Are main bearings the same as big end bearings?
In internal combustion engines, no. Main bearings and big-end bearings (also called rod bearings) are distinct components that perform different roles in supporting the crankshaft and connecting rods. They have different locations, sizes, installation requirements, and are not interchangeable.
This article explains how each bearing functions, where they sit in the engine, and what to know when diagnosing wear or planning a rebuild.
What each bearing does
Main bearings
Main bearings, or crankshaft main bearings, sit in the engine block and in the caps that clamp the crankshaft in place. They support the crankshaft along its axis and handle the radial loads generated as pistons move up and down and rotate the crankshaft. These bearings are typically shell-type inserts that ride in the main saddles and are torqued with the caps to keep the crankshaft properly aligned. In some designs, thrust surfaces are provided by the main caps or by an integrated thrust bearing within the main bearing set. Their primary job is to keep the crankshaft straight and allow smooth rotation with adequate oil film between the journal surfaces and the bearing shells.
Big-end bearings
Big-end bearings, or rod bearings, are located at the big end of each connecting rod, around the corresponding crankpin. They carry the rod’s end as it articulates with the rotating crankshaft, translating the pistons’ motion into crank rotation. Like mains, they are typically shell-type inserts housed in the rod cap and the rod’s inner surface. These bearings experience higher bearing speeds at the rod Journal due to the lever action of the connecting rod, and they require precise clearance and proper lubrication to prevent knocking or scuffing. Installations must preserve correct orientation so oil passages line up and the cap seats correctly.
Key differences at a glance
Before we dive into maintenance specifics, here is a concise comparison of the two bearing types:
- Location: Main bearings sit in the engine block between crankshaft mains; big-end bearings sit at the big end of each connecting rod around the crankpins.
- Function: Main bearings support the crankshaft itself; big-end bearings support the connecting rods and transfer piston forces to the crankshaft.
- Construction: Both are shell-type inserts, but they are sized, profiled, and matched to different journals; they are not interchangeable.
- Installation: Main bearings are clamped by the main caps; big-end bearings are clamped by the rod caps and must align with oil passages and thrust faces.
- Maintenance implications: Rebuilding typically involves replacing both mains and rod bearings in matched sets, with careful attention to clearance and orientation.
In short, while both are bearing shells and share operating principles, they serve different roles and must be treated as distinct components during servicing and replacement.
Maintenance and replacement considerations
When rebuilding or repairing an engine, bearing wear is evaluated with measurements of clearance and bearing surface condition. Plastigauge checks or micrometry help determine whether each bearing shell is within spec. Because mains and rod bearings wear at different rates and have different clearance targets, technicians typically replace them with the correct part numbers for each journal, ensuring proper orientation and seating. It’s also common to replace all bearings in a given engine rebuild to maintain balanced oil clearance across the cranktrain and reduce the risk of uneven wear.
Engine manufacturers and rebuild guides emphasize keeping track of which bearing goes where, as mixed components can lead to improper fit, oiling issues, or premature failure. If you’re shopping for replacements, use the OEM or precisely matched aftermarket parts and follow the torque specs and sequence recommended for your engine model.
Summary
Main bearings and big-end bearings are not the same; they sit in different locations, support different parts of the cranktrain, and require distinct sizes and installation practices. Understanding the distinction helps when diagnosing wear, ordering parts, and performing a rebuild to ensure reliable engine performance.
