diff --git a/libtest.png b/libtest.png
index d169c11..eeb7743 100644
Binary files a/libtest.png and b/libtest.png differ
diff --git a/libtest.scad b/libtest.scad
index 4a12c49..3d3d31a 100644
--- a/libtest.scad
+++ b/libtest.scad
@@ -225,7 +225,7 @@ inserts_y = 0;
 nuts_y = inserts_y + 20;
 washers_y = nuts_y + 120;
 screws_y = washers_y + 120;
-circlips_y = screws_y + 160;
+circlips_y = screws_y + 180;
 springs_y = circlips_y + 20;
 o_rings_y = springs_y;
 sealing_strip_y = springs_y + 20;
@@ -402,21 +402,21 @@ extrusions_y = panel_meters_y + 80;
 translate([x3, veroboard_y])
     veroboard_test();
 
-translate([x3 + 60, veroboard_y + 20])
+translate([x3 + 50, veroboard_y + 20])
     geared_steppers();
 
 translate([x3 + 160, ssrs_y])
     pcb_mounts();
 
-translate([x3 + 170, veroboard_y + 16])
+translate([x3 + 145, veroboard_y + 16])
     cameras();
 
+translate([x3 + 145, d_connectors_y - 10])
+    camera_housings();
+
 translate([x3, d_connectors_y])
     d_connectors();
 
-translate([x3 + 170, d_connectors_y - 10])
-    camera_housings();
-
 translate([x3, iecs_y])
     iecs();
 
diff --git a/readme.md b/readme.md
index a48dc59..f76fcf1 100644
--- a/readme.md
+++ b/readme.md
@@ -3484,6 +3484,7 @@ For an explanation of `screw_polysink()` see <https://hydraraptor.blogspot.com/2
 | `screw_radius(type)` | Nominal radius |
 | `screw_socket_af(type)` | Socket across flats |
 | `screw_socket_depth(type)` | Socket or slot depth |
+| `screw_thread_diameter(type)` | Thread diameter, if different from nominal diamter |
 | `screw_washer(type)` | Default washer |
 
 ### Functions
@@ -3526,12 +3527,14 @@ For an explanation of `screw_polysink()` see <https://hydraraptor.blogspot.com/2
 |   1 | `screw(M3_hex_screw, 10)` |  Screw M3 hex x 10mm |
 |   1 | `screw(M3_low_cap_screw, 10)` |  Screw M3 low cap x 10mm |
 |   1 | `screw(M3_pan_screw, 10)` |  Screw M3 pan x 10mm |
+|   1 | `screw(M3_shoulder_screw, 12)` |  Screw M3 shoulder x 12mm |
 |   1 | `screw(M4_cap_screw, 25)` |  Screw M4 cap x 25mm |
 |   1 | `screw(M4_cs_cap_screw, 25)` |  Screw M4 cs cap x 25mm |
 |   1 | `screw(M4_dome_screw, 25)` |  Screw M4 dome x 25mm |
 |   1 | `screw(M4_grub_screw, 8)` |  Screw M4 grub x  8mm |
 |   1 | `screw(M4_hex_screw, 30)` |  Screw M4 hex x 30mm |
 |   1 | `screw(M4_pan_screw, 30)` |  Screw M4 pan x 30mm |
+|   1 | `screw(M4_shoulder_screw, 16)` |  Screw M4 shoulder x 16mm |
 |   1 | `screw(M5_cap_screw, 30)` |  Screw M5 cap x 30mm |
 |   1 | `screw(M5_cs_cap_screw, 30)` |  Screw M5 cs cap x 30mm |
 |   1 | `screw(M5_dome_screw, 30)` |  Screw M5 dome x 30mm |
diff --git a/tests/png/screws.png b/tests/png/screws.png
index 856a400..a5f01f0 100644
Binary files a/tests/png/screws.png and b/tests/png/screws.png differ
diff --git a/tests/screws.scad b/tests/screws.scad
index a8c0200..16decc0 100644
--- a/tests/screws.scad
+++ b/tests/screws.scad
@@ -56,7 +56,7 @@ module screws() {
                     screw(screw, length);
             }
         }
-        translate([20, 40, -15])
+        translate([20, 60, -15])
             polysink_stl();
 }
 
diff --git a/vitamins/screw.scad b/vitamins/screw.scad
index 4a0e978..badf8e9 100644
--- a/vitamins/screw.scad
+++ b/vitamins/screw.scad
@@ -41,6 +41,7 @@ function screw_washer(type)           = type[9];     //! Default washer
 function screw_nut(type)              = type[10];    //! Default nut
 function screw_pilot_hole(type)       = type[11];    //! Pilot hole radius for wood screws, tap radius for machine screws
 function screw_clearance_radius(type) = type[12];    //! Clearance hole radius
+function screw_thread_diameter(type)  = type[13];    //! Thread diameter, if different from nominal diamter
 function screw_nut_radius(type) = screw_nut(type) ? nut_radius(screw_nut(type)) : 0; //! Radius of matching nut
 function screw_boss_diameter(type) = max(washer_diameter(screw_washer(type)) + 1, 2 * (screw_nut_radius(type) + 3 * extrusion_width)); //! Boss big enough for nut trap and washer
 function screw_head_depth(type, d = 0) =             //! How far a counter sink head will go into a straight hole diameter d
@@ -87,47 +88,50 @@ module screw(type, length, hob_point = 0, nylon = false) { //! Draw specified sc
     vitamin(str("screw(", type[0], "_screw, ", length, arg(hob_point, 0, "hob_point"), arg(nylon, false, "nylon"), "): ", description));
 
     head_type   = screw_head_type(type);
-    rad         = screw_radius(type) - eps;
+    shaft_rad   = screw_radius(type) - eps;
     head_rad    = screw_head_radius(type);
     head_height = screw_head_height(type);
     socket_af   = screw_socket_af(type);
     socket_depth= screw_socket_depth(type);
     socket_rad  = socket_af / cos(30) / 2;
     max_thread  = screw_max_thread(type);
+    has_shoulder = !is_undef(screw_thread_diameter(type));
+    thread_rad = has_shoulder ? screw_thread_diameter(type) / 2 : screw_radius(type);
     thread = max_thread ? length >= max_thread + 5 ? max_thread
                                                    : length
                         : length;
-    d = 2 * screw_radius(type);
-    pitch = metric_coarse_pitch(d);
+    thread_offset = has_shoulder ? thread : 0;
+    thread_d = 2 * thread_rad;
+    pitch = metric_coarse_pitch(thread_d);
     colour = nylon || head_type == hs_grub ? grey(40) : grey(80);
 
     module shaft(socket = 0, headless = false) {
-        point = screw_nut(type) ? 0 : 3 * rad;
-        shank  = length - thread - socket;
+        point = screw_nut(type) ? 0 : 3 * shaft_rad;
+        shank  = length - socket - (has_shoulder ? 0 : thread);
 
         if(show_threads && !point && pitch)
-            translate_z(-length)
-                male_metric_thread(d, pitch, thread - (shank > 0 || headless ? 0 : socket), false, top = headless ? -1 : 0, solid = !headless, colour = colour);
+            translate_z(-length - thread_offset)
+                male_metric_thread(thread_d, pitch, thread - (shank > 0 || headless ? 0 : socket), false, top = headless ? -1 : 0, solid = !headless, colour = colour);
         else
             color(colour * 0.9)
                 rotate_extrude() {
-                    translate([0, -length + point])
-                        square([rad, length - socket - point]);
+                    translate([0, -length + point - thread_offset])
+                        square([thread_rad - eps, length - socket - point]);
 
                     if(point)
                         polygon([
-                            [0.4, -length], [0, point - length], [rad, point - length]
+                            [0.4, -length], [0, point - length], [shaft_rad, point - length]
                         ]);
                 }
 
         if(shank > 0)
             color(colour)
                 translate_z(-shank - socket)
-                    cylinder(r = rad + eps, h = shank);
+                    cylinder(r = shaft_rad + eps, h = shank);
     }
 
     module cs_head(socket_rad, socket_depth) {
-        head_t = rad / 5;
+        head_t = shaft_rad / 5;
         head_height = head_rad + head_t;
 
         color(colour) {
@@ -173,7 +177,7 @@ module screw(type, length, hob_point = 0, nylon = false) { //! Draw specified sc
         }
         if(head_type == hs_grub) {
             color(colour) {
-                r = show_threads ? rad - pitch / 2 : rad;
+                r = show_threads ? shaft_rad - pitch / 2 : shaft_rad;
                 translate_z(-socket_depth)
                     linear_extrude(socket_depth)
                         difference() {
diff --git a/vitamins/screws.scad b/vitamins/screws.scad
index ab11d23..152361c 100644
--- a/vitamins/screws.scad
+++ b/vitamins/screws.scad
@@ -103,6 +103,9 @@ M8_hex_screw     = ["M8_hex", "M8 hex",         hs_hex,   8, 15,  5.65,   0,   0
 
 M3_low_cap_screw = ["M3_low_cap", "M3 low cap", hs_cap,   3, 5.5, 2,    1.5, 2.0, 18,  M3_washer, M3_nut,   M3_tap_radius,    M3_clearance_radius];
 
+M3_shoulder_screw= ["M3_shoulder", "M3 shoulder",hs_cap,  4, 7.0, 2.4,  1.5, 3.0, 6,   M4_washer, M3_nut,   M3_tap_radius,    2,   3];
+M4_shoulder_screw= ["M4_shoulder", "M4 shoulder",hs_cap,  5, 9.0, 2.4,  1.5, 3.0, 8,   M5_washer, M4_nut,   M4_tap_radius,    2.5, 4];
+
 M3_grub_screw    = ["M3_grub", "M3 grub",       hs_grub,  3,   0, 0,    2.5, 1.5, 0,   M3_washer, M3_nut,   M3_tap_radius,    M3_clearance_radius];
 M4_grub_screw    = ["M4_grub", "M4 grub",       hs_grub,  4,   0, 0,    2.4, 2.0, 0,   M4_washer, M4_nut,   M4_tap_radius,    M4_clearance_radius];
 M5_grub_screw    = ["M5_grub", "M5 grub",       hs_grub,  5,   0, 0,    2.4, 2.5, 0,   M5_washer, M5_nut,   M5_tap_radius,    M5_clearance_radius];
@@ -117,6 +120,7 @@ No8_screw        = ["No8", "No8 pan wood",      hs_pan, 4.2, 8.2, 3.05,   0,   0
 screw_lists = [
 [ M2_cap_screw,    M2p5_cap_screw, M3_cap_screw,    M4_cap_screw,    M5_cap_screw,    M6_cap_screw, M8_cap_screw],
 [ 0,               0,              M3_low_cap_screw],
+[ 0,               0,              M3_shoulder_screw, M4_shoulder_screw],
 [ M2_cs_cap_screw, 0,              M3_cs_cap_screw, M4_cs_cap_screw, M5_cs_cap_screw, M6_cs_cap_screw, M8_cs_cap_screw],
 [ M2_dome_screw,   M2p5_dome_screw,M3_dome_screw,   M4_dome_screw,   M5_dome_screw],
 [ 0,               0,              M3_hex_screw,    M4_hex_screw,    M5_hex_screw,    M6_hex_screw, M8_hex_screw],