If it's on a 216 or 235, I can tell you what the flywheel overall diameter will be (they're the same.)
But if you're getting a replacement clutch, just count the bolts holding the pressure plate on. If it has 6 bolts, it'll be a 9-inch clutch. If it has 9 bolts it'll be an 11-inch clutch. But you're not likely to find one of the 11-inch clutches on a 3100. They were used on big bolts mainly and are unobtanium now. If you can find one, plan on taking out a big loan to buy it.
