Although originally fitted with an EMD 8-567CR engine of , all were later fitted with 645 type "power packs" (piston & liner assemblies) for parts standardisation, while at the same time keeping their original power output for reliability reasons. They weighed and had a maximum speed of . Numbers B126-B129 were later rebuilt with an EMD 8-645E engine of ; as used in the 181 Class locomotives. The engine blocks were originally recycled from withdrawn rebuilt class B201s, but in their later years much swapping of engines occurred between classes 121, 141 and 181.