Connecting Rods and Rod Caps
Connecting rods and their matching caps are fastened to the engine’s crankshaft to convert linear motion into rotary motion. They are often constructed together as one piece. Then a process of breaking the cap away from the rod is done at the factory.
Connecting rods and caps must remain a matched set. Since the caps are matched with their corresponding connecting rods, it's important to keep the caps in order during disassembly. That's why, if no number is stamped, a mark with the cylinder’s corresponding number is made by the technician.
Marking the caps with their corresponding rods is done only when necessary. Making an indentation on a connecting rods shaft is a really bad idea. These rods are made of strong metals and often hardened to retain their strength as the engine wears.
Even scratching into one of these rods will weaken it, and could end in catastrophe. Most of us have seen a thrown rod protruding out of someone's engine block. So only mark them if you have to, the base is a good spot. Screw the nuts onto the bolts to keep the two parts together.