Though Honda was initially concerned with reliability and detuned the original 250cc motor, with the release of the Pro-X (pro-cross) cylinder and head in 1994, the 250R's displacement could be increased well into the 300cc range by changing the cylinder alone. Additionally, this and other aftermarket cylinders offered advanced port layout for more tuning options. With modifications to the transmission cases, later aftermarket cylinders and crankshafts permitted engine displacements to reach 500cc's. The last version of the Pro-X cylinder, as well as others, also added an exhaust operated powervalve, a feature that was left off the 250R but used almost universally by the competition, as well as in Two-Stroke motocross bikes since the mid-eighties.