the dimension is the diameter of the final drive shaft that the sprocket mounts too. I already ordered a set of the metal road wheels from Welshdragon. He must have had only one set because now there listed as out of stock. I think this is part of the problem with everyone waiting on Asiatam parts from Bill at RCC and everyone else who deals with Asiatam. They are just not supplying the dealers with either now enough parts or are just very slow in shipping them. I know that is the problem with RCC's orders for the gearboxes and other Asiatam parts. He just cant get them fast enough. And as far as the guys waiting for the upgraded turrets ,Bubbajoexx builds them for Bill. These are not just asiatam parts. These are custom turrets using Tamiya parts. Bubba gets them done as fast as he can.