The crankshaft of a four-cylinder engine usually has five main bearings. At the front of the crankshaft provision is made to locate and drive the crankshaft pulley and timing gear via keyways and securing bolts . The oil pump drive is located behind this, and then the first or front main bearing.
The big-end bearing for the first cylinder is fitted in between the crankshaft webs radiating from the main bearing journals . These webs form counter-balance weights to the big-end journal. One of the main bearings is usually fitted with a thrust washer to control axial movement of the crankshaft.
At the rear of the main bearing journal, at the back of the engine, a machined face is formed on the crankshaft as a mating surface for a sealing ring. This is the main oil seal at the back of the engine . In addition, there is a machined, threaded flange surface to accommodate mounting of the flywheel. For a four-cylinder engine, the big-end journals are paired and set at 180°. For most four-cylinder engines the firing order is 1, 3, 4, 2.
The crankshaft bearings are split-type, steelbacked shells with an alloy or coated bearing surface . Correct bearing types to engine manufacturers’ original specification must always be fitted. Bearing shell halves, when correctly fitted and tensioned in the bearing caps, form a perfectly round profile with equidistant clearance around the bearing journal. The bearings are ‘nipped’ and held in position
when fitted into the tightened bearing caps . The bearing shell is also fitted with a locating lug on the back that mates with a slot in the bearing locating ‘half-bores’ . This ensures that the bearing cannot rotate. Oil supply holes and slots are machined in the bearing surface to supply appropriate lubrication.