An A&P can only install approved parts - that includes screws of a
type approved by the manufacturer. The manufacturer builds the parts
under their manufacturing authority and not via the PMA process.

I have read that they can install
owner manufactured parts?

They can but the owner needs to show that the part is identical to the
original. Not better, not different identical. A writer for Lightplane
Maintenance documented how he made is own fiberglass rudder tip for his Cessna
right up to installing it and then removing it on advice from the FAA that it
did not meet their definition of an owner produced part. Of course your FAA
may vary depending on location.

You can of course by and install Stene's parts and in all likelihood most
future A&P IAs will not notice or ignore them. But you or a future owner might
well run into someone who is more than normally observant and determined to
document the condition of the aircraft and the owner might find himself having
to decide all over again if Stene's were really worth it.

John Dupre'