Every engine builder knows that the connecting rod establishes the all-important connection between the piston and the crankshaft. However, not all connecting rods (or con rods or just plain rods) are created equal. Let’s look at three features that I am sure you have observed.
Beam and Cap Mating
There are two basic methods of mating the rod cap to the rod beam.
Machined Mating Surfaces
For years this was the most popular way to mate a connecting rod beam and cap. But it has some downsides. To begin with, there needs to be a machining process of some sort. For the manufacturer, this represents an additional cost.
Secondly, because the connecting rod housing bore is machined after the cap and rod are mated, it is essential that anytime the rod is re-assembled, the cap and beam realign perfectly to preserve the precision of the machined housing bore. Manufacturers tried many different alignment techniques: dowels, rings, serrated bolts and studs, sawtooth, tongue and groove, and others. Some techniques work well, while others are difficult to realign precisely during reassembly.
Fractured Mating Surfaces
This style of rod is cast as a single piece. The bolt holes and threads are machined into the rod while it is still in one piece. Then the cap is literally broken off the beam with impact. The irregular fractured surface then becomes the means of aligning the rod and cap during reassembly. This is not only a less expensive manufacturing process but it also produces cap and beam alignment that is nearly perfect.
The primary drawback of this method of mating is the impossibility of correcting a rod that is out of round or has been stressed by a bearing failure. In these circumstances the rod is scrap and a replacement has to be used.
A second disadvantage (or rather just a simple fact) is the extreme care that must be taken when handling these rods. The fractured mating surfaces are quite delicate. Here are a few pointers.
o When disassembling, never rest the cap on the fractured surface. Some writers go so far as to say to not even touch them. Any abrasion on that delicate surface will compromise the mating when the rod is assembled.
o Never hammer on the fractured surface (or a machined surface for that matter) when removing the rod.
o After the rod is removed from the engine, carefully reassemble the cap and rod. First, blow any oil or dirt off the mating surface. Using only your hands, squeeze the cap and rod together and snug the fasteners by hand. Then tighten the fasteners securely with a wrench. Do not use an impact. When the rod is assembled and snug, it is much less likely to damage either the rod or cap in the typical shop environment. o Be double sure the caps and beams make a pair. Tightening the wrong cap to a rod instantly ruins two rods.
Angled Parting Line
Many high-horsepower diesels feature an angled parting line on the rod big end. Instead of the parting line being at 3 and 9 o’clock when the small end is at 12 o’clock, the parting line may be at 2 and 8 o’clock. The reason for this is simple. Rotating the parting line allows for a larger housing bore and connecting rod bearings. Take an angled rod and imagine it being parted ninety degrees from the beam. It will be obvious that the rod would not fit through the cylinder which would complicate installation, to say the least.
Tapered Small End
Many high horsepower engines also feature a tapered or wedge-shaped small end. This is done for increased small-end connection strength. Take a tapered con rod and install a wrist pin. Imagine the small end sides being parallel and observe the line of force. All of the force is transferred vertically through the sheer strength of the wristpin.
Now observe the line of force with the tapered rod. Because the piston pin bore is also tapered, the force-bearing surfaces on the rod and piston actually overlap. This greatly increases the sheer strength of the wrist pin.
Tapered rods do make installing wrist pin bushings and machining them more complicated. But with the right tools and equipment and a healthy dose of patience, it can be done precisely.
