A crankshaft works together with the pistons in an engine block to create the rotary motion required to get a car moving. Stress and vibrations can cause cracks in the crankshaft, leading to problems that can seriously damage the engine. Before buying a replacement crankshaft, drivers should understand the purpose of the part. They should check which type of crankshaft their vehicle requires and know how to select a quality part.
How crankshafts work
A crankshaft is part of the internal combustion engine. It connects to the pistons, which move up and down in one of the engine block cylinders. The crankshaft converts the two-way linear motion of the pistons into a rotary motion that propels the motor vehicle along the road. Connecting rods attach the pistons to the crankshaft. The crankshaft rides on special bearing material, which is a soft alloy of metals that provides a wear surface that is easy to replace. The crankshaft has oil passages drilled into it to allow the engine to feed oil to the bearings and connecting rods.
Types of crankshafts
Crankshafts differ in the number of cylinders they contain and the arrangement of those cylinders. Larger six- and eight-cylinder engines require a longer crankshaft design than four-cylinder engines. A straight-eight engine has a longer crankshaft than a V8 engine. There are three main types: cast, forged, and billet crankshafts.
Manufacturers can cast crankshafts in ductile iron. They define the shape by pouring molten metal into a sand mould that consists of a top and bottom half. Cast crankshafts are inexpensive to manufacture and fairly durable.
Modern manufacturers often prefer to forge crankshafts from steel bars, which are lighter than cast iron parts. They make the crankshafts using a set of dies, which they squeeze together tightly to make the metal denser. The dies are costly to manufacture, which makes forged crankshafts more expensive than cast versions; however, the forging process means that they are more robust than cast crankshafts.
Manufacturers produce billet crankshafts from high-grade steel, such as 4340 steel that contains aluminium, chromium, molybdenum, nickel, and other elements. Manufacturers use a dense, cylindrical steel blank and machine it into the exact specification required for the engine. Billet crankshafts are the most expensive because they take the longest to produce. They appear in high-performance cars, such as Ferraris.
Choosing a used crankshaft
Before buying a crankshaft, especially a used part, drivers should check the threads to ensure that they are not stripped, which can happen if the previous owner over-tightened the bolts. There should be no rust, chips, or indentations on the crankshaft. There should also be no welded-up holes or additional drilled holes, which indicates that the crankshaft had been rebalanced multiple times and the part could be weakened and more susceptible to cracks.
Large cracks are easy to spot, but drivers who are concerned about microfractures should check that the seller has thoroughly tested the part. Buyers should look for a shiny crankshaft because this indicates that there has been no exposure to extreme heat from a lack of oil.
If the crankshaft and the related components do not operate together perfectly, major engine damage can occur. The nature of a crankshaft's design makes it susceptible to stress and vibration, which can lead to cracks. Crankshaft damage and overheating can also result from poor engine maintenance that starves the bearings of oil.
A faulty harmonic balancer that revs the engine above the rate specified by the manufacturer can cause cracks in the crankshaft. An out of balance crankshaft can also become cracked. The most common areas for cracks are around the oil holes and the crank radius, which is the distance between the crank pin and crank centre. Drivers can remove minor cracks by first testing the crankshaft and then grinding the cracks away carefully; however, incorrect grinding can make the crack more severe, in which case the owner needs a complete replacement.
How to buy a crankshaft on eBay
You can find a replacement crankshaft or any related components, such as pistons and connecting rods, by searching the vast inventory of car parts on eBay. Enter the word "crankshaft" into the search box located on any page, and browse the list of results. If you know which type of crankshaft you need, then use specific keywords, such as "billet crankshaft". Read the item description carefully, especially if buying a used part, and ensure that you choose the right component for the make and model of car.