"Mark Hansen" wrote in message
...
I have a maintenance question.

I read somewhere that the mixture control should not bottom-out against
the firewall when full rich, as the pilot would not be able to tell if
the full-rich position was achieved, or if the knob just hit the firewall
stop.

However, I'm wondering if this is just a rule of thumb, or if there is
an actual FAR which dictates it, and which FAR that is?

While it's probably not a regulation, most service manuals specify that both
throttle and mixture knobs have a certain amount of "springback" instead of
the knob contacting the panel; the Piper Cherokee manual, for instance,
specifies 1/16" to 1/8" springback.