The pressure plate must be dead flat, and yours is conical.
I suspect you will also find that the clutch plate is also both not
flat and too thin -- if it's shiny, it has been slipping or had gotten
oil on it from a bad crankshaft or front transmission seal.
I would at the very least replace the clutch plate and pressure plate
assembly. You will also need to have the flywheel resurfaced flat.
I suspect you will also find broken springs in the center of the clutch
plate.
You may need a new pressure plate spring as well, the fingers may have
lost tension.
Chattering clutch is usually caused by bad pressure plate springs,
broken internal clutch plate springs, or oil on the clutch plate.
After it happens for a while, the flywheel will wear unevenly and
chattering will continue until all surfaces are flat and all the
springs are good.
Peter
