Concentric Slave Cylinders- ( Hydraulic Release Bearings)
This info is to help with the installation and correct setup of a Concentric Slave Cylinder also known as Hydraulic throw out bearings or hydraulic slaves. It is advised that only competent mechanics and technicians attempt to fit CSC's as incorrect fitting can damage clutch and engine parts.
First and foremost you must be familiar with what type of pressure plate you have. Generally in automotive clutches you have four basic types of clutches.
Push Type clutches-
- Flat Diaphragm
- Raised/ Rolled Finger Diaphragm and
- Reduced Diameter race clutches usually Twin or Triple plates
Pull Type Clutches- Will not work with a Concentric slave type setup.
In order to have you CSC work correctly and have good pedal feel it is important that you match the type of release bearing to the correct Diaphragm in your pressure plate
Flat Diaphragm- Must have a Bearing with a Radius
Raised/ Rolled Finger Diaphragm must have a Flat Face Bearing
Reduced Diaphragm- Must have a bearing with a radius.
Now that you have identified the correct bearing/ CSC combination you should try these tips to help setup your bearing correctly.
It is important that there is free play ( distance) between the face of the bearing and the diaphragm this is to allow for wear of the clutch disc which will in turn cause the fingers of the pressure plate to rise as the disc wears. If this is not set correctly then the pressure plate may not exert all it's clamping force on the clutch disc and may cause the clutch to slip and fail.
The amount of free play will depend on what style of clutch you are running. If you are running a standard large diameter with an organic disc- the disc is able to wear alot before the clutch will need replacing- a disc may wear over 2mm which can cause 3-6mm lift in the fingers where as a ceramic puck style disc may only need to wear 0.5-1mm before it needs to be replaced. Twin and Triple plates will also have minimal rise in the fingers and if fitted to a race vehicle service intervals may allow for this to be checked more often. You will be able to take a measurement of the clutch at static height before it is bolted down onto the flywheel and work out- what would be acceptable clearance.
If you would like to know how many mm it takes to release the clutch you can bolt the pressure plate complete with the clutch to the flywheel and place the setup into a bearing press / drill press and measure how many mm it takes before the disc moves under the pressure plate- Different clutch discs /Pressure plate combo's will have different release points eg Ceramic vs Organic.
Excessive clearance between the pressure plate and the release bearing may cause problems getting the clutch to release if the master cylinder isn't able to move enough fluid or there isn't enough pedal travel. Sometimes getting all this right may involve fitting and removing the unit a couple of times.
Depending on which style of CSC you have you will either need to use shims, a collar or studs to space the unit at the correct height. To work out these heights bolt the clutch and flywheel to the engine.
Take a measurement of the fingers in relation to the engine block or if you have a bell housing that unbolts from the gearbox bolt this to the engine and measure down to the fingers.
You will now be able to either measure from the gearbox face to the release bearing or if the bell housing doesn't remove from the face that mates the engine block down to the bearing. You will need to make sure there is a difference in these two measurements. Min of 3mm seems to work well- but will depend on what clutch you are using as above. You will need to space the CSC to get this right.
Once the CSC is installed and working the next important thing to consider is fitting a stop on the pedal. Over stroking of a clutch can cause the fingers to be pushed into the clutch disc hub damaging clutch components or stress on the crank can be applied causing premature engine thrust bearing wear. With the vehicle on a hoist or stands you can have someone actuate the clutch by depressing the pedal slowly and attempt to turn the drive shaft you will find the disengagement point of the clutch when the drive shaft starts to turn and then you can adjust the pedal stop so it's slightly after this point.
Make sure the bleed or the line with the bleed is the top most port to make sure all the air can be bled out of the system.