Mishnà su Osea 11:78

One who touches the yoke, the crossbar, the collar-piece, or the thick ropes, even when they are being used, he remains clean. If he touched the tail piece, knee or handle, he becomes unclean. If he touched the metal rings, the guides, or the flanks, he becomes unclean. Rabbi Judah says that he remains clean if he touched the guides since they only serve to increase the soil.