In all forms (except the aluminum ZL-1), the "rat motor", as it was later nicknamed (small-block engine being a "mouse motor"), was slightly heavier than the W-series model, with a dry weight of about . Aside from the new cylinder head design and the reversion to a conventional 90 degree cylinder head deck angle, the Mark IV shared many dimensional and mechanical design features with the W-series engine. The cylinder block, although more substantial in all respects, used the same cylinder bore pitch (bore spacing) of with a larger main bearing dimension, increased from the of the older engine (in fact, the shorter-stroke 348 and 409 crankshafts could be installed with the use of "spacer bearings" without modifying the crankshaft). Like its predecessor, the Mark IV used crowned pistons, which were castings for conventional models and impact extruded (forged), solid skirt types in high performance applications.