Are there alternatives to the circlip?

The vast majority of race engines, and propulsion engines in general, are of the reciprocating type and, for almost all of these, the piston pin is retained by (and has its end-float controlled by) a pair of round wire circlips. The subject of circlip design has been considered in a previous RET Monitor article; however, there are alternatives to the wire circlip which are either in current use or which have been used previously, either for racing or series production.

There are two methods of securing the piston pin which are variations on the same theme; mechanically speaking, it is the simplest method of piston pin control as it requires no extra components at all – that is, no circlips or any other method of retention. The use of mechanical interference has been widely used in production engines, and the two variations are to interfere the pin in either the con rod or the piston.

The easiest to achieve is interference between con rod and piston pin. There are three main reasons for this – there is only a small difference in thermal expansion coefficient between the pin and rod, there is a smaller difference in operating temperature between pin and rod compared to the temperature difference between pin and piston, and there is less interference length in the pin and rod contact. All of these mean that, if interference fitting is your chosen method, it requires lower levels of interference and lower levels of insertion force at room temperature. It may also be safer to have interference in the rod.

Assuming that a bronze bush or plain steel rod is used, there is less chance of damage or seizure than there would be with heavy interference between pin and an aluminium piston pin bore in the piston. Many years ago, the interference was achieved by having the small end of the con rod designed as a pinch clamp; the load on the clamp was applied by tightening a threaded fastener. Having a threaded fastener cyclically loaded in such a critical application was risky though, so this method was abandoned decades ago.

The method that is widely used for racing, especially in drag racing applications, is to replace circlips with polymer ‘buttons’. These are fitted into the piston pin and limit the pin’s travel in each direction when the buttons come into contact with the cylinder bore. There are usually only very small forces causing the piston pin to thrust one way or another, so the frictional losses due to the contact between the button and the cylinder bore should be low.

The method has proved successful over a number of years, but it can’t be considered to be the optimal solution in many cases, assuming that the optimal solution is the one that produces the lowest-mass solution. The pin length can be limited to that required to keep contact stresses low enough – this should be the same length as a pin retained by circlips.

In ‘slipper’ type pistons – those with only partial skirts on the thrust and anti-thrust sides of the piston – the buttons would have to be very long and would therefore be much heavier than a wire circlip. Drag race engines generally use ‘pot’ type pistons with a full skirt, and these need a lot of piston pin contact area owing to the huge combustion forces. The buttons are therefore comparatively light and short.

Another popular method of pin retention is the use of spiral locks. These are basically a helical design, being a spring of just less than two coils and made from flat material. They act in the same way as a circlip but are renowned for being very difficult to remove. Many engine builders would say though that this is not a disadvantage, especially those who have had a wire circlip ‘escape’ in the past.

Written by Wayne Ward