The connecting rod takes the force, which the piston receives, and transfers it to the crankshaft. Since it is constantly affected by compressing and pulling force, it needs to be sufficiently strong and rigid. There is an oil jet attached to the big end of the connecting rod for lubrication and cooling. Engine oil is supplied through the crankshaft engine oil hole. The connecting rod attaches to the bearing cap, so check the front mark as not to make a mistake when assembling the two parts.

When there is an appropriate oil film on the surface of the bearing, it can absorb the heavy load and shock from the rotating parts in the combustion stroke. This oil film prevents seizing and reduces output loss due to friction. There is an oil hole and oil groove in the upper half-bearing and it supplies oil to the bearing and main journal and lubricates. There is a locking lip to keep the bearing from rotating. The thrust washer absorbs the force applied to the crankshaft in the axial direction. There is an oil groove on the surface that touches the crankshaft. There is a tab on the bottom side thrust washer to keep it from turning. There are some engines that do not have the bottom side thrust washer.

1. Bearing size

There are several standard sizes for crankshaft bearings. This size code is inscribed on the back of the bearing. Use this code when selecting bearings to improve the crankshaft main journal oil clearance precision in order to prevent abnormal noise and seizing and to maintain good fuel economy. As the size code increases, the bearing thickness increases in micron units. Number of standard sizes and size coding differ according to the engine models.

2. Undersize bearing

When the crankshaft main journal is damaged or the oil clearance becomes larger, the main journal is ground and a thicker undersize bearing can be used. There are some engines that cannot accept an undersize bearing replacement. If this is the case, the crankshaft must be replaced.

The big end bore is machine-manufactured with the connecting rod and bearing cap. Due to unevenness in machine manufacturing precision, differences in big end bore size can occur. For that reason, there are several standard big end bore sizes. The size code of each bore is inscribed on a bearing cap. Use this code when selecting bearings to improve the connecting rod oil clearance precision in order to prevent abnormal noise and seizing and to maintain good fuel economy. As the size code increases, the big end bore size increases in micron units. Number of standard sizes and size coding differ according to the engine models.

The oil clearance for the crankshaft bearing and the connecting rod bearing is determined by the engine model. Each bearing should be selected to obtain the proper oil clearance with regard to the cylinder block main journal bore size and crankshaft main journal diameter, or the connecting rod big end bore size and crank pin diameter. As the bore size increases or the journal or pin diameter decreases, the thickness of the bearing to be used increases.

Oil clearance

Oil clearance is the gap between the bearing and the shaft. Oil covers the parts so that metal parts do not make direct contact with other metal parts. When the oil clearance becomes larger, abnormal sounds occur and oil pressure decreases, leading to seizing.

Bearing selection method

Use the following process to determine the correct bearing (size code).

A+B=C

ACylinder block main journal bore size code (or connecting rod big end bore size code)

BCrankshaft main journal size code (or crank pin size code)

CTotal number

Example:

ACylinder block main journal bore size code4

BCrankshaft main journal size code3

CA+B=4+3=7

Select the bearing with a size code of 3 from the chart on the left. In this case, the total number is used as is for the bearing size code from the chart. The selecting method varies by the engine model. And there are some engines where the total number becomes as it is for the bearing size code. There are some engine models that do not have a rank for the crankshaft main journal or crank pin. In this case, select a bearing with the same size code as the cylinder block main journal bore size code or connecting rod big end bore size code.