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Avoiding piston micro-welding

pistonsKeeping a piston’s top ring land from experiencing critical wear is a problem that every engine builder experiences, particularly in the hard fought world of NASCAR Sprint Cup Series competition. Dr Andrew Randolph of Earnhardt Childress Racing (ECR) Engines has worked endlessly to remedy the problem.

“As rings get closer and closer to the top of the piston, they get so hot that the aluminium melts and adheres to the piston. Not long after that, the piston begins to fail.” What Randolph and ECR have done to circumvent this problem is hard anodising on the top ring land of the piston. “It works pretty well, but it still wears away the groove. When that happens, the groove starts sagging and then the ring starts twisting in the groove.”

In order to circumvent this type of problem, Randolph works with his suppliers on utilizing different types of materials. “There are different types of hard anodising and other alternative approaches such as Keronite, an electrolysis coating. We’re always looking at different materials that can cope with the pressures, the temperatures and the power levels, all of which are going up. You work that ring harder and harder and harder,” he told me.

While the pistons, which are procured from BME and Capricorn, come without this type of ring land anodising, ECR usually “runs them through a machine to quantify exactly what the form is – how flat it is – and what the clearances are of the groove. Then we send the piston out for treatment.”

Through the 2009 season, ECR have tested four different types of hard anodised coatings from three different international vendors. “Everybody has this problem and there are different ways of looking at it as different people have different solutions. Is hard anodising a better solution? I like it; it’s been working pretty well,” Randolph admitted.

“The thing about hard anodising is that it is not a coating; it actually has depth to it. If you want to get half a tenth build-up of hard anodising, normally it will also sink into the metal about the same amount,” he said. “So it is tough stuff. It’s not a coating where pieces come off; it s a transformation of material properties.”

The hard anodising of a piston has an effect on wear. “Mileage is one of the factors we look at, and it varies from track to track. At Loudon, for instance, the duty cycle is pretty low, at about 50 percent; when you get out to Fontana the duty cycle is more like 75 percent. That is a lot higher average temperature and pressure that the ring is going to face, along with higher engine speeds,” Randolph said.

Keeping the top piston ring land intact and trying to avoid micro-welding is an ongoing dilemma. “This is one of our big constant obstacles – how to keep the top ring land and piston intact. It is not an easy thing to do.”


Written by Anne Proffit.

RCR piston
Photo caption: Hard anodising on the top ring land of the piston can lengthen the duty cycle of the piston and stop it from micro-welding.

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