I have a 30 that I bought in new short block form. Have no idea what the flywheel is from but it looks about right. I have a 25 from about 1999 that came as a whacker. There's a little difference in the cases. The 30 has the jug held on with #12 screws and on the 25 they are #10.

There was enough difference in everything to make me wonder about the validation of the only parts being different as the Piston/ring/jug? It was Warehouse Hobbies that was selling the 30 jugs to convert them?

Niether vibrates.

I don't think there is any counterweight to the flywheel for I balanced both mine out to Zero. They were not far off to begin with.

I made a 28 Mac into a 32 and I never had any trouble with it but the cases looked the same on them.

Enjoy,

Jim