A connecting rod on a diesel engine that runs with 720 rpm for 5,000 hours a year undergoes over 200 million load cycles every year. It is therefore hardly surprising that over time the lower bore in a connecting rod tends to become oval. Typically, the bore gets larger in the vertical direction and smaller in the horizontal one. This is a problem, particularly if during an engine overhaul the bearings are replaced by new ones. If the decreased clearance – in one direction – of an oval bore is compounded by new bearing shells with zero wear, then it may happen that the overall bearing clearance of the connecting rod bearing is insufficient. If so, then a bearing failure is almost inevitable.
To prevent this from happening, the lower connecting rod bores must be measured during every engine overhaul. If the ovality exceeds the maximum specified by the engine maker, or comes close to it, then the connecting rod must be either replaced or reconditioned. This is very, very important and yet it gets forgotten all to easily.
At QuantiServ we recondition hundreds of four-stroke connecting rods every year. Usually, we remove material at the mating surface of the two parts so that the bore becomes elliptic. Then we rebore it back to original dimensions so that standard-size bearing shells can be used.
On some engine types the situation is not so straight-forward, however. The connecting rods installed on certain engine types show a tendency to develop cracks in the serration. We therefore always check each connecting rod very thoroughly for even the tiniest signs of cracks.
If any cracks are present, then we mill off the entire serration on both halves and build it up again by welding. Thereafter, we redo the serration by CNC milling. QuantiServ have pioneered and industrialized this process and have achieved very good results. We have been able to give a second lease on life to hundreds of connecting rods that otherwise would had to be scraped.
To the very best of our knowledge, no one else has mastered this process yet. As said above, a connecting rod is one of the most highly loaded and therefore single most critical engine parts. There is absolutely no room for error.
We encourage engine owners and operators to come to us for connecting rod reconditioning and getting them done in a professional way, rather than taking chances and coming to us for crankshaft machining and/or metal stitching later on.
Cam Effect / Ridge Wear
If a four-stroke engine’s connecting rods are found to be oval, then in all likelihood the same is true for the crankpin. The phenomenon is called “cam effect” or “ridge wear” and it must be ruled out/corrected before new bearings are installed and the engine is restarted. The following picture shows a typical example of a crankpin affected by the “cam effect”.
Although less frequently performed, reconditioning or repairing of two-stroke connecting rods is also very well possible. Below is an example of a 50-bore, two-stroke connecting rod with damage sustained to the crosshead bearing bore. We repaired it in our workshop in the Netherlands in 2020.