Are TSO Instruments required in certificated aircraft? Altimeter,
airspeed , anything.....

Not unless it's used in operations for hire (rentals and instruction
don't count).

The problem is, if they're not TSO'd, on what basis are you installing
them? To be installed as an aircraft part, whatever you are installing
must be a standard part (not applicable to instruments - we're talking
hardware, wire, etc.), a TSO'd part, a TC'd/STC'd part, a PMA'd part,
or a part manufactured by the owner-operator. Otherwise it's not an
aircraft part, and will require a field approval.

So let's say you want to install a non-TSO'd altimeter, radio, whatever
into your certificated airplane.

Sometimes it's easy. Let's say you want to install a KX-170B. It's
not TSO'd. But it's manufactured as an aircraft part, with FAA
authorization. No problem.

Or let's say it's a Magellan 5000 VFR GPS. It has (believe it or not)
a Beech PMA. No TSO, but no problem either. It's an aircraft part,
the installation is considered a minor alteration these days (at least
in some FSDO's), so no worries.

But what if it's not? What if the manufacturer has no authorization
from the FAA of any kind? Not going to happen. Can't install it. Not
unless you get an STC or field approval.

Michael