 Posts
I've been considering going the V12 clutch, interesting thread. This is a concern I hadn't heard of yet.

So has anyone tried the 850 clutch master cylinder, maybe this would get the fluid dynamics correct for the pedal to slave piston travel ratio? Will the 850 master even be made to fit into the E39 set up?
 

·
Registered
Joined
·
8,722 Posts
I've been considering going the V12 clutch, interesting thread. This is a concern I hadn't heard of yet.

So has anyone tried the 850 clutch master cylinder, maybe this would get the fluid dynamics correct for the pedal to slave piston travel ratio? Will the 850 master even be made to fit into the E39 set up?
The sizing of master cylinders takes into account a number of different considerations that would not necessarily be addressed by swapping master cylinders, even assuming it's a straight mechanical swap, which I doubt. You would need to do some relatively complex brake bias calculations. In short, you are much more likely to screw things up by swapping master cylinders than by adjusting pin size or swapping slaves.

--Peter

Edit: Ignore this. I was thinking brake master cylinder. Big duh on my part!
 

·
Registered
Joined
·
154 Posts
FWIW, I reused the standard M5 clutch and it is a firm pedal but I like it. My wife daily drives the car and she never complains about it.
 

·
Registered
Joined
·
6,014 Posts
The sizing of master cylinders takes into account a number of different considerations that would not necessarily be addressed by swapping master cylinders, even assuming it's a straight mechanical swap, which I doubt. You would need to do some relatively complex brake bias calculations. In short, you are much more likely to screw things up by swapping master cylinders than by adjusting pin size or swapping slaves.

--Peter
Not really ;) the clutch system is pretty simple.
 

·
Registered
Joined
·
60 Posts
Discussion Starter #14
Would there be any way to tell if the slave cylinder is damaged. I do not see any signs of leaking and it is working just way down in the pedal travel. Perhaps I damaged it by bleeding it with the pin not secured but it did not pop out during the bleeding.
 

·
Registered
Joined
·
60 Posts
Discussion Starter #15
All I can tell you is that when I changed to the 850 slave and pin, my clutch was super light and engaged very early. The shop that did the install had some experience with this phenomenon and said they had swapped pins to accommodate a different engagement point. Theorize all you want. My advice is borne of experience. You need to bleed the slave some more or your damaged the slave when you installed it, which is not hard to do.

BTW, you absolutely can pop the pin out when bleeding with pedal pressure and that can absolutely damage the diaphragm. As me how I know!!

--Peter
Do you know which pins they tried or when you say switched pins do you mean used the 850 pin. I cant remember for sure but I don't think there was any length difference between the 850 pin and the M5 pin, they just had different tips.
 

·
Registered
Joined
·
60 Posts
Discussion Starter #17
depends on how much pressure you put on it while bleeding. if it didn't shoot the rod out you couldn't have had too much pressure on it.
I set the pressure bleeder to around 18 psi, this only made the fluid drip out once the bleed screw was opened. Pushing on the pedal however caused the fluid to shoot out.

So does the slave cylinder with high pressure somehow distort the diaphragm so that the pin can shoot out. I am still having a hard time understanding how the pin can shoot out if it is not sealing the fluid in the slave cylinder. When you pin shot out did fluid also shoot out?
 

·
Registered
Joined
·
8,722 Posts
I set the pressure bleeder to around 18 psi, this only made the fluid drip out once the bleed screw was opened. Pushing on the pedal however caused the fluid to shoot out.

So does the slave cylinder with high pressure somehow distort the diaphragm so that the pin can shoot out. I am still having a hard time understanding how the pin can shoot out if it is not sealing the fluid in the slave cylinder. When you pin shot out did fluid also shoot out?
No. If you pressured your slave and the pin did not pop right out, but you DID have fluid shoot out of the slave, you need a new slave.

--Peter
 

·
Registered
Joined
·
60 Posts
Discussion Starter #19
No. If you pressured your slave and the pin did not pop right out, but you DID have fluid shoot out of the slave, you need a new slave.

--Peter
I think there is a miscommunication, when I said fluid shot out I was talking about the bleed screw when it was open and I pushed on the pedal. I was asking you if when your pin shot out did fluid then follow it.

Which pin are you using?
 

·
Registered
Joined
·
8,722 Posts
I think there is a miscommunication, when I said fluid shot out I was talking about the bleed screw when it was open and I pushed on the pedal. I was asking you if when your pin shot out did fluid then follow it.

Which pin are you using?
Until I went with a Tilton clutch and flywheel with a hydraulic TOB, I used and loved the 850 csi slave and pin.

And no, no fluid followed my pin. If your pin didn't shoot out with pedal pressure, either you have air in the system that needs to be bled out or you have a damaged diaphragm in your slave.


--Peter
 
1 - 20 of 28 Posts
Top