It sounds like you've got a cracked piston skirt or a loose wrist pin. Since 235 rods are clamped to the wrist pin with a bolt, a worn wrist pin bushing isn't a possibility, but a loose clamp bolt is.
Another problem can happen if the wrist pin clamp bolt was torqued without supporting the pin- - - -that can twist the rod and cause an obbball-sounding knock as the piston twists back and forth as it travels from top to bottom of the cylinder. I always clamp a big tapered punch in a vise and slip the wrist pin over it before tightening the clamp bolt. Some people hold the big end of the rod in a vise, and put a lot of twisting force on a very flimsy rod. No Bueno!
Jerry
