Diesel engines and tractor units, and specially designed components therefor. a. Diesel engines for trucks, tractor units, and automotive applications of continuous power output of 400 BHP (298 kW) or greater (performance based on Society of Automotive Engineers J1349 standard conditions of 100 kPa and 25°C); b. Off-road semi-trailer wheeled tractor units of carriage capacity 9 t or more and specially designed components therefor; c. On-road semi-trailer tractor units, with single or tandem rear axles rated for 9 t per axel or greater and specially designed components therefor.