Big Ends, Conrods, or Connecting Rods


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In recent times, some car manufacturers are opting to replace historically dowelled and interlocking arrangements of con-rod big ends, with physical breaks. There are many mechanics looking at these today, with bewilderment.

Instead of machined faces, stress points are engineered and the casting is then broken. When bolted together, the big end bore or the conrod can be machined to finished size.

By sheer coincidence only, can these parts be inter-changeable, thereafter. In other words, the semi-circular caps that enclose one half of the plain white metal bearing, will only correctly fit its mating section of the original casting.

Some old school mechanics whose eyesight may be compromised, are in a difficult position, and even have issues with the disassembling procedures. The “crack" or “break" can be hard to notice, especially when it is not expected, and partially due to the fact that the “unit" was machined post-breaking.

The un-machined area of the casting has a rough texture as is normal, but masks to some extent, the true configuration of the assembly.

It gets a little more confusing. Instead of the historical physical “bend" or tab (out-turned section of the plain bearing, that use to stop them from spinning), these new types are in two halves, with no obvious means to prevent such spinning.

They rely on each half of the plain bearing, being the correct length, with respect to their circumference.

Those who still believe heavily on mechanical fastening opposed to modern glues for example may find it difficult to comprehend. And it is often these same people that cannot see the sense of it all.

The theory is that if each bearing half, when together, are slightly larger in diameter than the big-end bore of the connecting rod (con rod)in its correctly “torqued" state, then the outward force should be enough to drive the bearing. It actually makes sense too. If there happened to be more frictional force, inside the bearing (crankshaft diameter), then something is wrong anyway. Bear in mind that some lubricant should be there, as well as a lower frictional coefficient material that lines the big end bearings.

What is it all about?

Cost, speed or efficiencies!!

Each bearing half can be rolled to the correct length and wall-thickness, with one less folding operation for the tab.

The con-rod can be cast in one complete piece.

Mating halves will be less likely to be confused, but this will happen through human error, as many things do. The less number of operations, the simpler the process, and invariably the cost will come down. Whether profit margins increase, is another story.

An example of deliberate stress point incidence, to ensure that the fracture is within a certain area, has been around for years. It is with the fitting of a relatively large diameter “ball" if you like, within the relatively smaller outer-race bore of a “rose bush" or rose bearing. Common applications of these bushes/bearings are large angle, high-load, low speed, pivot points. Tractors, quarry equipment, fork-lifts, dumpers or any machinery with low precision, self alignment, high “swivel ability", and low cost rugged requirements.

Seamus Dolly and some mechanical articles are at


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