When we work with stuck valve caps, etc. on brass
instruments, rather than
apply force using something like a strap wrench, what works better is a
tangential blow with a hammer (rawhide head in the case of brass). The
concussive shock works where a steady torque does not--and the chances of
stripping threads or shearing bolts is much reduced.
Thay are hard to find, but there is a class of wrench called a
striking wrench. It is made exactly for this purpose - a wrench with a
cube on the end of the handle so you can whack it with a hammer.
--
Will