Glenn Ashmore wrote:
Is there any rule of thumb about how far can stretch a spring before it is
"sprung". i.e. permanently deformed?

I have found almost the perfect spring for my camera stabilizer project but
it is only 4" long and rated for 42 lb at 3.5" and I need it to extend a bit
more than 6". I am worried that if it bottoms out once it will never come
back to its original initial tension.

Get one and try it. Most likely, it will take this overtravel, but
that will shorten its life somewhat. The 6" extension will be about
the limit, I'd guess. Once you exceed the absolute elastic limit, the
spring will no longer contract fully, so you will know that you've
found the limit.

Jon