diff --git a/assembly.scad b/assembly.scad
index 1e67aac..eb55899 100644
--- a/assembly.scad
+++ b/assembly.scad
@@ -67,7 +67,7 @@ module moving_parts(xpos = cur_xpos, ypos = cur_ypos) {
     ysled_slider(spread = sled_ysize);
   
   translate([xsize - yrail_offset, cur_ypos, 0])
-    rotate([0, 180, 0]) 
+    rotate([180, 180, 0]) 
     ysled_slider(spread = sled_ysize);
   
   translate([yrail_offset, cur_ypos, 0])
diff --git a/parts.scad b/parts.scad
index d07f735..31942c2 100644
--- a/parts.scad
+++ b/parts.scad
@@ -13,14 +13,18 @@ corner_size=30;
 corner_offset = frame_size / 2 + wall_thickness;
 rail_diam = 8;
 // distance between centerline of rail and centerline of frame Y tube.
-yrail_offset = (frame_size + corner_size / 2) / 2;
+yrail_offset = (frame_size + corner_size) / 2;
 
 ysled_extralen = frame_size; // on each end
 
+// Both of these are distance from frame centerline
+belt_height1 = frame_size / 2 + wall_thickness + belt_width / 2 + 2;
+belt_height2 = belt_height1 + belt_width + 2;
+
 // calculated
 
 ysled_rail_spacing = (sled_ysize);
-
+motor_shaft_xoffset = (corner_size - wall_thickness) / 2;
 
 
 
@@ -83,11 +87,12 @@ module rail_holder(in_x) {
   }
 }
 
-// centered on 
+// centered on the rail.
 module ysled_slider(spread, associated_parts = $preview) {
   bom_item(label = str("ysled_slider(", spread, ")"));
   sled_len = spread + frame_size;
   sled_height = frame_size;
+  idler_spacing = spread / 3;
   
   colorize("green")
   render()
@@ -95,8 +100,11 @@ module ysled_slider(spread, associated_parts = $preview) {
   rotate([-90, 0, 0]) {
     difference() {
       union() {
-	translate([-frame_size/2, -frame_size/2, 0])
-	  cube([frame_size + 5, frame_size, sled_len]);
+        translate([-frame_size/2, -frame_size/2, 0])
+          cube([frame_size + 5, frame_size, sled_len]);
+        translate([pulley_sbend_offset - yrail_offset + motor_shaft_xoffset - pulley_clearance_rad - 4,
+                  -frame_size / 2, 0])
+          cube([frame_size + 5, frame_size, sled_len]);
       }
       
       cylinder(d = bushing_id * 1.1, h = sled_len);
@@ -116,6 +124,23 @@ module ysled_slider(spread, associated_parts = $preview) {
   }
   // TODO: add posts for the idlers
   
+  //color("#f00")
+  translate([pulley_sbend_offset  - yrail_offset + motor_shaft_xoffset, 0, 0]) {
+    translate([0, idler_spacing / 2, 0]) {
+      idler_post(belt_height = belt_height1);
+    }
+    translate([0, -idler_spacing / 2, 0]) {
+      *cylinder(d=3, h=30, $fs=1);
+      *idler(belt_height2);
+      rotate([0, 0, -90]) {
+      idler_post(belt_height = belt_height2);
+      }
+      
+    }
+    
+
+  }
+
   translate([0, -sled_len/2, 0])
   rotate([-90, 0, 0])
   if (associated_parts) {
@@ -153,6 +178,43 @@ module nema17_housing() {
   }
 }
 
+// belt height is the center of the belt.
+// Roration is in Z axis, from +x/+y
+module idler_post(belt_height, toothed=true) {
+  gap=0.5;
+
+  post_radius = pulley_clearance_rad + 4;
+  top_shelf_thickness = 4;
+  total_height = idler_zmargin.y + belt_height + top_shelf_thickness + gap;
+  slot_radius = pulley_clearance_rad;
+  slot_height = idler_zmargin.x + idler_zmargin.y + gap * 2;
+
+  difference() {
+    union() {
+      // outer shell
+      cylinder(h = total_height, r = post_radius);
+      translate([0, -post_radius, 0])
+      cube([post_radius, post_radius, total_height]);
+      translate([-post_radius,0,  0])
+      cube([post_radius, post_radius, total_height]);
+    }
+
+    translate([0,0,belt_height - idler_zmargin.x - gap]) {
+      cylinder(r = slot_radius, slot_height);
+      translate([-slot_radius, 0, 0])
+      cube([post_radius*2, post_radius * 2, slot_height]);
+      translate([0, -slot_radius, 0])
+      cube([post_radius * 2, post_radius * 2, slot_height]);
+    }
+
+    cylinder(d = 3, h=total_height + 1, $fn=60);
+    translate([0, 0, total_height - 2])
+    cylinder(r = 3, h = 3);
+  }
+
+  idler(height = belt_height);
+}
+
 module flcorner() {
   bom_item();
   rotate([0, 0, 0]) {
@@ -167,8 +229,8 @@ module flcorner() {
 	     -nema17_face/2 - frame_size/2 - wall_thickness,
 	     frame_size/2]) {
     nema17();
-    translate([0, 0, 2]) {
-      pulley(16,6,1,7);
+    translate([0, 0, -frame_size/2 + belt_height2]) {
+      active_pulley();
     }
   }
     
@@ -188,8 +250,9 @@ module frcorner() {
 	     -nema17_face/2 - frame_size/2 - wall_thickness,
 	     frame_size/2]) {
     nema17();
-    translate([0,0,2]) {
-      pulley(16, 1, 6, 7);
+    translate([0,0,belt_height1 - frame_size / 2]) {
+      rotate([180,0,0])
+      active_pulley();
     }
   }
 }
@@ -393,6 +456,9 @@ module xsled2() {
     
   }
 }
+
+
+
 // render()
 // xsled2();
 
diff --git a/stock_parts.scad b/stock_parts.scad
index 35fd58a..f06d705 100644
--- a/stock_parts.scad
+++ b/stock_parts.scad
@@ -60,10 +60,15 @@ module nema17(depth = 23.5) {
 // belts and pulleys
 belt_pitch = 2; // 2mm tooth width for GT2 = 2mm pitch
 belt_width = 6;
-belt_thickness = 3;
-tooth_height = 2;
+belt_thickness = 1.38;
+belt_pld = 0.254; // distance between line of fixed length and tips of gear teeth
+belt_backing = 0.63 - belt_pld;
+tooth_height = 0.75;
 
-module pulley(teeth, lrim, urim, belt_width, shaft=5, toothed=true) {
+pulley_od = 13;
+pulley_clearance_rad = pulley_od / 2 + 2.5;
+
+module pulley(teeth, lrim, urim, belt_width, shaft=5, toothed=true, od = 0) {
   bom_item(printable=false, label=str(
     toothed ? "pulley(" : "idler(",
     "teeth=", teeth, ", ",
@@ -73,10 +78,12 @@ module pulley(teeth, lrim, urim, belt_width, shaft=5, toothed=true) {
     "shaft=", shaft, "mm",
     ")"
   ));
-  inner_rad = belt_pitch * teeth / TAU;
-  outer_rad = inner_rad + belt_thickness;
+  nominal_rad = belt_pitch * teeth / TAU;
+  inner_rad = nominal_rad - belt_pld;
+  outer_rad = (od > 0) ? od / 2 : nominal_rad +  belt_backing;
   minor_rad = inner_rad - tooth_height;
   mid_rad = inner_rad - tooth_height / 2;
+  idler_rad = nominal_rad - belt_backing;
 
   dtheta = 180 / teeth;
 
@@ -105,7 +112,7 @@ module pulley(teeth, lrim, urim, belt_width, shaft=5, toothed=true) {
         linear_extrude(belt_width)
         polygon(tooth_points, convexity = teeth * 2 + 4);
       } else {
-        cylinder(r = minor_rad, h = belt_width);
+        cylinder(r = idler_rad, h = belt_width);
       }
 
       translate([0,0,lrim + belt_width])
@@ -117,4 +124,22 @@ module pulley(teeth, lrim, urim, belt_width, shaft=5, toothed=true) {
   }
 }
 
+idler_zmargin = [ 6.4 / 2 + 1.05, 6.4 / 2 + 1.05 ];
+
+module idler(height=0, toothed=false) {
+  // pulley(20, 1.05, 1.05, 6.4, toothed=toothed, od=18);
+  translate([0,0, height-idler_zmargin[0]])
+  pulley(16, 1.05, 1.05, 6.4, shaft=3, toothed=toothed, od=13);
+}
+
+active_pulley_zmargin = [ 9.5, 4.5 ];
+
+module active_pulley(height=0) {
+  translate([0,0,-active_pulley_zmargin[0] + height])
+  pulley(16,6,1,7, od=13);
+}
+
+// required distance between centers of pulley and idler for an S-bend with right angles.
+pulley_sbend_offset = (9.5 + 9.7) / 2 + 0.63;
+
 //!pulley(12, 6, 2, toothed=false);
\ No newline at end of file