Chattering and jerking can be caused by oil contaminated clutch linings (due to an oil leak on the back of the engine, usually the rear main crankshaft seal), or by damage to the clutch itself.
For the clutch to engage smoothly, the pressure plate must come down evenly on the disc. In other words, the surface of the pressure plate must be parallel to the flywheel otherwise it won't engage smoothly. If the flywheel is warped (which can happen as a result of overheating and abuse), it can make the clutch grab.
On rare occasion, a manufacturing defect in a new or rebuilt clutch can cause uneven engagement as can improper installation of the clutch. If someone allows the weight of the transmission to "hang" on the clutch, it can bend the clutch disk causing shudder, grabbing or chattering.
Damaged splines on the transmission shaft can cause the clutch disk to hang up and grab.
An often overlooked external cause of clutch chatter may be loose or broken motor mounts.
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