Yes, the diaphragm spring wants to be pretty much flat, not dished in much, they do different thickness pressure plates to help with this but I too have an extra plain plate in.
I also made a thin shim of the appropriate diameter to place under the diaphragm retaining circlip to take up any excess play in the plate stack.Yes, the diaphragm spring wants to be pretty much flat, not dished in much, they do different thickness pressure plates to help with this but I too have an extra plain plate in.