ok, well if you eally want to understand the difference in the make up of cast VS forged pistons, i can try my best to explain based on my knowledge of igneous petrology which is where magma cools in the earths crust to form rock.
The concept is the same, you have a molten solution of material, in the case of pistons, this mix is calculated and measured off and put into a big melting pot at the factory.
In cast pistons such as Namura, ProX yamaha OEM, a HYPEREUTECTIC process is used. All pistons are made of a mix of SILICA (SiO2)and ALLUMINIUM (Al), this makes an alloy when combined, the silicon makes it much stronger than if it were plain alluminium.
Now as a molten solution of SiO2 and Al cools, it reaches a "LIQUIDUS" This is illustarted in the diagram as the curved line from D - E and C - E.
T on the left is the temperature
In the upper region, marked liquid, all the mix is liquid and there are no soild metal crystals. When the melt cools and reaches the liquidus at H, crystals start to form. BUT only at a ratio of about 1% cryastal 99% liquid because the temp is still too high! the composition of H is about 80% Si02 and 20% Al.
As the mix cools further, we move down the liquidus curve. as me move, we draw a horizontal line from an arbitary point between H and E back to the line H - J. if we do this we will see that the proportions of crystals is increasing and the amount of liquid is decreasing, thus, we get more crystals as we drop the temp.
The composition of the crystals VARIES as we move along the liquidus with changing amounts of SiO2 and Al.
If we made pistons like this, we would have the crown of the piston made of 20%Al;80%SiO2 and the lower skirt being 90%Al;10%Al, this would be a very bad piston with variabke expansion and strenth.
HOWEVER, when we reach point E on the diagram, the crystals noe form at a constant rate and with a FIXED composition 45%SiO2;55Al on diagram. This E is the EUTECTIC POINT all the liquid is used up here and only soild of Al and SiO2 remain!
So, a cast piston uses This method of equal amounts of Al and SiO2, which is better than the old days when pistons very likely to have had variable compostion!
The advantages of using this Hypereuetecic story is that the piston will have less thermal expansion variance and therefore allows for closer tolerances and accurate fits, also less chance of heat sieze with a close cyl/piston clearance.
The downside is that the introduction of so much silica makes the piston brittle, hence, too loose a bore wil result in shattering the skirt.
A FORGED piston is adifferent animal, the process of which im not as clued up about, but i do know they use a NON MELTING process, which means the cooling of molten metal is avoided.
The piston once formed (how it is formed in the 1st place is similar to a cast piston) is subjected to sever pressure causing it to deform into a plastic state and not a liquid. this then results is the deforming of mineral grains.
The realignment of mineral grains via deformation results in the uniformly orientated mineral grains which improves strenth dramatically.
As far as i know, the forging process also make the piston less porous hence stronger
Forged are alot stronger and tolerant of about and loose bore knocking and slap.
HOWEVER, they are more dimensionally unstable, so too tight a bore can result in siezure as the piston does not expand uniformly.
and there you go a 5 min lecture on Petrology
it prolly is cuz its from the ghetto haha
