CFR > Title 40 - Protection of Environment > CHAPTER I—ENVIRONMENTAL PROTECTION AGENCY > SUBCHAPTER C—AIR PROGRAMS (Parts 50–99) > PART 86—CONTROL OF EMISSIONS FROM NEW AND IN-USE HIGHWAY VEHICLES AND ENGINES > SUBPART E—Emission Regulations for 1978 and Later New Motorcycles, General Provisions (§86.401-2006 to §86.449) > 86.419-78—Engine displacement, motorcycle classes.

86.419-78—Engine displacement, motorcycle classes.

(a) (1) Engine displacement shall be calculated using nominal engine values and rounded to the nearest whole cubic centimetre, in accordance with ASTM E 29-67.
(2) 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 tip seals times three times the number of rotors.
cc=(max. chamber volume − min. chamber volume) × 3 × no. of rotors
(b) Motorcycles will be divided into classes based on engine displacement.
(1) Class I—50 to 169 cc (3.1 to 10.4 cu. in.).
(2) Class II—170 to 279 cc (10.4 to 17.1 cu. in.).
(3) Class III—280 cc and over (17.1 cu. in. and over).
(c) At the manufacturer's option, a vehicle described in an application for certification may be placed in a higher class (larger displacement). All procedures for the higher class must then be complied with, compliance with emission standards will be determined on the basis of engine displacement.