The clutch on the MG Midget can only be accessed by removing the engine... typical! In most old cars the gearbox can be dropped to get to the offending item. Being an engine out job, I determined that this was a little too much to take on myself (at least in an acceptable time-scale).
The clutch itself is quite a simple single dry plate affair. It has a single drive plate with friction linings attached by rivets. The centre (hub) of the plate slides on spines machined on the gearbox output shaft. Drive from the plate to the hub is transmitted through springs to allow a cushioned take up of power. The plate is sandwiched between the rear face of the flywheel and a spring-loaded pressure plate carried inside a cover bolted to the flywheel. Drive is disengaged when the pressure plate is drawn backwards against the force of the thrust (pressure plate) springs. This movement is activated by levers inside the cover, ultimately connected to the clutch pedal hydraulically.
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| Clutch components, 1275cc MG Midget |
When the clutch pedal is pressed down, the clutch master cylinder forces hydraulic fluid through hydraulic pipes to the clutch slave cylinder. A piston in the slave cylinder is connected to the pivoted clutch release arm by a short pushrod. As the hydraulic fluid pushes the piston out, the release arm moves and the forked legs at the other end of the arm move forward against the clutch release bearing. The release bearing contains a carbon (graphite) block, that stands proud of its housing cup. This carbon-faced bearing pushes against the release bearing thrust plate. The thrust plate actuates the three clutch release levers radiating from the hub which are pivoted so as to move the pressure plate backwards, so by releasing the clutch plate. When the clutch pedal is released, the pressure plate springs force the pressure plate back into contact with the friction linings on the clutch plate and at the same time force the clutch plate against the flywheel (in the pressure plate - clutch plate - flywheel sandwich).
It was the carbon release bearing that failed in this instance. The garage tells me that lumps of carbon fell out of the casing when the gearbox and engine were separated. When the bearing failed the slave cylinder that actuates the other end of the clutch release arm over-extended, causing the piston to leave the cylinder and the clutch pedal to lose resistance and go to the floor. The slave cylinder was replaced.
The failure came as a bit of a surprise, and occurred when Mrs C was driving the car. The clutch was a little noisy, but no more than the gearbox, or what one might expect from a car of this vintage. There were certainly no nasty squeals when changing gear, a tell-tale sign of of a badly worn clutch release bearing. However, I did start to get strange noises from the starter motor just before the clutch failed. This could have been caused by bits of deteriorating carbon bearing becoming stuck in the teeth of the flywheel and the starter motor pinion. The starter motor has been now been checked and seems okay.
We've now done over 1,500 miles since buying the car, in around six months. Whilst this is not much by modern standards, it is still quite a lot for an old girl and a lot more than she was used to before I got hold of her! Wear of the clutch release bearing is accelerated if the clutch is ridden or held down for long periods in gear whilst the engine is running. Note to self: remember to put car in neutral at traffic lights and junctions!
Thanks to Alan and the team at Darley Dale Garage for getting "ABW" back on the road.
