I have a Sharps Bros lower I am building with a Geissele LPK.  All was fine until I tried to install the rear detent pin/spring.  The hole is threaded (4-40) the Detent pin and spring will not fit, no way.

This is the first build i've done with a threaded hole for the Rear detent pin/spring - I get the concept, but my pin/spring wont fit so there is no way for me to "lock 'em in"

I really don't care about the threads - the butt plate can hold the spring as it does for all my others.

What do I need to do to get the pin/spring to fit?  Do I run a drill thru it - do I tap it?

This seems a bit more complicated then needed.

Thoughts?

Doesn't fit as in the bore diameter is too small, or doesn't fit as in the spring is too long?

Bore diameter - I just used a 3/32 drill bit,  it opened up the whole enough.   I can't believe I did that but it worked.  Build is complete.

Check to see if the screw still threads into the hole. It may have been that the anodizing was the cause of th snug fit, and the drill bit cleared it out of the way. Having the screw retaining the spring and detent is a huge CONVENIENCE, although as you noted, not a requirement. Personally I like them and add them to lowers when I can.