The crankshaft is fitted into the engine block on a set of main bearings. There are caps which then bolt into the bottom of the block to hold the crank and bearings in place. Most all production motors use caps which are secured with 2 bolts, although some high performance motors which would see extreme rpm's and/or extreme cylinder pressures were produced with caps which required 4 bolts per cap to secure them to the block. In extreme service, 4 bolts will keep the cap from moving around in its saddle better than 2 bolts will. If we can keep the cap from moving, we can maintain a better control of bearing clearances and therefore oiling issues.