Some were air cooled and some liquid cooled - with a radiator.
The laws of physics dictate that you can compress a gas but you cant compress a liquid - so if the coolant gets hot and expands it will need a place to overflow to.
This is very probably the coolant expansion bottle you mention.
I know nothing of motorcycles - so I could easily be wrong.
Good Luck...
October 2015
howtomendit.com for free repair help, information and advice.
