Piston pins - material choices
Piston pins play a vital, literally pivotal, role in the reciprocating internal combustion engine. Mechanically they are an extremely simple component and, in their most basic form, could be a simple, short length of solid bar. Indeed, many production piston pins are not far removed from this simplest interpretation, being a very plain steel thick-walled cylinder. Racing pins are generally more complex than production car pins, as we wish to reduce mass to an absolute minimum and are less concerned with cost. Any deviation from a simple cylinder is costly, especially if we have to machine both ends of the component.
In chasing minimum mass, engineers look to decrease length and diameter, within the important constraint of having to maintain adequate stiffness. There are other constraints we need to respect, the most important being to ensure sufficient life in the component by keeping stresses within acceptable bounds for the material in question.
The most common material for piston pins in general is steel; the surface is often hardened to improve wear resistance, and the choice of hardening method will dictate the choice of material. There are two main choices for hardening - carburising (also known as case hardening) and nitride hardening. Both have a beneficial side-effect of imparting significant compressive residual stresses to the surfaces of the part, which in turn improve fatigue resistance compared to a part without these stresses.
Carburising steels are characterised by a low carbon content and additions of manganese, nickel and chromium; it is the low carbon content that allows diffusion of carbon into the surface. Nitriding steels have additions of elements such as chromium, aluminium and titanium, which are strong nitride formers. These nitriding steels are similar to - and in many cases the same as - steels that we would normally use to make racing crankshafts. There are a number of higher strength steels not necessarily designed for nitride hardening, but which it is certainly possible to nitride and which would make excellent candidates for piston pins.
The wear resistance of all kinds of steel pins has been improved by using hard, thin coatings. There are various kinds of DLC that have been shown to be beneficial, and pins with these kinds of coating have been in reasonably widespread use in many categories of motorsport for well over a decade.
Steel is not the only choice for piston pins though; a number of companies offer titanium pins. Titanium has a much lower density than that of steel, although its elastic modulus (a measure of stiffness) is also low compared with steel. Titanium Ti-6Al-4V is often used, although pins made from Ti-17 (Ti-5Al-2Sn-4Mo-2Zr-4Cr) are also commercially available.
We should not, however, expect to replace an optimised steel piston pin with one of the same dimensions made from titanium, and expect to find success. Titanium has particularly poor wear behaviour in sliding contacts, so titanium pins would need to be coated to achieve an acceptable level of durability. Again DLC is often used here, but pins are also marketed which have a titanium nitride coating.
A special note should be made of the very exotic pin developed many years ago for Formula One and Pro Stock applications, which used an aluminium beryllium inner and a steel outer sleeve. There is little doubt that this would have been extremely light and stiff.
Written by Wayne Ward