The 6-198 and 6-225 slant-six engines are inclined toward the right at an angle of 30° from the vertical. The engine uses in-line overhead valves and wedge shaped combustion chambers with adjustable mechanical tappets through 1980.The 1981 and later engines employ hydraulic lifters. The lubrication system consists of an externally mounted rotor type oil pump on the lower right-hand side of the block. The semi-series flow cooling system contains an aluminum water pump body with a pressed-in ball bearing and seal assembly.
The 6-198 was used in limited numbers on early D100 models. The 6-225 was the standard engine on all other models.
The 6-251 is and in-line 6 cylinder engine of the L-head, or "flathead'' design, with all the valves and valve actuating mechanism located in the block. The are no moving parts in the cylinder head, giving it a thin flathead appearance. The exhaust valve seats are replaceable. The block and head are cast iron. The camshaft rides in 4 bearings and is driven by crankshaft via a timing chain.
The diesel engine is a Mitsubishi built, in-line 6 cylinder with overhead valves. The diesel engine is entirely metric, therefore requiring the use of metric tools.
All V6 and V8 engines used are valve-in head type engines with wedge shaped combustion chambers. All are equipped with hydraulic tappets. The 318 and 360 lubrication system is a rotor type oil pump mounted on the rear main bearing cap and a full-flow, throwaway element filter located on the lower right-hand side of the block. The 400 and 440 oil pump and filter are on the outside of the block at the front of the engine.