The TT is not a clone of the OS. The OS has (I think) a wedge head with two parallel valves. The TT .91 has a himi or penthouse head with two valves slanted toward each other. Also the TT has much longer rocker arms which give more valve lift. It is also a lot more powerfull than the OS or Magnum versions. The designer of the OS Surpass quit and worked on the design of the TT four strokes, so they are what OS would have had if he had stayed.