I just put an 87 degree t-stat in my '82 diesel.  The t-stat that came
out was of unknown temp.  It says "ROBERTSHAW CONTROLS" on it and
337/437 - 160.
The needle moved from just below the light to just above.  The diesel
now actually has heat!  No difference in the way it runs.  Mileage still
around 27.5 (first tank after change).
Mike Pidcoe