Can I put a liquid cooled 81 honda cr 250 head jug and piston on a fl 250 bottom end with the fl 250 connecting rod ????

not without a lot of welding

the fl250 uses 4 studs to hold down the head and cylinder

it can be done but lots of work,, have to build up the cases with lots of weld material as the studs need to be move out and then all machined back to what you need ,,, some dollars unless you can do it yourself,,
have one of the first HONDASAKI motors myself and it took lots of time welding and machineing ,, and building fixtures to hold the case halfs for accurate machineing ,,

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.

thank you very much for all your input and advice I was thinking about putting a 1/8 pieace aluminum plate between the crank case and cylinder but lt looks like that's not going to work but it would have been sweet though ( more power ) thanks again hoser