ORIGINAL: blw

The Alpha is a .40. The TPS is a .455 and was marketed as a .40 by Horizon/Evolution for a while. The Alpha and .46NT share the same crankcase, bearings, carb, and cylinder. You can turn a .40 into a .45 by using the .46NT piston and liner. The crankshafts are slightly different. The crank pin on the .46 is slightly farther out. The .40 is better balanced but you could turn it into a complete .46NT by swapping crankshafts.

I remember my .46NT as taking a couple of tanks of fuel to smooth out and fly like it was broken in. I put it on a plane that previously had a TT Pro .46 and OS .46FX in it. The Evo had more power than those two on the first flight and it just got better. Of the two engines, my .455 is the better one.