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Piston pin design features

In its most basic form the piston pin is simply a piece of round bar. However, very few engines – other than very basic, low-budget units – are equipped with solid gudgeon pins as standard. Of course, in the rarified world of racing, there are some applications where conventional technology won’t suffice, and we can also find solid piston pins in some tractor pull engines.

It is usual though for engines in production cars and motorcycles, and almost all race engines, to be equipped with hollow pins, as these offer sufficient stiffness to resist excessive ‘ovalisation’, which is the term many use to describe the crushing deformation that can cause a pin to bind in its bore. The amount of ovalisation that can be tolerated depends on the clearance between the pin and the bores into which it fits. Larger clearances can tolerate larger deflections without binding, and these larger allowable deflections can lead to a lighter pin. However, this has to be weighed against the maximum stresses in the pin which may increase with more flexible designs.

The bore of a racing piston is very often equipped with a taper at each end. The amount of taper again dictates the amount of ovalisation and the stresses at the ends of the pin. Excessive tapering will lead to a pin with insufficient stiffness and strength, so we need to temper our ambition in the pursuit of low component mass.

At the ends of the outside diameter there are often chamfers and radii to which you may have paid little heed; they are often very tightly controlled and play a critical part in the function of the piston pin and the ease with which it can be installed. The external chamfers may actually be double-angled.

A shallow-angle chamfer is often added to aid easy fitment of the piston pin, allowing the part to slide smoothly through the piston and rod bores. This chamfer is normally short so as not to effectively shorten the pin to bore contact. A more substantial but carefully controlled chamfer, often machined at a 45º angle, will in the case of plain round wire circlips often form the surface that is in contact with the circlips. In such designs the chamfer machining is critical, as the relationship between the chamfers on each end of the circlip dictates the amount of piston pin ‘float’ (potential axial movement) that is possible.

Where any such chamfers meet the outside diameter, it is often the case that the resulting sharp edge is ‘broken’, with either a machined radius or using an alternative abrasive mechanical process. This prevents the likelihood of any material being removed from the pin bores in the piston, or any other damage (burring or scoring) being caused to the bore during assembly.

Surface finish is another important design feature, if you going to design your own pins. It is important though that the surface finish is very fine – piston pins are often very hard, and make excellent cutting tools if the surface finish is too rough.

Written by Wayne Ward

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