Print this chapterPrint this chapter

LO 7-7 Identify and explain the purpose, function, and construction features of connecting rods

1. Connecting Rods

1.4. Fracture Split Connecting Rods

LO 7-07: Identify and explain the purpose, function, and construction features of connecting rods.

 

Fracture Split Connecting Rods

One common feature of connecting rods is that the rod ends can have a 45–60 degree offset or split of the rod cap. This is to accommodate larger connecting rod journal sizes while still allowing the connecting rod to pass through the cylinder during assembly Figure 51. If the rod were not split at an offset angle, the big end would not pass through the cylinder due to the dimensions of the rod. The mass of the big end can also be somewhat minimized by using this feature. A split fracture connecting rod is the most common type of connecting rod manufactured today. These connecting rods have had the halves of the big end of the connecting rod and cap parting surfaces split by fracturing, rather than using extensive machining of the mating surfaces between the rod and the bearing cap Figure 52.

  Fig07_51

FIGURE 07-51: An offset split rod permits a larger crankpin diameter and a stronger rod cap retention.

   Fig07_52

FIGURE 07-52: Split fracture rods give a coarse-grainy appearance to both rod cap surfaces.

  

 

Using a split fracture connecting rod offers the following advantages:

  • More precise alignment of the rod and cap than a machined rod and cap
  • Improved control of oil clearances and oil film thickness
  • Reduced production cost because machining steps are eliminated
  • Greater retention strength between the cap and the rod

Split fracture connecting rods allow the rod and cap to be separated along a predetermined fracture line. Instead of cutting the rod cap away from the rod and then machining and reboring the rod and cap, a hydraulically operated wedge pushes the two pieces away from one another. A line, scribed where the split should take place, provides a weakened point to control the split Figure 53. Because the big end of the connecting rod is fracture-split, the rough-edged big end and cap will resist movement better and are aligned more precisely than a conventional flat machined surface. Split fracture rods can be made about 17% lighter, which reduces engine vibration while maintaining the strength of traditionally manufactured rods. Split fracture rods are produced more economically and with less energy.

   Fig07_53

FIGURE 07-53: The appearance of the parting lines of a fracture split rod.

 

©2026 - CDX