Some of the valve covers on my car were disappearing over time and I wanted to replace them to keep dirt out of the valves. I wasn't looking for anything special and it turned out to be the cheapest strain I could find online. When I placed them on the valve stems they showed some resistance as if the threads were worn out. It was the right size thread, it seems the quality is a little low. The next valve stem caps I buy will definitely be plastic so I feel safe because they don't hurt.