Thin shell bearings are made of a layer of white metal (or bimetal/tri-metal overlay) bond to a steel back in the form of two half shells. Compared with the older method of cast/white-metal-lined bearing housing (a "white metal flooded" bearing), they offer:

1. Greater fatigue strength: the thin layer of white metal (~0.3 to 0.5 mm) bonded to a rigid steel backing gives a much higher fatigue/load capacity, so higher specific loads and higher peak pressures can be sustained, allowing higher MEP engines.
2. Less white metal required and lower cost of the expensive overlay material.
3. Ease of replacement: shells are standardised and easily interchangeable without remetalling/re-machining the bearing housing; worn shells are simply discarded and new ones fitted.
4. Improved heat flow: the steel backing conducts heat away rapidly from the loaded zone to the housing/oil, reducing bearing temperature.
5. Better dimensional accuracy and conformity (precision bored) giving correct oil film, clearance and alignment; less tendency to fatigue cracking and wiping that plague thick white metal.
6. They can be made of composite materials (e.g. trimetal with a soft overlay on a bronze/lead-bronze) giving a surface tolerant of misalignment plus a strong backing.
7. Reduced maintenance and the ability to use higher oil pressure and load.

The shells are prevented from rotating by locating them firmly in the bearing housing/housing bore. Because the shells are a shrink/interference fit (fit with a small amount of surface pressure when the two shells are tightened - the "crush"), half shells are held in the bearing housing. In addition, a projection/lug or dowel (a small rectangular or semi-circular "nip/locator" tang) at one end of each shell engages with a corresponding groove in the bearing housing bore, so the shells cannot rotate or move axially (each half is restrained by the tang in its groove on the parting face). The correct crush/tightening of the bearing cap also clamps the shells in place, together with the tang preventing rotation and axial movement.

Axial location of the bearing (locating the shells along the shaft axis) is important so that the bearing half-shells cannot slide along the housing, which would misalign the oil holes/grooves with the oil supply passages, cut off lubrication, cause overheating, and lead to wiping/seizure of the journal. It is achieved by the locating tang/lug of each shell fitting into a machined groove in the housing bore, and by the shells being clamped axially (with correct end clearance) between the housing shoulders. The shells are also designed so that the oil groove and hole coincide with the supply passages in the housing, and their axial position is fixed so lubrication is always delivered to the oil groove. Incorrect axial location would starve the bearing of oil and cause failure.