79 Water Cooled YZ 125 F Kit .

[over the bars]

 Check this out. Im not sure what the water pump runs off ?   Maybe battery driven ?  .  ?   

  http://cgi.ebay.com/ebaymotors/Yamaha-79-YZ125F-WATERCOOLED-KIT-Vintage-MX-MotoCross-_W0QQcmdZViewItemQQhashZitem3cb1c5ed6bQQitemZ260680576363QQptZMotorcyclesQ5fPartsQ5fAccessories

[oldfart]

Thermo syphon ..... very much the same way a solar collector works.  Barrel water heats up and rises to radiator which is air cooled , thus you have a natural convection loop.

Frosty, am I correct

[oldfart]

Just about had it right    go to contents and click on     simple thermosiphon.
 http://en.wikipedia.org/wiki/Thermosiphon

[monaro308]

If i remember correctly US Dirt bike mag ran a story and test on something like that when the YZ J  was released.
I think there was another on a welded up finned barrel on a YZ465 or 250.....i'd have to hunt my old mags.

[over the bars]

  Thanks fellas, I should have paid attention in science class instead of drawing moto x or that should be scramble bikes on the back cover of my exercise book.   

[TT5 Matt]

it's still got a aircooled head which has less fins/surface area than the std head,why bother at that price?

[Lozza]

Could buy a water cooled engine or $1500 buys a welder and a lot of aluminium