and a reproductively of bore circularity will be improved in the following finish processing step s 4 for processing large end portion bore processing main rod body 14 and cap 15 will be fastened and clamped together again by two bolts 17 under the same torque condition as assembled condition and the upset portion 20 c remaining at cap lower part 28 will be retained clamped by a jig 21 under these conditions a horning processing will be done as a finishing process on large end portion bore 12 a for a bearing surface of large end portion 12 thereafter the semi assembled product will under go a suitable cleaning process and is dispatched to an engine assembly factory in an engine assembly process step s 5 in an engine assembly factory as shown in fig3 while piston 1 is being connected via piston pin to small end 11 of main rod body 14 main rod body 14 will be introduced with large end portion 12 r advancing ahead in a direction of arrow y 1 into cylinder bore 2 of cylinder block cb from its upper side and large end portion 12 r of main rod body 14 will be seated on crank pin 4 of crankshaft 3 disposed below the cylinder block cb then as shown in a direction of arrow y 2 cap 15 will be placed on crank pin 14 from beneath and both parts 14 and 15 will be fastened to a prescribed torque to be finally assembled within an engine as described above in one or more embodiments of the.