I have a couple of fins which l have noticed when tightening have threaded through.they are brass & my Question is how do you tap them out .they seem to be seized & l don't want to damage or crack the insert holes on my fin
Support with a couple of blocks and tap them out with a hammer and punch. They'll come out just fine.
Replace the nut and all will be fine .they need a good wack...sometimes ...get a bolt the same size as the nut .and hit that ,
^^^^ Yes but with support like shifu said
Get a nut a tiny bit bigger, place fin over that so it is supported all the way around the barrel nut, then bang out with a bolt or bit of steel rod as a drift.
Second option is over vice with jaws opened just enough - but that only supports two sides of the hole and everyone can find a nut on the floor of Bunnings (well maybe not the incompetents who work there but anyway...) , not everyone has a vice :)
spray with some wd40 and leave a few minutes before the operation. Also one side of the barrel usually has a screwdriver slot, you can start loosening it up by rotating it..
If the thread is just corroded and the fin screws are getting too tight, just use a thread tap to clean them out. 
Some Multifins fins used to come with the brass threaded with both UNC and Metric threads. 
. They worked just fine.
If you need to relpace the brass barrel nuts, just remember that there are quite a few different sizes used by different fin manufacturers. They need to be a nice slight interference fit to stop them falling out. If you can buy the right size brass rod you can make your own with a drill and a thread tap.
