diff --git a/tests/regression_tests/implicit/diamond/inputs_true.dat b/tests/regression_tests/implicit/diamond/inputs_true.dat
index 6e80d2f8f2..693ee5bc9b 100644
--- a/tests/regression_tests/implicit/diamond/inputs_true.dat
+++ b/tests/regression_tests/implicit/diamond/inputs_true.dat
@@ -10,12 +10,12 @@
   <geometry>
     <cell id="1" material="1" region="-7 1 -2 3 -4 5 -6" universe="1"/>
     <cell id="2" material="void" region="7 1 -2 3 -4 5 -6" universe="1"/>
-    <surface id="1" type="x-plane" boundary="periodic" coeffs="-0.5" periodic_surface_id="2"/>
-    <surface id="2" type="x-plane" boundary="periodic" coeffs="0.5" periodic_surface_id="1"/>
-    <surface id="3" type="y-plane" boundary="periodic" coeffs="-0.5" periodic_surface_id="4"/>
-    <surface id="4" type="y-plane" boundary="periodic" coeffs="0.5" periodic_surface_id="3"/>
-    <surface id="5" type="z-plane" boundary="periodic" coeffs="-0.5" periodic_surface_id="6"/>
-    <surface id="6" type="z-plane" boundary="periodic" coeffs="0.5" periodic_surface_id="5"/>
+    <surface id="1" type="x-plane" boundary="vacuum" coeffs="-16.0"/>
+    <surface id="2" type="x-plane" boundary="vacuum" coeffs="16.0"/>
+    <surface id="3" type="y-plane" boundary="vacuum" coeffs="-16.0"/>
+    <surface id="4" type="y-plane" boundary="vacuum" coeffs="16.0"/>
+    <surface id="5" type="z-plane" boundary="vacuum" coeffs="-16.0"/>
+    <surface id="6" type="z-plane" boundary="vacuum" coeffs="16.0"/>
     <surface id="7" type="implicit" coeffs="0.0 0.0 0.0 1.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0 1.0" isovalue="0.0">
       <function>
         <add>
@@ -26,7 +26,7 @@
                   <scale value="6.283185307179586">
                     <x/>
                   </scale>
-                  <constant value="1.0"/>
+                  <constant value="8.0"/>
                 </div>
               </to_cache>
             </sin>
@@ -37,7 +37,7 @@
                     <scale value="6.283185307179586">
                       <y/>
                     </scale>
-                    <constant value="1.0"/>
+                    <constant value="8.0"/>
                   </div>
                 </to_cache>
                 <to_cache id="2">
@@ -45,7 +45,7 @@
                     <scale value="6.283185307179586">
                       <z/>
                     </scale>
-                    <constant value="1.0"/>
+                    <constant value="8.0"/>
                   </div>
                 </to_cache>
               </sub>
diff --git a/tests/regression_tests/implicit/diamond/results_true.dat b/tests/regression_tests/implicit/diamond/results_true.dat
index e5cf6cceec..a9144eca5e 100644
--- a/tests/regression_tests/implicit/diamond/results_true.dat
+++ b/tests/regression_tests/implicit/diamond/results_true.dat
@@ -1,2 +1,2 @@
 k-combined:
-9.438052E-01 4.404392E-03
+2.372756E-01 1.317173E-03
diff --git a/tests/regression_tests/implicit/diamond/test.py b/tests/regression_tests/implicit/diamond/test.py
index ec29a809e4..329004f07c 100644
--- a/tests/regression_tests/implicit/diamond/test.py
+++ b/tests/regression_tests/implicit/diamond/test.py
@@ -15,17 +15,15 @@ def implicit_diamond_model():
     material.set_density('g/cm3', 16.0)
  
     # Diamond
-    x0 = openmc.XPlane(-0.5, boundary_type="periodic")
-    x1 = openmc.XPlane(+0.5, boundary_type="periodic")
-    y0 = openmc.YPlane(-0.5, boundary_type="periodic")
-    y1 = openmc.YPlane(+0.5, boundary_type="periodic")
-    z0 = openmc.ZPlane(-0.5, boundary_type="periodic")
-    z1 = openmc.ZPlane(+0.5, boundary_type="periodic")
-    x0.periodic_surface = x1
-    y0.periodic_surface = y1
-    z0.periodic_surface = z1
+    x0 = openmc.XPlane(-16.0, boundary_type="vacuum")
+    x1 = openmc.XPlane(+16.0, boundary_type="vacuum")
+    y0 = openmc.YPlane(-16.0, boundary_type="vacuum")
+    y1 = openmc.YPlane(+16.0, boundary_type="vacuum")
+    z0 = openmc.ZPlane(-16.0, boundary_type="vacuum")
+    z1 = openmc.ZPlane(+16.0, boundary_type="vacuum")
     box = +x0 & -x1 & +y0 & -y1 & +z0 & -z1
-    impl  = openmc.TPMS.from_pitch_isovalue("diamond", 1., 0.)
+    
+    impl  = openmc.TPMS.from_pitch_isovalue("diamond", 8., 0.)
  
     fuel_cell = openmc.Cell(region=-impl & box, fill=material)
     void_cell = openmc.Cell(region=+impl & box)