Always good to see that someone is inquisitive as to the inner workings of things like fuel control units.
So to clear up some confusion on this post you must understand there are 2 makes of fuel controls used on the PT-6.
The smaller engines use the Bendix fuel control - which has a needle valve in a contoured seat to meter the fuel to the engine. This is the system as explained in the FreeBee publication.
The larger engines use a Woodward fuel control unit that uses a 3D cam and the rotating / lifting (Coffee cup with a square hole reference) fuel metering valve.
The 2 fuel controls operate differently.
So to answer the original question as to how the fuel metering valve is held against the minimum flow stop during engine start the answer is simple. When the valve train is installed the torque tube and metering valve is set to pre-load the valve against the min flow stop.
The information was in the United Turbine document - page 47
reference start
BELLOWS ASSEMBLY
The bellows assembly consists of an
evacuated (acceleration) bellows and a
governor bellows connected by a
common rod. The end of the
acceleration bellows opposite the rod is
attached to the body casting. The
accelerating bellows provides an
absolute pressure reference. The
governor bellows is secured in the
body cavity and its function is similar to
that of a diaphragm. Movement of the
bellows is transmitted to the metering
valve by a cross shaft and associated
levers.
The cross shaft moves within a torque
tube which is attached to the cross shaft
near the bellows lever. The tube is
secured in the body casting at the
opposite end by means of an
adjustment bushing. Therefore, any
rotational movement of the cross shaft
will result in an increase or decrease in
the force of the torque tube. The torque
tube forms the seal between the air and
fuel sections of the control. (The torque
tube is positioned during assembly to
provide a force in a direction tending to
close the metering valve. The bellows
act against this force to open the
metering valve.) Py pressure is applied
to the outside of the governor bellows.
Px pressure is applied to the inside of
the governor bellows and to the outside
of the acceleration bellows.
Reference end
The torque tube will transmit the force from the air side to the fuel side - sealing is done with an o-ring which is a simple effective seal method. The torque tube is not a torsion tube - ie. - it is not a spring and simply serves to transmit the forces.
Having had many of these fuel control units apart I can attest that the valve is pre-loaded exactly as the manual says.
Hope this helps.
author of the FreeBee Bendix FCU document.