RE: .o49 engines by cox
I have purchased more than 40 of them at that price. Just ordered 10 of them this week. They were selling for about $29.00 a little more than a year ago. Cox has stopped making fuel (gas) engines. So these are closeout deals, I called about 2=3 months ago they said they still hade 30,000 left but with everyone selling them on e-bay for $10.00 or more plus some hobby shops also selling them some day they will be all gone.
They are a great sport engine for RC or C/L from the factory they spin a 6x3 prop about 12,000 to 13,000 rpm on average. That means you have about 9 = 11 oz of static thrust. With slight modification you can easy get over 15,000 rpm which is about 14 oz of thrust. anything from 10=20 oz should fly ok with them from 100=250 sqin of wing area. I fly high wing plane at 12 =14 oz and the make good fliers.
These are good little sport engines, and well worth the $7.00 but almost everyone that I have bought needed the piston reset. You can find out about that by doing a search on cox piston reset. They will usually work without a reset, but you add years to their life by doing so, and if you modify them to get more than 18,000 rpm they won't last a long time without a reset of the piston