diff --git a/source/Boltfactory.py b/source/Boltfactory.py
index 5c3c99f..cb73eb9 100644
--- a/source/Boltfactory.py
+++ b/source/Boltfactory.py
@@ -188,6 +188,14 @@ class add_mesh_bolt(Operator, AddObjectHelper):
         description='Diameter of the Counter Sink Head',
         unit='LENGTH',
     )
+    bf_CounterSink_Head_Angle: FloatProperty(
+        attr='bf_CounterSink_Head_Angle',
+        name='Head angle', default=1.5708,
+        min=0.5, soft_min=2.62,
+        max=2.62,
+        description='Included Angle of the Counter Sink Head',
+        unit='ROTATION',
+    )
     bf_Cap_Head_Height: FloatProperty(
         attr='bf_Cap_Head_Height',
         name='Head Height', default=3 * createMesh.GLOBAL_SCALE,
@@ -369,6 +377,7 @@ class add_mesh_bolt(Operator, AddObjectHelper):
                 col.prop(self, 'bf_Pan_Head_Dia')
             elif self.bf_Head_Type == 'bf_Head_CounterSink':
                 col.prop(self, 'bf_CounterSink_Head_Dia')
+                col.prop(self, 'bf_CounterSink_Head_Angle')
             col.separator()
         # Shank
         if self.bf_Model_Type == 'bf_Model_Bolt':
@@ -524,6 +533,7 @@ def BoltParameters():
         "bf_Hex_Head_Height",
         "bf_Hex_Head_Flat_Distance",
         "bf_CounterSink_Head_Dia",
+        "bf_CounterSink_Head_Angle",
         "bf_Cap_Head_Height",
         "bf_Cap_Head_Dia",
         "bf_Dome_Head_Dia",
diff --git a/source/createMesh.py b/source/createMesh.py
index 8d0fa5c..5522461 100644
--- a/source/createMesh.py
+++ b/source/createMesh.py
@@ -778,7 +778,7 @@ def Create_Dome_Head(HOLE_DIA, HEAD_DIA, SHANK_DIA, HEIGHT, RAD1, RAD2, FACE_OFF
     return sVerts, faces, Dome_Height
 
 
-def Create_CounterSink_Head(HOLE_DIA, HEAD_DIA, SHANK_DIA, HEIGHT, RAD1, DIV_COUNT):
+def Create_CounterSink_Head(HOLE_DIA, HEAD_DIA, SHANK_DIA, HEAD_ANGLE, RAD1, DIV_COUNT):
 
     HOLE_RADIUS = HOLE_DIA * 0.5
     HEAD_RADIUS = HEAD_DIA * 0.5
@@ -788,8 +788,9 @@ def Create_CounterSink_Head(HOLE_DIA, HEAD_DIA, SHANK_DIA, HEIGHT, RAD1, DIV_COU
     faces = []
     Row = 0
 
-    # HEAD_RADIUS = (HEIGHT/tan(radians(60))) + SHANK_RADIUS
-    HEIGHT = tan(radians(60)) * (HEAD_RADIUS - SHANK_RADIUS)
+    # As per ISO 10642 the angle of the head is not constant with nominal diameter.
+    # Between 20 and 22mm shank the head angle changes 90 to 60.
+    HEIGHT = tan((1.5708 - HEAD_ANGLE / 2.0)) * (HEAD_RADIUS - SHANK_RADIUS) + RAD1
 
     FaceStart = len(verts)
 
@@ -2418,7 +2419,7 @@ def Bolt_Mesh(props, context):
     elif props.bf_Head_Type == 'bf_Head_CounterSink':
         Head_Verts, Head_Faces, Head_Height = Create_CounterSink_Head(
             Bit_Dia, props.bf_CounterSink_Head_Dia / GLOBAL_SCALE,
-            props.bf_Shank_Dia / GLOBAL_SCALE, props.bf_CounterSink_Head_Dia / GLOBAL_SCALE,
+            props.bf_Shank_Dia / GLOBAL_SCALE, props.bf_CounterSink_Head_Angle,
             props.bf_CounterSink_Head_Dia / GLOBAL_SCALE * (0.09 / 6.31),
             props.bf_Div_Count
         )
diff --git a/source/presets/operator/mesh.bolt_add/_default.py b/source/presets/operator/mesh.bolt_add/_default.py
index f0f29de..8902af0 100644
--- a/source/presets/operator/mesh.bolt_add/_default.py
+++ b/source/presets/operator/mesh.bolt_add/_default.py
@@ -18,6 +18,7 @@ op.bf_12_Point_Head_Height = 3.0 / 1000.0
 op.bf_12_Point_Head_Flat_Distance = 3.0 / 1000.0
 op.bf_12_Point_Head_Flange_Dia = 5.72 / 1000.0
 op.bf_CounterSink_Head_Dia = 6.300000190734863 / 1000.0
+op.bf_CounterSink_Head_Angle = 1.5708
 op.bf_Cap_Head_Height = 3.0 / 1000.0
 op.bf_Cap_Head_Dia = 5.5 / 1000.0
 op.bf_Dome_Head_Dia = 5.599999904632568 / 1000.0
diff --git a/source/presets/operator/mesh.bolt_add/_m3.py b/source/presets/operator/mesh.bolt_add/_m3.py
index f0f29de..8902af0 100644
--- a/source/presets/operator/mesh.bolt_add/_m3.py
+++ b/source/presets/operator/mesh.bolt_add/_m3.py
@@ -18,6 +18,7 @@ op.bf_12_Point_Head_Height = 3.0 / 1000.0
 op.bf_12_Point_Head_Flat_Distance = 3.0 / 1000.0
 op.bf_12_Point_Head_Flange_Dia = 5.72 / 1000.0
 op.bf_CounterSink_Head_Dia = 6.300000190734863 / 1000.0
+op.bf_CounterSink_Head_Angle = 1.5708
 op.bf_Cap_Head_Height = 3.0 / 1000.0
 op.bf_Cap_Head_Dia = 5.5 / 1000.0
 op.bf_Dome_Head_Dia = 5.599999904632568 / 1000.0
diff --git a/source/presets/operator/mesh.bolt_add/_m4.py b/source/presets/operator/mesh.bolt_add/_m4.py
index 8513f9f..aa7548b 100644
--- a/source/presets/operator/mesh.bolt_add/_m4.py
+++ b/source/presets/operator/mesh.bolt_add/_m4.py
@@ -18,6 +18,7 @@ op.bf_12_Point_Head_Height = 4.0 / 1000.0
 op.bf_12_Point_Head_Flat_Distance = 4.0 / 1000.0
 op.bf_12_Point_Head_Flange_Dia = 7.22 / 1000.0
 op.bf_CounterSink_Head_Dia = 9.399999618530273 / 1000.0
+op.bf_CounterSink_Head_Angle = 1.5708
 op.bf_Cap_Head_Height = 4.0 / 1000.0
 op.bf_Cap_Head_Dia = 7.0 / 1000.0
 op.bf_Dome_Head_Dia = 8.0 / 1000.0
diff --git a/source/presets/operator/mesh.bolt_add/_m5.py b/source/presets/operator/mesh.bolt_add/_m5.py
index 1a43a52..62e3917 100644
--- a/source/presets/operator/mesh.bolt_add/_m5.py
+++ b/source/presets/operator/mesh.bolt_add/_m5.py
@@ -18,6 +18,7 @@ op.bf_12_Point_Head_Height = 5.0 / 1000.0
 op.bf_12_Point_Head_Flat_Distance = 5.0 / 1000.0
 op.bf_12_Point_Head_Flange_Dia = 8.72 / 1000.0
 op.bf_CounterSink_Head_Dia = 10.399999618530273 / 1000.0
+op.bf_CounterSink_Head_Angle = 1.5708
 op.bf_Cap_Head_Height = 5.0 / 1000.0
 op.bf_Cap_Head_Dia = 8.5 / 1000.0
 op.bf_Dome_Head_Dia = 9.5 / 1000.0
diff --git a/source/presets/operator/mesh.bolt_add/_m6.py b/source/presets/operator/mesh.bolt_add/_m6.py
index 33d82d7..4587d59 100644
--- a/source/presets/operator/mesh.bolt_add/_m6.py
+++ b/source/presets/operator/mesh.bolt_add/_m6.py
@@ -18,6 +18,7 @@ op.bf_12_Point_Head_Height = 6.0 / 1000.0
 op.bf_12_Point_Head_Flat_Distance = 6.0 / 1000.0
 op.bf_12_Point_Head_Flange_Dia = 10.22 / 1000.0
 op.bf_CounterSink_Head_Dia = 12.600000381469727 / 1000.0
+op.bf_CounterSink_Head_Angle = 1.5708
 op.bf_Cap_Head_Height = 6.0 / 1000.0
 op.bf_Cap_Head_Dia = 10.0 / 1000.0
 op.bf_Dome_Head_Dia = 12.0 / 1000.0
diff --git a/source/presets/operator/mesh.bolt_add/_m8.py b/source/presets/operator/mesh.bolt_add/_m8.py
index 8f7bb49..dac570c 100644
--- a/source/presets/operator/mesh.bolt_add/_m8.py
+++ b/source/presets/operator/mesh.bolt_add/_m8.py
@@ -18,6 +18,7 @@ op.bf_12_Point_Head_Height = 8.0 / 1000.0
 op.bf_12_Point_Head_Flat_Distance = 8.0 / 1000.0
 op.bf_12_Point_Head_Flange_Dia = 13.27 / 1000.0
 op.bf_CounterSink_Head_Dia = 17.299999237060547 / 1000.0
+op.bf_CounterSink_Head_Angle = 1.5708
 op.bf_Cap_Head_Height = 8.0 / 1000.0
 op.bf_Cap_Head_Dia = 13.5 / 1000.0
 op.bf_Dome_Head_Dia = 16.0 / 1000.0
diff --git a/source/presets/operator/mesh.bolt_add/m10.py b/source/presets/operator/mesh.bolt_add/m10.py
index faf892a..fbe9ba7 100644
--- a/source/presets/operator/mesh.bolt_add/m10.py
+++ b/source/presets/operator/mesh.bolt_add/m10.py
@@ -18,6 +18,7 @@ op.bf_12_Point_Head_Height = 10.0 / 1000.0
 op.bf_12_Point_Head_Flat_Distance = 10.0 / 1000.0
 op.bf_12_Point_Head_Flange_Dia = 16.27 / 1000.0
 op.bf_CounterSink_Head_Dia = 20.0 / 1000.0
+op.bf_CounterSink_Head_Angle = 1.5708
 op.bf_Cap_Head_Height = 10.0 / 1000.0
 op.bf_Cap_Head_Dia = 16.0 / 1000.0
 op.bf_Dome_Head_Dia = 20.0 / 1000.0
diff --git a/source/presets/operator/mesh.bolt_add/m12.py b/source/presets/operator/mesh.bolt_add/m12.py
index 8ffc693..43e6816 100644
--- a/source/presets/operator/mesh.bolt_add/m12.py
+++ b/source/presets/operator/mesh.bolt_add/m12.py
@@ -18,6 +18,7 @@ op.bf_12_Point_Head_Height = 12.0 / 1000.0
 op.bf_12_Point_Head_Flat_Distance = 12.0 / 1000.0
 op.bf_12_Point_Head_Flange_Dia = 18.27 / 1000.0
 op.bf_CounterSink_Head_Dia = 22.0 / 1000.0
+op.bf_CounterSink_Head_Angle = 1.5708
 op.bf_Cap_Head_Height = 12.0 / 1000.0
 op.bf_Cap_Head_Dia = 18.5 / 1000.0
 op.bf_Dome_Head_Dia = 24.0 / 1000.0