Engine bearings: the layer stack, and the 20 mm rule

A bearing is not a plain bush. It is a steel back, a bonding layer, and a running layer chosen for load. Measuring it wrong is more common than wearing it out.

What the layers do

The steel back carries the crush and holds the shape. The intermediate layer bonds. The running layer is the soft, sacrificial surface that embedds dirt and tolerates a brief loss of oil.

Aluminium-tin alloys (such as AlSn20Cu) are common for main and rod bearings in this class of engine. Copper-based alloys carry higher load. Ceramic-reinforced layers are used in heavily loaded industrial engines.

Measure at 20 mm, with the caps torqued

Bearings are made slightly oval on purpose so they hold tension in the housing. Measuring them loose, or at the parting line, gives a wrong number you will chase forever.

Torque the cap to spec, then measure wall thickness about 20 mm from the parting line. Compare with the journal to get the oil clearance.

Reading the wear pattern

Wear at the centre of the shell means the journal is bowed or the housing is distorted. Wear at both edges means the journal is tapered. A single bright patch means dirt was embedded there. Scoring across the whole face means the oil was contaminated or the pump was weak.

Never fit new bearings onto a scored journal — the crank has to be ground or replaced first.