diff --git a/gearfunc/_Classes.py b/gearfunc/_Classes.py
index 4272695..3dc1752 100644
--- a/gearfunc/_Classes.py
+++ b/gearfunc/_Classes.py
@@ -95,8 +95,12 @@ class involute_gear():
         obj.addProperty(
             "App::PropertyAngle", "beta", "gear_parameter", "beta ")
         obj.addProperty(
-            "App::PropertyLength", "backlash", "gear_parameter", "backlash in mm")
-        obj.addProperty("App::PropertyPythonObject", "gear", "test", "test")
+            "App::PropertyLength", "backlash", "tolerance", "backlash")
+        obj.addProperty(
+            "App::PropertyBool", "reversed_backlash", "tolerance", "backlash direction")
+        obj.addProperty(
+            "App::PropertyFloat", "head", "gear_parameter", "head_value * modul_value = additional length of head")
+        obj.addProperty("App::PropertyPythonObject", "gear", "gear_parameter", "test")
         obj.gear = self.involute_tooth
         obj.simple = False
         obj.undercut = False
@@ -107,8 +111,10 @@ class involute_gear():
         obj.beta = '0. deg'
         obj.height = '5. mm'
         obj.clearance = 0.25
+        obj.head = 0.
         obj.numpoints = 6
         obj.backlash = '0.00 mm'
+        obj.reversed_backlash = False
         self.obj = obj
         obj.Proxy = self
 
@@ -120,7 +126,8 @@ class involute_gear():
         fp.gear.pressure_angle = fp.pressure_angle.Value * pi / 180.
         fp.gear.beta = fp.beta.Value * pi / 180
         fp.gear.clearance = fp.clearance
-        fp.gear.backlash = fp.backlash.Value
+        fp.gear.backlash = fp.backlash.Value * (-fp.reversed_backlash + 0.5) * 2.
+        fp.gear.head = fp.head
         fp.gear._update()
         pts = fp.gear.points(num=fp.numpoints)
         rotated_pts = pts
@@ -223,7 +230,7 @@ class cycloide_gear():
         obj.addProperty("App::PropertyAngle", "beta", "gear_parameter", "beta")
         obj.addProperty(
             "App::PropertyLength", "backlash", "gear_parameter", "backlash in mm")
-        obj.addProperty("App::PropertyPythonObject", "gear", "test", "test")
+        obj.addProperty("App::PropertyPythonObject", "gear", "gear_parameter", "test")
         obj.gear = self.cycloide_tooth
         obj.teeth = 15
         obj.module = '1. mm'
@@ -298,7 +305,7 @@ class bevel_gear():
                         "gear_parameter", "number of points for spline")
         obj.addProperty(
             "App::PropertyLength", "backlash", "gear_parameter", "backlash in mm")
-        obj.addProperty("App::PropertyPythonObject", "gear", "test", "test")
+        obj.addProperty("App::PropertyPythonObject", "gear", "gear_paramenter", "test")
         obj.gear = self.bevel_tooth
         obj.m = '1. mm'
         obj.teeth = 15
diff --git a/gearfunc/_involute_tooth.py b/gearfunc/_involute_tooth.py
index bafff44..29c3295 100644
--- a/gearfunc/_involute_tooth.py
+++ b/gearfunc/_involute_tooth.py
@@ -25,7 +25,8 @@ from ._functions import nearestpts, rotation, reflection, trimfunc, norm, transl
 import numpy as np
 
 class involute_tooth():
-    def __init__(self, m=5, z=15, pressure_angle=20 * pi / 180., clearance=0.12, shift=0.5, beta=0., undercut=False, backlash=0.00):
+    def __init__(self, m=5, z=15, pressure_angle=20 * pi / 180., clearance=0.12, shift=0.5, beta=0.,
+                       undercut=False, backlash=0.00, head=0.00):
         self.pressure_angle = pressure_angle
         self.beta = beta
         self.m_n = m
@@ -34,16 +35,18 @@ class involute_tooth():
         self.shift = shift
         self.clearance = clearance
         self.backlash = backlash
+        self.head = head        # factor, rename!!!
         self._calc_gear_factors()
 
     def _calc_gear_factors(self):
+        print(self.head)
         self.pressure_angle_t = arctan(tan(self.pressure_angle) / cos(self.beta))
         self.m = self.m_n / cos(self.beta)
         self.c = self.clearance * self.m_n
         self.midpoint = [0., 0.]
         self.d = self.z * self.m
         self.dw = self.m * self.z
-        self.da = self.dw + 2. * self.m_n + 2. * self.shift * self.m_n
+        self.da = self.dw + 2. * self.m_n + 2. * (self.shift + self.head) * self.m_n
         self.df = self.dw - 2. * self.m_n - \
             2 * self.c + 2. * self.shift * self.m_n
         self.dg = self.d * cos(self.pressure_angle_t)
@@ -146,7 +149,7 @@ class involute_tooth():
     def _update(self):
         self.__init__(m = self.m_n, z = self.z,
                 pressure_angle = self.pressure_angle, clearance = self.clearance, shift = self.shift,
-                beta = self.beta, undercut = self.undercut, backlash = self.backlash)
+                beta = self.beta, undercut = self.undercut, backlash = self.backlash, head = self.head)
 
 
 class involute_rack(object):