The black ones were the last series, I have two black .40s, a black .30, a black .50 and three silver .61s. They are basically 9,000 to 9,500 rpm engines so they need to be propped to that rpm. I ran 10x6 three blades on the .40s when they were on a Kombat Twin. All below the .50 were bushed engines, the .50 and .61 were ball bearing. I also have a gasoline (petrol) .61 that turns a Graupner 12x6 at about 9,000. The Mercos will run all day long without so much as a hiccup, 5% nitro or no nitro is the rule for them. I picked these up in the early 90s