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i did have 285mm 4pots, new discs are 24mm thick (only available from hispec at £180odd a pair)
i have since converted to 280mm 172/182 discs (same thickness and offset) just a fair wack cheaper
21 turbos are 5stud IIRC and only 21mm thick discs (i think)
i looked into this, can't remember the exact details, but something was different about them
offset is basically where the hub face of the wheel is in relation to the centre line on that wheel
ET40 means the hub face of the wheel is 40mm from the centre line
ET30 means its only 30mm from the centre line (hence the wheel sticks out further)