Rod Knock

[NOT_aerosmith]

I saw videos of these engines on youtube with this "Rod Knock". I am curious as to what it is, what causes it, and if it is common with these engines.

[bill buttermore]

Rod knock is caused when the connecting rod strikes the crankshaft instead of sliding around it. The bottom, or big end of the connecting rod is lined with a thin, soft bearing in the form of a split shell, and is separated from the smooth surface of the crankshaft journal by a layer of oil under pressure supplied from the oil pump. The layer of oil is critical - without it, the bearing would quickly overheat, distort, and fail, allowing the harder metal of the connecting rod to gouge and damage the crankshaft journal. The rod can then become welded to the journal. If the engine is still running, this can result in bent or broken connecting rods and broken engine blocks. The space between the bearing and the journal is referred to as the oil clearance. In a new engine, this space is quite small. As the bearing wears with age, this space increases, and eventually reaches a point where the oil pressure in the engine drops. When the space gets large enough that it can no longer be completely filled and cushioned with engine oil, the rod begins to tap or knock at either end of its stroke.

Google "rod bearing" to see what a connecting rod and rod bearings look like.

Failed, damaged or badly worn rod bearings (which result in rod knock) can be caused by allowing the oil level in the engine to drop to the point where the bearing is starved for oil. Low oil level also allows the temperature of sliding parts to increase making them more susceptible to damage. Revving an engine immediately upon start-up, before the oil pump has a chance to fill the oil clearances can quickly cause rod bearing damage. Engines run at high speed and under heavy loads are more likely to suffer rod bearing damage.

IMO, these engines are about average in terms of the tendency to develop rod knock.

[NOT_aerosmith]

Thank you for that thorough description, I understand it now.