40 C.F.R. § 205.153 Engine displacement.
Title 40 - Protection of Environment
(a) Engine displacement must be calculated using nominal engine values and rounded to the nearest whole cubic centimeter, in accordance with American Society for Testing Materials (ASTM) E 29–67. (b) For rotary engines, displacement means the maximum volume of a combustion chamber between two rotor tip seals minus the minimum volume of that combustion chamber between those two rotor seals times three times the number of rotors.
cc=(Maximum chamber volume−minimum chamber volume) × 3 × number of rotors.
Title 40: Protection of Environment
PART 205—TRANSPORTATION EQUIPMENT NOISE EMISSION CONTROLS
Subpart D—Motorcycles
§ 205.153 Engine displacement.