Straightening a Crankshaft

When it comes to repairing a crankshaft, there is a certain sequence of processes that must be followed. Once the crankshaft has been cleaned and checked for straightness, any significant runout (bend) in the crankshaft must be corrected with a straightening process. There are two primary techniques used for straightening, which are described below.

Hydraulic Press Straightening

Hydraulic Crankshaft PressMany automotive machine shops use a hydraulic press (pictured left) to straighten crankshafts. This is accomplished by placing the crankshaft in v blocks and applying pressure on the crankshaft journal, on the opposite side of the bend, to essentially bend the crankshaft back into its normal position. This process can be more difficult on cast steel crankshafts as they are more likely to crack under pressure. To avoid the potential of cracking, some shops apply heat to the center main journal to help it relax so that it goes back into its natural position with less force. 4340 forged steel crankshafts, for example, are less likely to break when using a hydraulic press to correct the bend since this type of steel is much softer than cast steel.

As noted previously, using a hydraulic press to straighten a crankshaft carries some risk. The potential for cracking cast steel is high, especially when the journal is bent .030” or more. Using heat does reduce this risk, but still many automotive machinists prefer to use a much safer straightening method that is known as a stress relieving operation. Hydraulic presses can also over-straighten a crankshaft, which significantly reduces is strength and reliability in performance applications.

Stress Relieving Process

Many automotive machinists have adopted the use of a stress relieving process that requires v blocks, a brass tipped chisel and a large hammer. By using the hammer to hit the chisel inside of the damaged journal’s radius, on the low spot, the steel is forced to relax. While laborious, there is absolutely no risk of breaking a crankshaft using this process. The one problem with stress relieving a bent crankshaft is that the force of hitting a chisel into the radius can leave small gouges. Using a brass tipped chisel does help to eliminate the marks in the radius, but some will still exist. However, these gouges normally are removed when the crankshaft is ground and no adverse impact to the crankshaft’s strength is observed.

Starrett Dial IndicatorAs with all straightening processes, the crankshaft is rotated in v blocks and the runout is measured with a dial indicator (pictured right). Once the crankshaft is straight enough to grind, which does also depend on how out of round the journal itself is, then the crankshaft can move onto the next phase of the repair process.

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