The outside of the cylinder shows no damage or signs of crushing in a vice etc but the inside was quite badly gouged which has been improved by polishing so I don't think it needs to be honed. The piston will drop under it's own weight for about half it's travel but as soon as the rear gets passed the cap threads it tightens up. I've checked and the rear of the piston seems to be just clear of the cylinder walls so it is the front end causing the binding. My lathe is not set up at the moment so can't test for concentricity just now but I reckon that is the problem as there is nothing I can see or feel in the cylinder which could account for the scraping.