When replacing the disc you must make sure the flywheel and pressure plate are both straight. If not, they must be reground straight before a new disc is installed. It is common for the pressure plate to wear or warp when in service. If you have warpage on the pressure plate and you install a new disc it will only touch on the outside edge of the new disc, only providing a small amount of the holding power. If this occurs then the clutch may slip badly and burn the flywheel and pressure plate.