I'M working on my sons Ariens riding mower with a B and S engine 31C707-0603-G1 and suspected a bad head gasket. Ordered it and when removing the push rods I noticed one was bent. It was the lower which I think is the intake. Pulled the head, replaced the gasket, which looked fine and ordered new push rods. Question is the bent one is aluminum while the top is steel. Why aluminum and why did it bend. I was thinking about this on the way home and am wondering if the valve is stuck. New push rods should be here in 3 days so I will check to make sure the valve isn't stuck. Seems strange to have one aluminum rod.