Collets - surface treatments and coatings
Valve collets (also known as valve keepers, valve cotters or valve locks) are not something that engine tuners often turn to in order to make their engines perform better or improve reliability. While collets for the general automotive market are very price-sensitive, decisions on the quality of racing collets are not driven purely by the desire to save, say, 0.1 cents per piece. So, where collets for passenger cars are stamped from sheet steel or wire, many of those produced for racing are fully machined for reasons of precision.
Collets are generally too small and light to make a significant difference to reciprocating valvetrain mass; there is much greater scope to reduce mass by looking to components such as the valve, springs and spring retainers. However, the engineer should not be drawn into thinking that these simple components have suffered from a lack of development. Their interface with the valve and the spring retainer are critical to the reliable function of the valvetrain. Stress concentration and fretting where poor design choices have been made or incorrect materials have been chosen are two reasons why collets can cause valves to fail.
Surface treatments and coatings also play an important role in providing a suitable collet. Coatings can be an important way to reduce problems owing to material incompatibility or simply using a material whose surface is not suited to the application. For example, if the lock and retainer materials are too similar, the materials are soluble in one another, and there can be problems with the materials seizing.
There are situations where both aluminium and titanium are used for collets, and in both cases it is probably wise to consider coatings to prevent damage during installation and service. In particular, in the case of titanium collets being used with titanium valves and retainers, there is a danger that cold-welding of the surfaces could take place with very little load. Protecting the interface by coating one of the components must be considered. It is quite often the case that the valve will be coated with a hard, thin engineering coating such as chromium nitride, but sometimes titanium collets are also coated, especially where there is no coating in the bore of the retainer.
Even when there are no particular concerns over the collet material, it is common to find that collets have been subjected to ‘oxide’ surface treatments to prevent corrosion. We should not confuse the term ‘oxide treatment’ with rust though – the process here is what we might usually call ‘black oxide’ or ‘blackodising’, where a very thin layer of the surface is converted to an oxide known as magnetite, which is then subsequently treated with oil. The oil is absorbed into the component surface, providing a degree of corrosion prevention.
A number of companies who offer bespoke collets to race engine suppliers and builders say they will coat or surface treat collets to suit customer requirements, and such coatings will include soft metal coatings such as silver. The aim of coating or surface treating the collet is not to reduce friction – it is desirable to have a high friction coefficient at the interface between the collet and the retainer in order to reduce movement.
Written by Wayne Ward