Substantial modification of a marine diesel engine means:
.1 For engines installed on ships constructed on or after 1 January 2000, substantial modification means any modification to an engine that could potentially cause the engine to exceed the applicable emission limit set out in regulation 13. Routine replacement of engine components by parts specified in the Technical File that do not alter emission characteristics shall not be considered a “substantial modification” regardless of whether one part or many parts are replaced.
.2 For engines installed on ships constructed before 1 January 2000, substantial modification means any modification made to an engine which increases its existing emission characteristics established by the Simplified Measurement method as described in 6.3 in excess of the allowances set out in 6.3.11. These changes include, but are not limited to, changes in its operations or in its technical parameters (e.g., changing camshafts, fuel injection systems, air systems, combustion chamber configuration, or timing calibration of the engine). The installation of a certified Approved Method pursuant to regulation 188.8.131.52 or certification pursuant to regulation 184.108.40.206 is not considered to be a substantial modification for the purpose of the application of regulation 13.2 of the Annex.
Source: Resolution MEPC.177(58), amendments to the Technical Code on Control of Emission of Nitrogen Oxides from Marine Diesel Engines, 10 October 2008, International Maritime Organization. Regulatory Guidance