I seem to be having problems with water circulation throughout my 1996 polaris sport 400 engine.  The fan does not turn on when the engine is hot and the water is not circulating.  I just changed my thermal switch on my head.  I think it is the water pump but if it is, what in the water pump would cause these situations?  Where is the second thermal switch, if there is one?  Also, is the water pump a difficult task to change on a 96 polaris sport 400?  

if your water is not circulating the water pump is bad.

check the coolant temperature sensor, that is what tells the fan to come on.
if the unit overheats most times the coolant temperature sensor will burn up.

and it is possible to buy a new one that is bad.

but if you bought the new sensor and ran the atv and it got hot because the water pump wasn't working it is very possible that the new sensor burned out as the engine overheated.

it's kind of hard to determine whether it would be hard for you.
for me it is simple because i have changed a ton of these.
it would depend on your mechanical ability, just take your time and make sure you lay everything out so that it goes back together the same way it came off.  

I have a 1999 Polaris sport 400 two stroke and when i got the bike it was full of water. so i drained all the fulids but i cant find out how much oil goes into the motor bock i know it isnt much but i dont know what kind of oil and how much any ideas? Thanks  

should be here in the manual if your manual is uploaded here.

http://manuals.dqt.be/index.php?option=com_remository&Itemid=36