Originally Posted by
fietsbob
But generally JIS arms wont go onto an ISO spindle More than 1/2 way.
other way round and the ISO arms bottom out at the end of the square taper without being quite right..
You have this backwards. JIS and ISO square tapers share the same 4 degree included angle but ISO tapers have slightly
smaller dimensioned squares at the ends. So, a JIS crank will go too far up an ISO spindle and may bottom out before it gets firmly seated. Conversly, an ISO crank won't go far enough up a JIS spindle to fully support the crank and to achieve a proper chain line.
Here is Sheldon Brown's article on these tapers:
http://www.sheldonbrown.com/bbtaper.html