I agree the pistons are different as you can tell that when looking at the replacement parts on STS Micro Engines.com. I believe you; I just will never get a T if they are that much tighter as I have enough problems starting my B with over 3 gallons on it. You say you have both P/S does this stand for pull starter and if so you most have customized them your self as they do not offer either with a pull starter stock. Yes the 30, 28 and E21 can be bought stock with a pull starter. Even a cast piston has to be machined. Please do not take any of this wrong just making conversation and trying to learn all I can and thanks for all the input.