I had an FG a few years ago-very capable little body...
I also had a few 50 1.8 Series E lenses over the years. It seems many of the Series E lenses would get slack in the focus helicoid with heavy use. It would be best to find one that looks great cosmetically-that's a sign that it didn't get used much.
my name is Noel and I have 3 SG.
...
one if my series E leness was so loose in the heliciod it rattled when you wound on
if you are handy and have
rubber glove
pill box plastic about 50mm diameter
small cruicifirm screw driver
darning needle
small swiss file
Six inches of double sided pressure sensitive tape
then unscrew name plate ring using glove as friction surface and pill box as wrench
you can undo all six screws and stick them to tape you dont need to undo all of them but it should not matter if you note where they came from.
The focus ring will then remove
and you can unscrew both inner and outer heliciod you should mark the entry points but I did not.
My heliciod was devoid of any lube...
I used a medium PTFE loaded auto grease on inner and outer.
I set the outer heliciod out a turn to allow the inner heliciod greater engagement which was not necessary with the heavy grease. The outer has fine slow pitch treads the inner coarse pitch interrupted threads.
When you reassemble one of the aperture followers will click in easily one needs to be guided in from the mount end with a darning needle. No force is needed just patience.
You need to set infinity on a star or moon. I needed to rebuild at succession of heliciod start points to get a full range you should be ok if you marked the start points
Don't try to loosen of the mount screws they are glued in... with a darning needle you don't need to.
No trace of slack now
I have two series E, a pancake AIs and a long nose not easy to distinguish their photos.
Lots of my other nikkors need new lube think Nikon were very sparing in the factory.
So many problems so little time.