A cross-bolted bearing is a bearing, usually a crankshaft

Crankshaft

The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...

 main bearing

Main bearing

In a piston engine, the main bearings are the bearings on which the crankshaft rotates, usually plain or journal bearings.All engines have a minimum of two main bearings, one at each end of the crankshaft, and they may have as many as one more than the number of crank pins...

 of a piston engine. Most bearing caps are retained by two bolts, one on each side of the bearing journal, and parallel to the cylinder axis. A cross-bolted bearing has one or more additional bolts, at right-angles to both the cylinder and crankshaft axes. These bolts may be either a single through bolt passing through the crankcase from one side to the other, or else two blind bolts threaded into the bearing cap from each side.

Cross-bolting has two advantages: it couples the two sides of the crankcase

Crankcase

In an internal combustion engine of the reciprocating type, the crankcase is the housing for the crankshaft. The enclosure forms the largest cavity in the engine and is located below the cylinder, which in a multicylinder engine are usually integrated into one or several cylinder blocks...

 together, making it stiffer overall and less prone to twisting. Secondly it helps to hold the bearing cap down against the downward force from the piston. This second advantage is usually only possible with blind bolts, as the dowel effect of a through bolt is less effective.