I have problems with Nelson F33 irrigation heads on my wheel-line. We are pumping dirty river water (by necessity) and the grit wears the rotary shaft seals completely out and the heads actually jettison out of the base portion. I am able to remove and replace the shaft and seals BUT do not know where to find the shaft and seals. I have search the internet for sources, but no luck so far. Any help would be appreciated.