I thought someone said the internals were identical, I'm assuming the piston/sleeve or conrod or crank has to be different though since the displacement is different between the two. The .28 is absolutely insane, I'd definitely try the .25 as well if I had a choice. It probably gets slightly better fuel mileage and still will have plenty of power to get around most tracks in a timely manner. Even if amain doesn't stock .25 parts...they can get anything from Russel and he probably has it in stock so it won't be more than a few days for amain to get the correct parts in stock. I wouldn't base your decision solely on that if I were you...if you're more interested in the .25 get it and don't look back. In the event of needing new internal parts simply call one of the retailers and for them getting the correct parts shouldn't be an issue. Russel ships stuff out usually the same day payment is sent so it won't take long for you to get it.