The McCoys were loosely fit to begin with and some had little compression. They have a cintered iron piston and would wear out the fit pretty quickly even if they started out well. Having said that they are a great buy. Many are still quite new since the owners often didn't know how to start or run them and very many were made and sold due to their low price. We have learned that with today's fuels they can run with very little wear and perform well. On the pure castor oil fuel of their era you might get 10 to 20 run-hours before they were shot. Today a mix of 28-30% oil, 50-50 mix of castor/Klotz red oil will make them last indefinitely as long as you don't run them too lean or hot. Many old engines you can buy are frozen in old oil and can be ruined trying to get them loose if not careful. However these old McCoys seldom get frozen due in part to materials and loose fits. I've bought maybe twenty off eBay in the last few years and only two or three weren't usable. You always take a chance when you can't put your hands on them before you buy .
Dave