240mm.....you can try to get an aftermarket one with an adapter for the Porsche alternator. Maybe Oliver Knuf (
www.powertuningparts.de ) can help you with that. I know that the 240mm ring is used for the Porsche 911 cooling kits in Germany. You might also be able to get a 911 Turbo ring and turn it down to 240mm because I believe it is the smallest ring of the Porsche ones but I might be mistaken.
Alex