The main bearing caps are what hold your crankshaft to the engine block. Every combustion event tries to violently shove the crank downward and sideways — the caps are what stop it from launching out the bottom of your engine. How those caps are bolted determines how much power the bottom end can survive.

Two-Bolt Mains: The basic design — each cap is held down by two vertical bolts, one on each side of the crank journal. Cheap to manufacture, adequate for stock power levels (typically up to 400-500 hp in a small-block V8). The weakness is cap walk: under heavy load, the cap can flex side-to-side because nothing resists lateral movement. The Chevy 350 in your grandpa's truck uses two-bolt mains and runs forever at 250 hp.

Four-Bolt Mains: Adds two more vertical bolts outboard of the original pair. The extra clamping force resists vertical separation under high cylinder pressure. GM's LS3 and the Ford 5.0 Coyote both use four-bolt mains on the inner caps (where loads are highest), and two-bolt on the end caps. Good for sustained power around 700-800 hp naturally aspirated.

Cross-Bolted (Splayed) Mains: The serious stuff. In addition to vertical bolts, two more bolts come in horizontally from the sides of the block, threading into the cap at an angle. This locks the cap against lateral movement — no more cap walk. The Hemi 6.2 Hellcat, Nissan VR38DETT (GT-R), and Porsche's flat-six Mezger all use cross-bolting because they're making 600+ hp with boost.

Rule of thumb for clamping load: A 1/2"-13 ARP main stud torqued to 110 ft-lb generates roughly 19,000 lbf of clamp force. A two-bolt cap = 38,000 lbf holding the crank. A four-bolt cap = 76,000 lbf. Now consider: peak cylinder pressure in a boosted engine can hit 2,500 psi across a 4" bore — that's 31,000 lbf shoving the piston down on every power stroke. You can see why two-bolt mains start losing the fight above 600 hp.

The aftermarket fix: A main cap girdle — a single billet plate that bolts across all the caps, tying them together so they share loads. Common upgrade for built LS engines. Doesn't equal cross-bolting, but it dramatically reduces cap walk for ~$400.