  We present a method for measuring thermal expansion under tunable uniaxial
stresses, and show measurements of the thermal expansion of Mn$_3$Sn, a room
temperature antiferromagnet that exhibits a spontaneous Hall effect, under
uniaxial stresses of up to 1.51 GPa compression. Measurement of thermal
expansion provides thermodynamic data about the nature of phase transitions,
and uniaxial stress provides a powerful tuning method that does not introduce
disorder. Mn$_3$Sn exhibits an anomaly in its thermal expansion near $\sim$270
K, associated with a first-order change in its magnetic structure. We show this
transition temperature is suppressed by 54.6 K by 1.51 GPa compression along
[0001]. We find the associated entropy change at the transition to be $\sim$
0.1 J mol$^{-1}$ K$^{-1}$ and to vary only weakly with applied stress.
