The old clutches were 13", the "new and improved" clutches are 11"... Just to give you an idea, most Mack Trucks have 14" clutches (although they are double disc units). Try and get an original 13" clutch if you can, they have alot more surface area. I replaced the original clutch in my 1990 first gen at 240,000 miles only because I had to do a rear main seal and figured since I had to take the old clutch out, a new one was going back in. The old one still had PLENTY of life left and that truck worked HARD every day, heavy loads and big trailers.