40 CFR 205.155 - Motorcycle class and manufacturer abbreviation.
(a) Motorcycles must be grouped into classes determined by separate combinations of the following parameters:
(1) Engine type:
(i) Gasoline - two stroke.
(ii) Gasoline - four stroke.
(iii) Gasoline - rotary.
(2) Engine displacement.
(3) Engine configuration:
(i) Number of cylinders.
(ii) Cylinder arrangement (i.e., in line, opposed, etc.).
(4) Exhaust system:
(i) Muffler: (A) Type, (B) Location, (C) Number.
(ii) Expansion chambers: (A) Location, (B) Size.
(iii) Spark arrestors.
(iv) Other exhaust system components.