I think one of the points Tim was refering to, but didn't quite make
was that unless a specific FAR (or possibly a TC Data Sheet) requires a
TSO'd instrument you are not required to install only TSO'd instruments
in certified aircraft.

I believe the FAR's require TSO'd Transponders and ELT and for certain
IFR Operatations TSO'd instruments are required.

However the G-meter is a great example. If anyone ever finds a TSO'd
G-meter please let me know. In fact if you can even find the TSO for
G-meters please let me know. As best as I can tell the FAA has never
bothered to write a TSO for G-meters. Why should they as there is no
FAR that requires TSO'd G-meters. Again please prove me wrong if you
can.

If only TSO'd instruments can be installed in certified aircraft then I
don't think it is possible to put legally put a G-meter in a certified
aircraft unless it is listed on the TC data sheet. But I can find no
FAR that says non-TSO'd instruments can not be installed.

Tim is right in that many FAA inspectors do not seem to understand
this anymore, and it is easy to find FAA inspectors will argue the
above with you.

Brian

BTW: I didn't get into who can install such equipment and what is the
required for paperwork to do so. This can be almost, if not a longer
discussion.