There are three different manufacturing processes used in order to make crank shafts. 1. Casting. 2. Forging. 3. Billet machined. The first two are common in the Volkswagen/Audi range. The last one being more common in super cars or race cars. Questions often arise as to which type crank shaft is fitted to an engine you have opened given the fact that both 1 and 2 are fitted to power plants we're used to dealing with. Ill go through them all below, how to recognise them, how they are made, why the manufacturing process makes them recognisable, and the hardening process carried out for each type of crank which can also effect appearance. Ill also go through why one is better than the other and why. 1. Cast Cranks. These are around for a long time and are found In a lot of engines and in both petrol and diesels. As the name suggests these are cast and made from Malleable Iron. The shape being defined by a sand mould as with many other engine parts. These are pretty cheap to make and hold up fairly well too so they are a common choice for manufacturers. A sand mould is made comprising of a top and bottom half, a pattern is made in wood or other material and this forms the required shape the mould halves contain once they are brought together. The molten metal flows into this mould relying on gravity alone. Both flat plane (single plane) and cross plane cranks can be made this way fairly easily. A flat plane crank is one where the journals are 180 degrees apart common in all in-line four engines. Therefore only two mould halves are needed to make them as the pattern can be withdrawn from the sand mould without locking. This leads to fairly quick production times. A cross plane crank on the other hand needs a mould of multiple parts because the journals and counter weights are not symmetrical either side of the parting line(where both mould halves meet) Therefore withdrawing the pattern from just two halves would be impossible. This isn't a major issue all the same as once the moulds are figured out production is just as fast as for a single plane crank. Here is a single plane crank common to in-line four engines. Notice how it is symmetrical so only two moulds halves are needed to cast it. It has just one casting line along its centre. Here is a cross plane crank common in V engines. Notice the journals are 90 degrees apart. This makes a two mould cast impossible as the pattern could not be withdrawn from the two mould halves so a more complex mould build is needed. On older cast cranks of this type many different parting lines can be seen in different locations on the cranks due to multiple moulds being used to build the finished mould. Its pretty easy to tell if your crank shaft is cast by looking at it a bit closer. On a cast crank you will see the parting line easily. This line is created where both mould halves come together. The line will be easy to see, defined, and approx 2mm wide. On an in-line 4 flat plane crank it falls on the centre line of the crank also. I may as well mention grain structure now at this stage too, metals have grain structure, the easiest way to describe this without going into too much boring detail is to think of each grain as an arrow. When a part is cast such as the crank above your spilling in all these arrows into the mould at once, and under atmospheric pressure. This as you can imagine leads to slight chaos within the mould. The arrows have no order, direction, or organised layout. They also aren't as close to each other as they could be. This in turn effects the over all strength and properties of the finished crank. But it is a quick process and the crank can be cast to pretty much near its final dimensions so minimal machine time is required. Mould manufacture is also fairly cost effective with this process. Below is a close up of a VW cast crank, notice the defined parting line and slightly rough texture the metal has taken when poured into the sand mould. And a close up on the counter weight showing sand like texture. And the picture below showing the micro structure. Notice how the carbon has clumped in some places, and also the general randomness of the material itself. Even on a cut cast crank I have you can see the carbon present on the cut face. This is the main reason why cast has such good wear properties as it is self lubricating to agree due to this carbon/graphite. Cast cranks can be flame hardened to improve wear resistance in particular areas. Flame hardening involves aiming gas flames to particular areas which require hardening. This is sometimes done on a jig having multiple flame jets aimed at, in this case the various bearing journals. Once the desired temperature is reached the crank is then quenched in molten salt and oil. Water is rarely used in this direct contact case as it extracts the heat so fast as cracking may occur. After flame hardening the casting then consists of a hard, wear-resistant outer layer of martensite and a core of softer grey Iron. Flame-hardened castings are stress relieved at 150 to 200C after quenching to remove any tension caused from the quenching process. It is at this stage that the final journal grinding can occur. Below is an example of a flame jig, although it is for a camshaft the principles are the same. Aiming the jets at just the bearing areas ensures that the crank still remains ductile in other stressed areas. That pretty covers the basics of cast cranks and how to recognise them, now onto forged cranks. 2. Forged Cranks. These are a more robust crank than a cast crank for a few reasons. They are more commonly found in higher stressed engines and come standard in some 16v engines and almost all of the 1.8T engines. I do believe they feature in the new fsi engines too, although I have not yet got the pleasure of getting my hands on one>yet. A forge crank is made in a totally different way to a cast one. A set of dies are machined to the approximate shape of the crankshaft as below. These dies sit in a very large hydraulic press having a clamping force of many many tons. A hot bar approximately 150mm in diameter is placed onto the bottom die and the dies are closed. The bar is of high grade steel alloy containing all the various metals needed in order for the finished crank to fulfil its job. One benefit of this is the metal does not need to have certain properties required for casting, fluidity when molten, etc. Once the dies are closed the metal is squeezed in very tightly, this has the effect of making the metal more dense, packing the arrows closer together if you like, and also given the fact the entire bar is pressed into the shape of the dies the grain structure also follows the shape of the crank throughout too. The material is then both compacted and aligned better than with the casting process. Below is a set of forging dies for a flat plane crank, these dies are extremely costly to manufacture, one of the reasons why forged stuff is dearer than cast counter parts. Notice the four posts at the corners, these act as position and as limit stops. Here you can see the dies fitted to the press and the heated blank within. The rough forging being withdrawn. The dies are pressed together until the limit stops on the dies come into contact, once this happens the blank has been completely pressed and any excess is squeezed out the gap between the dies. It is this excess metal, or flash that makes a forged crank very easy to recognise. This flash is quite thick, sometimes as much as 10mm, as a result it has to be ground off before any finish machining can be done. This so called part line then ends up being quite wide and can be recognised instantly over a cast cranks faint part line. Here is a better view of the excess metal where it has been pressed out once dies were full. The grinding of this flash also adds to the cost of a forged crank. On a close up of a forged crank I have here the wide flash line can be seen easily. One of the main tell tale signs of a forged part. Forged on the right. A forged crank will also have smaller counter weights for the simple fact that the material is more dense and therefore heavier than that of the cast type. And the picture below showing the micro structure. Notice how its tighter and more organised looking than the cast shot. These type cranks are also hardened like the cast types but a different process is now used. Its known as Induction hardening. Its straight forward and a quicker and cleaner way of doing it as opposed to flame hardening. A coil is placed around journal. Current is passed through this causing the journal to heat up, once at the correct temperature coolant is then passed though the coil assembly and it rapidly cools the metal in this particular place causing hardening of the metal to a pre-set depth as with flame hardening. This also eliminates the danger of either overheating or burning the surface of the metal as with a flame hardening. Induction hardening also causes the surrounding journal material to take on colours, similar to the colours sometimes seen when a crank has been heated due to lack of oil. This colouration is a characteristic of this hardening process and can be alarming when seen for the first time on your own engine. The colouration can be seen below. Of course, forging has its limits too, single plane cranks can be forged easily as only a top and bottom die are needed to stamp a single plane crank as shown again below. A cross plane crank due to its 90 degree journal positioning cannot be stamped using the normal two die method so a change has to be made to the hot forging before it is cooled. The cross plane crank is stamped as a normal single plane crank in the beginning, that way it can be removed from the dies and a simple two die set up works fine. Before the forging cools the crank is placed into another machine which grips the big end journals and turns them 90 degrees and into their correct positions as shown below. Doing so this has the effect of twisting the already formed grain structure at the main journals. Although single plane forging is extremely tough, this is sometimes frowned upon as causing a disrupted or weak spot in a cross plane forging. This is where the billet crankshaft comes in.