Thanks you guys Ed and eugene I already have three of these engines and yes they are of the old series. I've tryed other props on them and they mostly like the 18/10 prop. Most of them are broken in and one has had a problem with MOTHER earth( need to send it in and get a new crank and prop adapter. Two of these will go on my 101" B-25 now and for my TFg P-51 I think I will either get a Brillie 46 or a DA 50. But I do have a 3W engine that I think is a 28 with a pitts style muffler and electronic ignition that I will be selling here real soon if I don't put it on my CE that now has one of these 41s on it. It is rated at 2.75 horses and will turn a 20/10 prop,