the crankcases are the same between the 18cc and 24cc engines (at least on the 4 bolt backplate versions), so most of them probably interchange, makes it very easy to plop a new piston/jug on and test other sizes

the threads on the crank are 5/16-24 like homie's

I have one 21, a scored to heck 18, a good 18 (I keep it around as a weedeater, works good), a good 25, a scored 25, a 24 that needs a ring, and a good 24, they all seem to have most of the same parts, I've ran one 24 with the plates removed from the muffler, it isn't all that loud and works fine, it cuts the weight of the muffler nearly in half