There were two engine types in the Ford engine family known as the 335 series, the 351 Cleveland (351C) and the “M-block.” There were several minor variants of the 351C engine (including the basic 2V and several high-performance 4V versions), but there were only two M-block variants — the 351M and the 400. From 1971 to 1982, Ford manufactured millions of M-block engines for use in mid-sized cars, full-sized cars, luxury cars, and light-duty trucks.
The 400 featured a unique engine block design with the big-block (429/460) bell housing bolt pattern, and the same bore diameter (4.00") and bore spacing (4.38") as the 351C and the later Windsor-class small blocks (i.e., 289, 302, and 351W).
Compared to the 351C block, the 400’s deck height is over 1" taller (10.297" vs. 9.206"), the 400 has larger crankshaft main bearing journals (3.00" vs. 2.75"), and the 400 uses longer connecting rods (6.58" vs. 5.78"). The 400 achieves its additional 49 cubic inches of advertised displacement with a 4.00" stroke of the crankshaft (compared to the 3.50" stroke of the 351C). The 400 has the longest stroke of any Ford pushrod V8 engine.
Both engine types in the 335 series (351C and M-block) share many design characteristics, but the only major component that is interchangeable between 351C and M-block engines is the cylinder head. Several internal components and accessories interchange, including camshafts, timing sets, water pumps, fuel pumps, oil pans, distributors, and thermostat housings. All 335-series engines are externally balanced with a 28 oz-in imbalance on the crankshaft (the same as 302 and 351W engines up to 1982).