Experienced car restorers will tell you that while finding a classic car from the Fifties or Sixties whose engine can still turn over is highly unusual, one whose clutch is in perfect working order would be pretty close to a miracle!
From around the mid-Thirties till the end of the Sixties, a single plate coil spring clutch, usually produced by Borg and Beck, was fitted to most European and British popular cars. Since then, most clutches have been diaphragm operated.
Compared to other automotive devices, the clutch on cars was and remains decidedly low-tech, meaning that the task of replacing and adjusting the clutch requires neither exceptional skill nor access to sophisticated equipment.
The role of the clutch is to transfer power from the flywheel to the gearbox input shaft through a pressure plate.
When pressed, the plate, which is spring-mounted, pushes against the flywheel; and the release bearing, which in turn moves the pressure plate to release the clutch plate – and disengage the clutch – when the pedal is pressed.
Problems occur when the clutch doesn't fully disengage, that when the familiar grinding occurs during gear changes.
When assessing a clutch's state of health, the first step should be to check the pedal-arm bushings and the actuating cable. In the case of vehicles fitted with hydraulic systems, the slave and master cylinders should be examined.
At the heart of the clutch assembly is the clutch disc.
The disc is composed of friction material, similar to brake-lining material, which lies on each side and a splined hub that slides back and forth on the transaxle's splined input shaft.
Pressing on the clutch pedal allows the clutch disc to spin freely between the flywheel and pressure plate, allowing the engine, flywheel and pressure plate to spin on their own, while the splined input shaft and transaxle freewheel with the disc.
The clamping force that engages the clutch comes from a large spring, or series of springs, located inside the pressure plate called a diaphragm. Depressing the clutch pedal presses on the diaphragm spring, causing the pressure plate to move away from the clutch disc, and the clamping force diminishes until the engine can spin independently of the transaxle.
In cars fitted with a hydraulic system, pressing on the clutch pedal moves a piston in the master cylinder, displacing fluid, causing the piston in the slave cylinder to move, operating the clutch.
A clutch-release bearing placed at the end of the release fork applies even pressure to the diaphragm spring, and the pressure plate moves back, releasing the clutch disc. Disengaging the clutch causes a pressure reduction on the spring, allowing the clamping force on the disc to return.
One familiar problem is oil leaks from the engine that reach the surface of the clutch, causing it to slip.
Extra care should be taken to ensure that the oil seal has not sprung a leaking, which means that it should be replaced without delay.
Before replacing the seal, it will be wise to mark the flywheel so it can be reinstalled on the crankshaft in the correct index
Specific models of classic cars were fitted with "stepped" flywheels, meaning that the clutch disc rides on one surface and the pressure plate bolts to a surface that's stepped above the clutch surface.
These stepped flywheels can wear out over time and will need regular maintenance and often repair.
While repairs can be effected, the only safe and sure alternative is to replace the worn flywheel with a new one.
When all of these checks have been completed, it's time to install the replacement clutch.
Before installing a new or reconditioned flywheel, the crankshaft flange must be examined to ensure that it is spotless. After the flywheel has been positioned on the proper flange, the bolts must be tightened in a star pattern in line with the appropriate torque specification.
Installing the clutch disc and pressure plate requires a lot of attention to detail. The first rule is to remember that the disc is designed to go on in one direction with the damper hub that protrudes from the friction surface pointing in the direction of the transaxle.
Secondly, the clutch disc needs to be installed to align with the pilot bushing in the crankshaft. A specialised item of equipment. a "clutch alignment tool" will be required to carry out his delicate procedure.
To install the disc, it must be slid onto the alignment tool, which is then held in place over the clutch disc until all the bolts are fixed using a torque wrench in a star pattern according to the manufacturer's specifications. After this stage is complete, the alignment tool can be removed.
Once everything has been closed up, the new clutch will be ready for action, and another step in the restoration process will be over.
Having a well-maintained clutch makes for a pleasant driving experience, taking very little time and will not make too large a dent in the restoration budget.
If it turns out that they are in any less than perfect condition, they should be immediately replaced.