I got another thought ...
Take cap off and install like 4-5 nickels under cap to force spring down more to apply more pressure to retainer so it can be tightened up. It will require more force to thread cap on . But this should do the trick . Afterwards take out nickels and spend wisely
If this does not work , for sure take apart and thread bolt in peice and see what's wrong .
PM me for email as than I can send pix of examples , heck I would tear mine apart to show ya what's what . Been doing this my entire life on those planet
Rail
Best of luck.
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Living life to the fullest to avoid regrets later ,,,,or I am a complete loon ! Either way I am good