(a)(1) Engine displacement shall be calculated using nominal engine values and rounded to the nearest whole cubic centimeter.

(2) For rotary engines, displacement means the maximum volume of a combustion chamber between two rotor tip seals, minus the minimum volume of the combustion chamber between those two rotor tip seals, times three times the number of rotors, according to the following formula:

cc = (max. chamber volume − min. chamber volume) × 3 × no. of rotors

(b) Motorcycles will be divided into classes and subclasses based on engine displacement.

(1) Class I—0 to 169 cc (0 to 10.4 cu. in.).

(i) Class I motorcycles with engine displacement less than 50 cc comprise the Class I-A subclass.

(ii) Class I motorcycles with engine displacement 50 cc or higher comprise the Class I-B subclass.

(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 and compliance with emission standards will be determined on the basis of engine displacement.

[69 FR 2437, Jan. 15, 2004, as amended at 81 FR 73979, Oct. 25, 2016; 86 FR 34366, June 29, 2021]