Hi Gofli
I don't pretend to be an expert on all Poulans, but it seems to me that they use the same crankcase, crankshaft, and flywheel on several models. The piston and the cylinder seems to be what varies to change the displacement. The ports, especially the intake port, seems to be noticeably smaller on the smaller displacent engines. I have 2 - 21cc and 2 - 25cc engines and another cylinder and piston that I got with some other parts. This is my experience with Poulans. It looks as though the cylinders from the 21s would interchange with the 25s. As far as the new 25cc engine with the 2 bolt crankcase, I am not sure about it's cylinder interchanging with the older versions. As far as having work into a crankcase, that might not be a big deal, because trimming a FL-25, the one I posted photos on, is not hard at all. I have done two and I could do another one in less than 1/2 hour. They are cake to do, they are almost as easy to trim as the Toro 25.4cc engines in Dave Wallace's thread. The first 25cc FL-25 I did runs very well, after I ported the intake and exhaust, added 2 -1/2" pipes to the muffler and installed a bigger 2 needle carb. If you want to stick with Poulan, I suggest keeping an eye out for a FL-25, they still show up on ----.