Why did your crankshaft break?

A broken crankshaft is not a common occurrence. Cranks can break due to a casting or forging flaw but this is very rare. Today’s quality control systems for forging and machining crankshafts are state of the art. Very seldom do you here of a new crankshaft breaking “out of the box”. Usually a crank will break because of another failure. In a diesel engine the amount torque created during a failure such as a spun bearing can cause a crank to snap. Diesel crankshafts are more prone to breakage because diesel crankshafts are extremely hard.

All diesel crankshafts are heat treated. Some diesel cranks are nitrided. Nitriding is a process that introduces nitrogen into the surface of the steel at an extremely high temperature. This heat treating is done to increase wear resistance, improve fatigue and obtain a high surface hardness. Because of this process the crankshaft is extremely hard but also brittle. When an engine has a bearing failure, the excessive clearance can cause the crank to have a “jump rope” effect inside the engine. The crankshaft can bend severely or even break in some cases. In cases where a nitrided crankshaft is bent, it is usually not recommended to straighten. Attempts to do so could cause in internal and external cracks which would result in another crankshaft failure.

So, you have a broken crankshaft and it needs replacing. What now? Well you could just buy a new or remanfactured crank and install it, but it's not that simple. Remember the “jump rope” effect inside the engine? Well that not only causes damage to the crank but also causes damage to the housing bores of the connecting rods and mains in the engine block. Because the crank jumps around in the engine, the crankshaft can pull on the housing bores of the connecting rods and engine block. The massive amount of torque from the jumping crank can also stretch the bolts and distort the bores. Main caps can become loose in the registers causing misalignment.

It is never a good idea to just swap the crankshaft out and put a new one back in without properly checking these housing bores. The most accurate way to check the housing bores is by using a dial bore gauge. Calipers won’t cut it here. In many case the engine must be pulled out of the machine in order to get an accurate measurement. If you do not have a dial bore gauge then you should strip the block and bring it with the connecting rods to your local machine shop. They can check all of the housing bores to see if they are out of round. If they are, usually they can correct them. By not doing this you are risking another crankshaft failure. Housing bores that are out of round will give you improper bearing clearance. Improper bearing clearance can cause premature bearing failure and in a worst case scenario, another broken crankshaft.

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