The use of Spirolox clips for piston pin retention

Tuesday, September 30, 2014

Tags :  pistons-rings

In most cases, piston pins are retained with some form of clip. There are occasions when people have preferred to use plastic buttons that can contact the cylinder bore, but these are really a niche product. Buttons were covered in a previous article. From motorcycles to freight trucks, for road transport and racing, circlips of one form or another reign supreme, at least in terms of numbers used.

In many instances, piston pins are kept in their intended position with round-wire clips of varying design. Some are free to rotate within their grooves, while others are designed with features that locate in an appropriate hole or slot in the piston. For racing use, round-wire circlips are usually manufactured from a high-strength wire, and these have been covered in another previous article.

However, as usual, there are alternatives, and the Spirolox clip (often referred to as Spiralock clips or Spiral locks) was mentioned in the article on alternatives to round-wire clips, but not discussed. There are two types – single and dual coil. The single coil is essentially a circlip formed from a small, flat bar, rather than round wire. However, the double-coil type is often used for more demanding applications. Compared to a round-wire clip, the beam section of the Spirolox type gives the single-coil clip much greater stiffness, owing to a greater moment of inertia. It is the double-coil type that is normally fitted in order to retain piston pins.

To ease fitting and help take up any axial clearance in the piston, the double coil is strained axially, so that the flat clip is opened up into a helix. This is then carefully fed into the groove, sometimes with a special tool but often only with fingers. Because the clip has been strained plastically, it retains some elasticity and therefore tries to resume its ‘expanded’ form, thus taking up any clearance in the groove. As this design of clip is flat on its outside diameter, it does not work in the same way as a round-wire clip, which centralises itself within its groove, which is also machined with a round cross-section.

It is common to find two of these clips on each side of the piston pin, although this choice is by no means universal. However, if the piston is provided with a groove designed for two clips to be used then two clips should be used in order to prevent excessive axial movement of the piston pin.

Compared to a round-wire circlip, the Spirolox clip has much more abutment area and bears against the end face of the piston pin. It is the combination of size and position of the circlip groove the chamfers that controls the maximum piston pin end-float where a round-wire clip is used, but this doesn’t apply with Spirolox clips, so the chamfers on the pin are solely for ease of fitting. During manufacture, it is much easier to control the length of a pin than to control the distance between gauge diameter on opposite ends of the pin.

Written by Wayne Ward

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