challenger wrote:
they look really close
They don't even look slightly close.
The 81-84 ATC250R looks closer and its a mile off from matching the FL250 bottom end.
Below is a cylinder gasket from a 1983 ATC250R laying on top of the Mossbarger cylinder.
You can see the ATC250R cylinder has a lot bigger transfer port area.
The stock FL250 was 17 hp and a stock ATC250R was 27 hp.
The older MR250 and the CR250 cylinders off the motorcycles shared the same bolt pattern as the FL250 and bolted right on and the CR250 made more power than the MR and FL cylinders, the Mossbarger setup is the old air cooled CR250 cylinder with a water tank built around it to carry away the heat better.