Find the difinitive answer here:
http://en.wikipedia.org/wiki/Formula_TT*
The TT Formula was divided into three categories:Formula I - four-stroke from 600 to 1000 cc and two-stroke 350 to 500 cc
Formula II - four-stroke from 400 to 600 cc and two-stroke 250 to 350 cc
Formula III - four-stroke from 200 to 400 cc and two-stroke 125 to 250 cc
*Source - Wikipedia