6-bolt main refers to the number of bolts holding the main caps to the block of an internal combustion engine, and therefore holding the crankshaft in place. A main cap refers to the locating support for the crankshaft of a piston engine. The main cap holds the crankshaft in the engine with fasteners of two, four, or six bolts. The common logic is, the more bolts, the better.
In general, 6-bolt main designs are considered the strongest under all conditions. In most cases the "mains" are held in by four bolts from the bottom extending upward into the block, two on each side of the crankshaft, and two cross bolts coming from the left and right side pan rails into the side of the main caps to provide additional lateral support at excessive engine speeds. All aluminum block Ford Modular engines feature 6-bolt main caps. All GM LS engines have 6-bolt mains.