Have you ever wondered what the green coating is on the skirts of some of our Cosworth pistons? If you have, then you are not alone! The coating is actually a fluoropolymer material that has the tradename Xylan, and Cosworth have been using this coating for a number of years on a wide range of parts, and not just the piston. Oil pump and scavenge pump bodies also received the Xylan treatment to help prevent wear from rotors.
Not all Cosworth pistons have this coating – pistons from earlier engines like the BDA, for example, were designed without any skirt coating. But the increases in piston speed in their race engines meant that some form of coating was necessary to help prevent scuffing of the skirts against the cylinder bore walls.
In the early ’90s Cosworth started to coat the skirts on pistons from several different race engine categories, including those destined for Formula 1 like this one from the race-winning HB engine (the coating also looks great when etched with the driver’s name!). Cosworth soon started Xylan coating nearly all of its race and high performance engine pistons, including the ones we sell today for the YB engine.
What is Xylan?
Xylan was developed by DuPont in 1969, primarily for kitchenware utensils as an alternative to Teflon. Its excellent wear properties meant that it was soon adopted by the automotive industry, where it found its way into numerous different applications.
Xylan is in effect a composite material comprising of a dry film lubricant contained in a matrix with high-temperature organic polymers. This creates what can be termed as a plastic alloy that has excellent surface characteristics and is easy to apply. Xylan has a very low coefficient of friction, so its perfect for the interface between the skirt and the cylinder bore. It also provides exceptional wear resistance, and quite often Cosworth would strip engines down and see that the Xylan coating was completely unmarked. This is in part due to another benefit of Xylan – it has excellent surface adhesion.
The Cosworth Process
As with any coating, good preparation of the substrate surface is vital. In the case of pistons, the skirts need to be completely clean and free of any oil. Once cleaned, the skirt area is masked off with special tape which can simply be peeled off after coating.
The piston xylan coating is then sprayed on in one of Cosworth’s special custom-made spray booths onto the skirt. Each pass of the spray adds around 5 microns, so the thickness can be carefully built up to the required level. Most piston skirts only need a couple of passes to get the thickness up to the required level of between 6 and 12 microns.
The pistons are then left to dry – normally this can take 24 hours. Once dry, the pistons are good to go, with no other treatment necessary.
If you’d like to know more about the Xylan coatings that Cosworth use on piston skirts then get in touch.