40 CFR 90.109 - Requirement of certification
(a) An engine's crankcase must be closed.
(b) For purposes of this section, “crankcase” means the housing for the crankshaft and other related internal parts.
(c) Notwithstanding paragraph (a) of this section, the Administrator will allow open crankcases for engines used exclusively to power snowthrowers based upon a manufacturer's demonstration that all applicable emission standards will be met by the engine for the combination of emissions from the crankcase, and exhaust emissions measured using the procedures in subpart E of this part. This demonstration may be made based upon best engineering judgment. Upon request of the Administrator, the manufacturer must provide an explanation of any procedure or methodology used to determine that the total CO emissions from the crankcase and the exhaust are below the applicable standard for CO.