cylinder liners

Hi 

I have just been starting some of my bikes up and tinkering in general and noticed a little smoke from 
the exhausts on the Nc , it could be that it has just been sat for a couple of years?

But it got me thinking , has anyone ever put new liners in an exiting set of barrels . 
I know they come out because several years ago I put some parts in an oven to cure the paint work
and the liners had dropped out as the ali expanded 

I assume they could be turned from cast iron /Meehanite