roblem: Core Shift and Overheating

The small-block Chevrolet debuted in 1955 as a 265 cubic inch engine with 3.750-inch cylinder bores. By 1970, the small block had grown to 400 cubic inches with 4.125-inch cylinder bores. The exterior dimensions of the block, however, remained unchanged, so it was a matter of squeezing more stuff into a limited space.

The predictable result was problems with core shift and overheating.

The big-bore factory blocks are infamous for cooling problems. It's not just the siamesed cylinder barrels that are the root cause, although the steam holes in the decks of 400-ci blocks were intended to forestall coolant circulation problems. The real issue is coolant flow around the cylinder barrels. With thick and thin areas on the cylinder walls, boring a 400 factory block can be risky business.


The Dart SHP block has scalloped outer water jacket walls that solve both problems simultaneously. First, there is more space for coolant to circulate around the cylinder barrels. Moving the water jacket outward also allowed the thickness of the cylinder walls to be increased. The SHP block has a  .230" minimum wall thickness at 4.165" diameter, the recommended maximum bore size.