40 CFR §205.155

1. (a)Motorcycles must be grouped into classes determined by separate combinations of the following parameters:
   1. (1)Engine type:
      1. (i)Gasoline—two stroke.
      2. (ii)Gasoline—four stroke.
      3. (iii)Gasoline—rotary.
      4. (iv)Other.
   2. (2)Engine displacement.
   3. (3)Engine configuration:
      1. (i)Number of cylinders.
      2. (ii)Cylinder arrangement (i.e., in line, opposed, etc.).
   4. (4)Exhaust system:
      1. (i)Muffler: (A) Type, (B) Location, (C) Number.
      2. (ii)Expansion chambers: (A) Location, (B) Size.
      3. (iii)Spark arrestors.
      4. (iv)Other exhaust system components.