|The 351M is sort of a hybrid between a 351C and the 400. The 351C and 351M have the same bore and stroke - 4.00" x 3.50" (also shared by the 351W and the 352). The 351C, 351M, 351W, and all other
small blocks share the same bore spacing and cylinder head bolt pattern. Cleveland style heads were first used on the 1969 Boss 302. |
The 351M has the higher deck block of the 400.
This makes the engine wider and heavier than a 351C. As a result, 351M has longer connecting rods.
The 351M uses the larger 351W / 400 sized crankshaft journals. Stronger in the heavy duty truck sense, but more friction for performance applications
at high RPM.
| ||351C ||351M ||400 ||351W |
|Deck height ||9.206" ||10.297" ||10.297" ||9.503"* |
|Rod length ||5.780" ||6.580" ||6.580" ||5.956" |
|Main journal ||2.749" ||3.000" ||3.000"
|Rod journal ||2.311" ||2.311" ||2.311" ||2.311" |
||1.647" ||1.947" ||1.647" ||1.769" |
* 1969/70 351W deck is 9.480"
The 351C was available with both 2-barrel and 4-barrel heads, with very different port sizes. The 4V size was the choice
for high performance builders in the '70s. The 351M is available only with the smaller port, 2V heads. These are more practical on a street engine where low end torque is desired.
The 351C uses the Small
Block (289/302) bellhousing bolt pattern. The 351M and 400 use the Big Block (429/460) bellhousing bolt pattern, with the exception of a fairly rare 1973 block.
351C blocks can be machined for 4 bolt mains.
The 351M/400 uses thinner wall block so isn't as strong after an overbore, and more prone to overheating.
351M/400 blocks cast at Michigan Casting Center before March 1977 had casting problems, resulting
in cracking of the water jacket inside the lifter gallery. Blocks with date codes of 7C01 or later should be OK.
Cleveland family engines use a distributor that will physically interchange with those of the 429/460 big block.