diff --git a/readme.md b/readme.md
index 32d0154..73245b9 100644
--- a/readme.md
+++ b/readme.md
@@ -223,6 +223,7 @@ Individual teeth are not drawn, instead they are represented by a lighter colour
 | Function | Description |
 |:--- |:--- |
 | ```belt_pitch(type)``` | Pitch in mm |
+| ```belt_pitch_height(type)``` | Offset of the pitch radius from the tips of the teeth |
 | ```belt_thickness(type)``` | Total thickness including teeth |
 | ```belt_tooth_height(type)``` | Tooth height |
 | ```belt_width(type)``` | Width in mm |
@@ -231,7 +232,7 @@ Individual teeth are not drawn, instead they are represented by a lighter colour
 | Function | Description |
 |:--- |:--- |
 | ```belt_length(points, gap = 0)``` | Compute belt length given path and optional gap |
-| ```belt_pitch_height(type)``` | Offset of the pitch radius from the tips of the teeth |
+| ```belt_pitch_to_back(type)``` | Offset of the back from the pitch radius |
 
 ### Modules
 | Module | Description |
@@ -244,7 +245,7 @@ Individual teeth are not drawn, instead they are represented by a lighter colour
 | Qty | Module call | BOM entry |
 | ---:|:--- |:---|
 |   1 | ```belt(GT2x6, [ ... ])``` |  Belt GT2 x 6mm x 128mm |
-|   1 | ```belt(GT2x6, [ ... ], 80, [0, 0.81])``` |  Belt GT2 x 6mm x 694mm |
+|   1 | ```belt(GT2x6, [ ... ], 80, [0, 0.314])``` |  Belt GT2 x 6mm x 696mm |
 |   1 | ```belt(T2p5x6, [ ... ])``` |  Belt T2.5 x 6mm x 130mm |
 |   1 | ```belt(T5x10, [ ... ])``` |  Belt T5 x 10mm x 130mm |
 |   1 | ```belt(T5x6, [ ... ])``` |  Belt T5 x 6mm x 130mm |
diff --git a/tests/png/belts.png b/tests/png/belts.png
index 9553d20..2475485 100644
Binary files a/tests/png/belts.png and b/tests/png/belts.png differ
diff --git a/vitamins/belt.scad b/vitamins/belt.scad
index 920b9fc..0ec3196 100644
--- a/vitamins/belt.scad
+++ b/vitamins/belt.scad
@@ -34,7 +34,9 @@ function belt_pitch(type)        = type[1]; //! Pitch in mm
 function belt_width(type)        = type[2]; //! Width in mm
 function belt_thickness(type)    = type[3]; //! Total thickness including teeth
 function belt_tooth_height(type) = type[4]; //! Tooth height
-function belt_pitch_height(type) = belt_tooth_height(type) + type[4]; //! Offset of the pitch radius from the tips of the teeth
+function belt_pitch_height(type) = type[5] + belt_tooth_height(type); //! Offset of the pitch radius from the tips of the teeth
+
+function belt_pitch_to_back(type) = belt_thickness(type) - belt_pitch_height(type); //! Offset of the back from the pitch radius
 
 function no_point(str) = chr([for(c = str) if(c == ".") ord("p") else ord(c)]);
 //
@@ -56,24 +58,26 @@ module belt(type, points, gap = 0, gap_pt = undef, belt_colour = grey(20), tooth
 
     module shape() rounded_polygon(points, tangents);
 
+    ph = belt_pitch_height(type);
+    th = belt_tooth_height(type);
     module gap()
         if(gap)
-            translate(gap_pt)
+            translate(gap_pt + [0, -ph + thickness / 2])
                 square([gap, thickness + eps], center = true);
 
     color(belt_colour)
         linear_extrude(width, center = true)
             difference() {
-                offset(thickness - belt_pitch_height(type)) shape();
-                offset(-belt_pitch_height(type) + belt_tooth_height(type)) shape();
+                offset(-ph + thickness ) shape();
+                offset(-ph + th) shape();
                 gap();
 
             }
     color(tooth_colour)
         linear_extrude(width, center = true)
             difference() {
-                offset(-belt_pitch_height(type) + belt_tooth_height(type)) shape();
-                offset(-belt_pitch_height(type)) shape();
+                offset(-ph + th) shape();
+                offset(-ph) shape();
                 gap();
             }
 }