A connecting rod on a diesel engine that runs with 720 rpm for 5000 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 bearings can be used.
On some engine types it 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 cracks are present, then we will mill off the entire serration on both halves and build it up again by welding, before milling a completely new serration. 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 best of our knowledge, no one else has mastered this process yet but us. As said above, a connecting rod is one of the most highly loaded and therefore is one of the single most critical engine parts. There is absolutely no room for error.
We encourage ship 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.