I had one apart probably 10 years ago.  I think it was this sight but might be wrong.  Mine was stuck and could not be turned with your fingers.  There was a small screw on the side that when removed let out several very small ball bearings.  Once they were out it came apart.  I cleaned it up, reinstalled the bearing and it worked fine.  So, if yours has the small screw it probably comes apart the same way.  I wonder if the tiny bearings are bad.  
If you pull the spring, like shown in my picture, beware that you will need to readjust the standing position detent. The nut opposite the loop locks the spring into position, so leave it loose until the sight is adjusted for your gun.  
To adjust, install the spring through the base clevis and into the sight stem base.  I use a piece of leather under a small vice grip to compress the spring.  Once the spring is through the base, release the vice grip.  Turn the spring until it locks into the detent in the sight stem base.  Install the sight on the gun and lock the spring in place with the nut when the standing position is correct.
Bob

gnoahhh wrote on Apr 4^th, 2024 at 9:19pm:


If it's one of the lower priced Lymans that didn't have the locking collar for the stem this is a common problem. Those with the lock collar didn't have this issue.
It's usually caused by years of wear, and either the stem, collar, or both are well worn and need replacement.

This is the locking collar style that never have loose stem issues:
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I had the same problem.

I removed the stem and used a small punch to dimple the side of the stem (near the base).
The dimples flared it enough so that it was tight when put back into place.

Although I have some old guns with Marbles and Lyman tang sights, and think they're kinda neat, they are not my first choice for any accurate shooting. 
I also prefer almost any vernier or even graduated peep tang sights over them. The typical tang sights are all tighter and better adjustments than the Lyman or Marbles base model sights. Until you get up to the micrometer style Marbles and Lymans with good windage and elevation adjustments, the rest are just sort of plinker sights.