I would want to check first before possibly truncating data:

select max(len(code)) from stkitem

or

select *

from stkitem

where len(code) > 20

If you do try:

update stkitem

SET Csimplecode=left(Code,20)

Here is the link for MS: http://technet.microsoft.com/en-us/library/ms177601.aspx

Mike