Bit of an old thread but I found it using the search.
I'm also trying to get a rear crank seal but viton say they can't supply the size I need, 79.38x95.25x11 (that's the dimensions marked on the seal I have at the moment) the best they can do is an imperial equivalent which is quite close on the I.D and O.D but only works out around 8.5mm on the thickness.
Does that sound right?