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My first 8" Newtonian used valve springs in the primary mirror cell, but like most things with springs it never remained collimated for long as the wing nuts on the back would shift. Not a great idea IMHO.
In the end I removed the springs and used lock nuts on the inside of each threaded rod, opposing the wing nuts. A bit more fiddly to adjust during collimation, but once done it DID remain collimated - well enough that I could remove the entire cell from the tube to clean the mirror, put it back and no adjustment was needed.
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