My PD I-pilot's lower unit is loose - swivels around the shaft. It appears that it is threaded to the shaft, but I'm not sure. Tried to screw it back on and nothing happens. I don't want to get rough with it because the shaft is composite material and the lower unit is steel. How do I tighten it and align it with the head? Thanks for your help. (And yes I did hit a couple of stumps last weekend)