Camshaft Bearing

Definition and Function

The camshaft bearing (cam bearing) is a crucial component that supports the rotation of the camshaft. As an integral part of the engine valvetrain, its primary function is to keep the camshaft precisely positioned during high-speed operation. By minimising friction and vibration during rotation, it ensures the valves open and close with exact timing and lift, directly influencing engine power output and running refinement.

Structure and Types

Camshaft bearings are broadly categorised by structure into two main types: plain bearings (journal bearings/bushings) and rolling element bearings. In modern passenger car engines, plain bearings are widely utilised, where bearing shells are fitted directly into the cylinder head bearing journals, allowing the camshaft journals to rotate smoothly within them. These bearings are typically of a split-type design for straightforward installation.

Another configuration is the needle roller bearing, which falls under the rolling element bearing category. Its typical design integrates a thick outer-ring needle roller bearing with a mounting stud featuring internal oil feed galleries. The outer ring profile comes in cylindrical or crowned/spherical variants, with full-complement roller designs tailored for heavy-load, low-speed applications, whilst eccentric variants allow fine-tuning of alignment via the stud axis. To facilitate fitting, the bearing head is engineered with a slotted or hex socket drive. While needle roller bearings offer lower load capacities compared to plain bearings, their significantly lower rotational drag allows them to operate on minimal lubrication, slashing starting torque by around 90% and rotational drag by roughly 50%. Consequently, replacing conventional plain bearings with rolling element units has emerged as a key engineering approach to boosting engine fuel efficiency in recent years.

Materials

The choice of material for camshaft bearings directly impacts their durability and operational reliability. Plain bearings generally feature a bi-metal construction: a steel backing to provide structural strength, bonded to a bearing alloy lining (namely Babbitt or white metal) to deliver optimal surface properties. Bearing alloy typically comprises a tin- or lead-based matrix alloyed with elements like antimony and copper, where hard intermetallic compounds resist wear while the softer matrix accommodates embeddability and conformability. Babbitt metal delivers superb wear resistance, low friction, compressive strength, and resistance to seizing. Certain high-performance bearings incorporate an ultra-thin electroplated overlay atop these layers to further enhance surface durability and performance.

Common Faults

Common camshaft bearing failures primarily include abnormal wear, unusual knocking noises, and bearing seizure (wiping/melting).

Abnormal wear is the most frequent issue. Root causes include: natural wear and tear from prolonged mileage; blocked oil galleries and insufficient engine oil pressure/supply; degraded, low-quality, or contaminated engine oil; and abrasive metallic debris (such as aluminium and vanadium particles) entering the oil clearance, causing severe scoring. Furthermore, misaligning the bearing oil holes with the cylinder head/block oil supply passages during installation will starve the journal of lubrication.

Unusual noises serve as a telltale sign of bearing wear. When excessive bearing-to-journal clearance develops from wear, the resulting play induces impact and vibration, producing a distinct, sharp metallic tapping or knocking sound. Technicians can diagnose this by using a mechanical stethoscope across the camshaft bearing locations. The noise is distinct at idling, pronounced under mid-range engine speeds, and may become muffled or erratic at higher revs—often pointing to excessive end float or loose bearing fitment.

Bearing burn and seizure represents a catastrophic failure. When clearance is overly tight or oil starvation occurs, frictional heat can rapidly melt the bearing material and weld it to the camshaft journal, resulting in complete camshaft seizure.

Maintenance and Replacement

Camshaft bearing service centres on clearance measurement and replacement. The standard operating clearance between the camshaft journal and bearing typically ranges from 0.02 to 0.10 mm, with a maximum allowable service limit of 0.10 to 0.20 mm. Exceeding this limit necessitates rectification or outright replacement. Measurement procedures follow the same principles used for crankshaft main bearing clearances (such as using Plastigauge).

It is important to note that most modern engines do not offer undersized/oversized repair bearings for camshaft journals. Once clearances exceed service limits, the camshaft and bearing shells usually have to be replaced as an assembly. If the bearing journals or cylinder head bearing saddles are excessively scored or worn, replacing the entire cylinder head or block assembly may be required. Only in rare engines where journal regrinding is permitted can the journals be machined and paired with matching undersized repair bearings.

During installation, bearings require a precise interference fit within their housing bores. Technicians must strictly ensure that the bearing oil feed holes align precisely with the engine oil passages. Camshaft end play (axial clearance) must also be checked and set to factory specifications, typically measured using feeler gauges. If thrust flange wear causes excessive axial float, a standard-thickness thrust flange should be installed; technicians must never alter thrust washer/spacer thickness arbitrarily, as this alters camshaft alignment and disrupts valve timing accuracy. As of July 2026, industry standard repair practices mandate strict adherence to OEM technical specifications for all inspection and overhaul procedures.