Forged pistons are durable, light weight and can take a right good pounding when operating. This is why we love them and can’t get enough of them. To create something so strong often takes a fire and brimstone approach and here we take a look at the stages in the process of manufacturing a forged piston from scratch.
Pistons are the point in the engine that horsepower is produced. Everything else in the engine is just an aid to get more air, fuel and spark on top of the piston as quickly as possible, and then extract it as efficiently as possible. The piston as to put up with all this and more and then has the unenviable task of converting the generated power into a rotational force.
Now everyone you ask seems to know that forged pistons are the shit and if you want loads of power and torque then forgies are a must, but why is this the case? When the question is posed, most just shrug their shoulders, or say “Cause they are forged”, without actually knowing what that actually means.
Here we take a look at a Ross forged piston being made from a billet bar of 2618 aluminium. Not just any piston though, this is an SR20DET piston which is destined for a bullet proof bottom end. You can see the quality as well as the machining skill and precision that has gone into making this piston and this care and accuracy can be exhibited right across the range of ACL and Ross Racing Pistons.
One thing to note is that different pistons are manufactured in different ways. There is a great deal of science and measurements for different engines and applications that need to be noted in order to have a piston that will work properly and not damage itself or other engine internals.
Thanks to Automotive Componets Limited (ACL) and Ross Racing Pistons for supplying the photos of the SR20 piston manufacturing process. ACL can be contacted through their new website www.aclperformance.com.au or call them on (03) 8791 2700 to gain access to your local state outlet.
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| Ross 1:
Here are piles of 2618 aluminium billet bar which is waiting to be cut to the correct length. Before this stage the aluminium has been transformed from bauxite and was then heated and forced through a die. This creates the grain structure needed. |
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| Ross 2:
This is what piston builders call a ‘biscuit’. A biscuit is a short length of saw off billet. They weigh a tad bit more than the forging. |
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Ross 3:
Here the biscuits have just been cooked in the oven. With 750 degrees F the aluminium biscuits don’t exactly raise. Instead they are ‘plastic’ but are not yet melted. At this stage they are ready to be forged in the press. |
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Ross 4:
The upper male punch comes down into the female cavity with many tonnes of force. The male punch forms the crown of the piston while the female cavity shapes the outside. |
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Ross 5:
The biscuit is ready to get punched. In one single stroke it is done. |
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Ross 6:
The biscuit is fresh from being forged. The fin surrounding the forging is necessary to ensure that the whole forging is filled completely so that it is strong. The fin is removed using a band saw when the forging has cooled down sufficiently. Now the forging can be heat treated and blasted on the inside to guarantee uniformity and appearance. |
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Ross 11:
Now that the piston is forged the machining process is next. The forging is plain so that there is no limit to the dimensions that the piston can be shaped to. |
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Ross 12:
The outside diameter is rough turned on a lathe. This is to make sure that the outside diameter is concentric with the inside of the piston. Once this is done, all the holes need to be drilled. The pin hole and pin lock grooves, lock removal indents and annular groove for oil spreading are all drilled at this stage. The oil return holes are also drilled where the oil ring groove will later be machined. |
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Ross 13:
Now the piston is starting to take on a more familiar shape. The CNC lathe work has been done at this stage. The dish for lower compression on the top has been finished also. The outside diameter has been machined one half millimetre larger than the bore size and the ring grooves have been machined to a fine, accurate finish. The ring lands are also finished 0.6 millimetres smaller than the cylinder bore size. |
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Ross 14:
The mill slots at the end of the pinholes and valve pockets have been fashioned with a horizontal CNC milling machine. |
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Ross 15:
The excess material at the bottom of the piston called dummy material has been removed below the pinhole. The piston is now almost complete and only needs a few machining operations. |
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Ross 16:
The crankshaft counterweight clearance is cut into the bottom of the piston. |
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Ross 17:
The aluminium has been extracted from the sides of the pinbosses. This will give the rods the correct piston to rod side clearance. The pin oil holes that connected the annular grooves with the oil returns have now been drilled at this stage. |
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Ross 18:
The profile and cam have been expertly machined on the piston. From here it is a matter of passing a final inspection and then it can be fitted into the nearest SR20! |
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