The 3 mil is made by Vito's. It comes with stuffer blocks, a spacer plate and two paper base gaskets. Case trenching isn't required with this crank because the big end of the rod is smaller than stock. The 4 mil stroker is made by Hot Rods. This kit requires that the cases be trenched and the head machined to accept the additional stroke. The 4 mil uses a standard size rod. Both kits will benefit from cylinder porting to compensate for the additional stroke. If you decide to go with the 4 mil, the cases will need to be set up on a mill and have 3 mm removed from the surface of the crankcase. This needs to be done on center to the outer race of the bearing, not the surface of the crankcase.