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post #1 of 12 Old Feb 22nd, 2014, 7:37 am Thread Starter
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Increase Clutch Spring Pressure

Now that I have replaced the clutch & have a better understanding of how the clutch slave cylinder works with the clutch diaphragm spring I think you can increase the spring pressure to compensate for the clutch disc wear.

I think if you install shims between the clutch slave cylinder & the transmission to compensate for the clutch disc wear you can gain the lost clutch diaphragm spring pressure back. You could stack the BMW 23122352156 gasket or make your own thicker gasket. Gasket material is available in many different thickness sizes.

Dave Selvig
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post #2 of 12 Old Feb 22nd, 2014, 8:33 am
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Re: Increase Clutch Spring Pressure

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Originally Posted by saddleman View Post
Now that I have replaced the clutch & have a better understanding of how the clutch slave cylinder works with the clutch diaphragm spring I think you can increase the spring pressure to compensate for the clutch disc wear.

I think if you install shims between the clutch slave cylinder & the transmission to compensate for the clutch disc wear you can gain the lost clutch diaphragm spring pressure back. You could stack the BMW 23122352156 gasket or make your own thicker gasket. Gasket material is available in many different thickness sizes.
You think the slave is actually taking some of the spring pressure when the clutch lever is released as the clutch wears? That would be an stunningly stupid design if it is the case.

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post #3 of 12 Old Feb 22nd, 2014, 8:52 am Thread Starter
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Re: Increase Clutch Spring Pressure

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You think the slave is actually taking some of the spring pressure when the clutch lever is released as the clutch wears? That would be an stunningly stupid design if it is the case.
Like most hydraulic clutch systems there is no adjustment for clutch disc wear.

Dave Selvig
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post #4 of 12 Old Feb 22nd, 2014, 8:58 am
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Re: Increase Clutch Spring Pressure

Did you measure or get a sense of how much preload pressure is present? Are you suspecting excessive pressure is held against the pressure plate? I know on all the vehicles I've worked on with hydraulic clutches (many), there is preload when installing the slave cylinder, but it's abated by the fluid returning to the MC. Steve
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post #5 of 12 Old Feb 22nd, 2014, 9:41 am
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Re: Increase Clutch Spring Pressure

the piston in the slave sits in a different position when the clutch is engaged as the clutch wears, the only issue is if the piston bottoms out and therefore could no longer allow the clutch to engage. As JZ has said the fluid will rise in the master as the clutch wears exactly the opposite as the brakes where the fluid level goes down as the pistons stay further out when the pads wear down. Hydraulic clutches in cars - fluid in reservoir goes down as clutch wears. The K clutch is fairly unique in design.

the adjustment in both systems is the amount of fluid between the master cylinder piston and the slave piston when the system is at rest

Gary
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post #6 of 12 Old Feb 22nd, 2014, 9:55 am
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Re: Increase Clutch Spring Pressure

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Like most hydraulic clutch systems there is no adjustment for clutch disc wear.
Most I've seen were like disk brakes. The "adjustment" is the fluid level in the master cylinder.

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post #7 of 12 Old Feb 22nd, 2014, 9:58 am
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Re: Increase Clutch Spring Pressure

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the piston in the slave sits in a different position when the clutch is engaged as the clutch wears, the only issue is if the piston bottoms out and therefore could no longer allow the clutch to engage. As JZ has said the fluid will rise in the master as the clutch wears exactly the opposite as the brakes where the fluid level goes down as the pistons stay further out when the pads wear down. Hydraulic clutches in cars - fluid in reservoir goes down as clutch wears. The K clutch is fairly unique in design.

the adjustment in both systems is the amount of fluid between the master cylinder piston and the slave piston when the system is at rest
Exactly. That was my comment about a stupid design if the slave piston reaches end of travel and is thus always carrying load from the diaphragm spring.

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post #8 of 12 Old Feb 22nd, 2014, 10:05 am
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Re: Increase Clutch Spring Pressure

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Originally Posted by gary45 View Post
the piston in the slave sits in a different position when the clutch is engaged as the clutch wears, the only issue is if the piston bottoms out and therefore could no longer allow the clutch to engage. As JZ has said the fluid will rise in the master as the clutch wears exactly the opposite as the brakes where the fluid level goes down as the pistons stay further out when the pads wear down. Hydraulic clutches in cars - fluid in reservoir goes down as clutch wears. The K clutch is fairly unique in design.

the adjustment in both systems is the amount of fluid between the master cylinder piston and the slave piston when the system is at rest
!0-4, same as an automotive style diaphram or finger clutch. Still, Is JZ's contention that the clutch is still serviceable despite having reached a disc thickness that causes the slave to be fully compressed ? Steve
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post #9 of 12 Old Feb 22nd, 2014, 10:57 am
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Re: Increase Clutch Spring Pressure

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Exactly. That was my comment about a stupid design if the slave piston reaches end of travel and is thus always carrying load from the diaphragm spring.
highly unlikely unless the engineers were asleep during design, once the disk hits minimum it is likely prone to slipping, personally I think the spring could be
stronger but ease of use and clutch bite are a trade off, if they had put a larger diameter slave on to compensate for a stronger spring would have required a larger master to move enough fluid to disengage properly - you would feel it with the left hand

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post #10 of 12 Old Feb 22nd, 2014, 3:38 pm
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Re: Increase Clutch Spring Pressure

In my case the slave was not bottomed out and the disc was still "inside" the wear limit but close to the edge. All my slip was caused by disc thickness being so thin (and a tiny bit of engine oil from the o-ring). This was always a 5th gear forced slip that could only be done once per attempt interval of 20 minutes or so before I could induce it again.

While I don't disagree with Dave's assertion, I don't think there would be much to be gained by shimming the slave. I believe the spring has a finite limit to what it can compress on disc thickness, slave or no slave present.

John
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post #11 of 12 Old Feb 22nd, 2014, 6:31 pm Thread Starter
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Re: Increase Clutch Spring Pressure

All I know is that my disc was .010" from the wear limit & it would not engage until the clutch lever was released. I think if I had shimmed the slave cylinder out I could have changed the point that the clutch lever would start to engage the clutch disc & prevent some slipping.

Dave Selvig
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post #12 of 12 Old Feb 22nd, 2014, 9:08 pm
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Re: Increase Clutch Spring Pressure

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All I know is that my disc was .010" from the wear limit & it would not engage until the clutch lever was released. I think if I had shimmed the slave cylinder out I could have changed the point that the clutch lever would start to engage the clutch disc & prevent some slipping.
If you try, let us know. I think the shims would just change the fluid level in your master cylinder as it accommodates the shims. Now if the piston is bottoming in its bore before the disk spring has applied its full force, then I agree that shims would help. I just can't believe BMW got the design that wrong, but anything is possible!

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