I'm just wondering what the difference is between these two motors? Do they mount differently, or anything like that? I also would like two know if I could take a 86 cylinder and put it on the 84???

 

The '84 ATC250R is air cooled. '85 and '86 ATC250R were water cooled. The 85/86 engine will bolt into the TRX frame since they are basically the same engine. The cylinders are also interchangeable.
The air cooled engine is totally different. If you have ever raced a water cooled R, you probably got smoked. The bore and stroke of the air and water cooled is totally different. The cylinder bolt spacing is different also. Basically, there is no way to get it to fit.
Swapping the entire engine is also a bad idea because you would have to find somewhere to mount the radiators. The mounting brackets would also have to be made. I also believe the engine is slightly longer on the water cooled.
Hope this helps.

 

OK that tells me what I need to know. I've been looking for a 250r motor that needs some work done to it to buy for my project quad I plan building. I finally sold my Polaris 400 Sport and have some $$ to spend.