i have a 2000 ds 650 over heated broke down on me while down shifting put new head gasket in it runs like new but there is antifreeze mixing with the oil in it changed it twice still runs but still mixes do u know what the problem might be thanks

 

Water Pump Seal is my first guess.

Do a search. Plenty on here.

f650.faq has a nice set of instructions on how to fix.

I suggest getting the parts from your BMW dealer. Same engine as the F650 bike. The impeller is already mounted on the shaft. Makes life easier.